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CN CJENKRAL- PHYSIOLOGY

QUE3 LOEB

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THE DECENNIAL PUBLICATIONS OF
THE UNIVERSITY OF CHICAGO

THE DECENNIAL PUBLICATIONS

ISSUED IN COMMEMORATION OP THE COMPLETION OP THE FIRST TEN
YEARS OP THE UNIVERSITY'S EXISTENCE

AUTHORIZED BY THE BOARD OP TRUSTEES ON THE RECOMMENDATION
OP THE PRESIDENT AND SENATE

EDITED BY A COMMITTEE APPOINTED BY THE SENATE

EDWARD CAPPS

STARR WILLARD CUTTING ROLLIN D. SALISBURY

JAMES ROWLAND ANGELL WILLIAM I. THOMAS SHAILER MATHEWS

CARL DARLING BUCK FREDERIC IVES CARPENTER OSKAR BOLZA

JULIUS STIEGLITZ JACQUES LOEB

THESE VOLUMES ARE DEDICATED

TO THE MEN AND WOMEN

OF OUR TIME AND COUNTRY WHO BY WISE AND GENEROUS GIVING

HAVE ENCOURAGED THE SEARCH AFTER TRUTH

IN ALL DEPARTMENTS OP KNOWLEDGE

STUDIES IN GENERAL PHYSIOLOGY

STUDIES IN GENERAL
PHYSIOLOGY

BY

JACQUES LOEB

FORMERLY OF THE DEPARTMENT OF PHYSIOLOGY

NOW PROFESSOR OF PHYSIOLOGY IN THE

UNIVERSITY OF CALIFORNIA

t

Mil.*

THE DECENNIAL PUBLICATIONS
SECOND SERIES VOLUME XV

PART I

CHICAGO : THE UNIVERSITY OF CHICAGO PRESS
LONDON : T. FISHER UNWIN, PATERNOSTER SQUARE

1905

Coi> U r i ijl, I 1H05
BY Tin; rxivicitsiTY <>K i IIK \<,

PREFACE

I SHOULD not have had the courage to offer these
volumes to the public, had not requests repeatedly come to
me from physicians and biologists to render my publications,
which are widely scattered, more easily accessible. There-
fore, when the editor of the "Decennial Publications of the
University of Chicago' 1 invited me to make a contribution
to the series, I mentioned to him, not without hesitation,
the idea of collecting and republishing my papers on
General Physiology. Through his initiative and kind as-
sistance the idea has been carried out.

No one will expect that a collection of papers on very
diverse subjects can form attractive reading matter. Yet I
may mention, by way of an apology, that, in spite of the
diversity of topics, a single leading idea permeates all tin-
papers of this collection, namely, that it is possible to get
the life-phenomena under our control, and that such a control
and nothing else is the aim of biology. Thus the reader
will notice that in a series of these publications I have tried
to find the agencies which determine unequivocally the
direction of the motion of animals, and he will also notice
that I consider a complete knowledge and control of these
agencies the biological solution of the metaphysical problem
of animal instinct and will. In taking up the problem of
regeneration I started out with the idea of controlling these

O O

phenomena, arid considered it my first aim to find means by
which one organ could at desire be caused to grow in the place
of another organ. Thus the experiments on heteromorphosis
originated. As far as the problem of fertilization is con-
cerned, it seemed to me that the first step toward its solution
should consist in the attempt to produce Iarvo3 artificially
from unfertilized eggs in various classes of animals.

ix

PREFACE

It seemed desirable that the reader should be spared an
undue amount of repetition, and for this reason a number
of publications are omitted from this collection, and those
printed are in many cases shortened. Among the papers
which have been omitted are the preliminary notices and all
those papers of which I am not the sole author. Occasion-
ally I have made additions in the form of footnotes. Such
footnotes have always been marked by the addition of [1903]
at the end.

Only a small number of these papers appeared originally
in English, namely, VII, XXI, XXVI-XXXV, and XXXVII.
The other papers were translated from the German by Pro-
fessor Martin H. Fischer, to whom I wish to express my
sincere thanks. The credit as well as the responsibility for
the translation belongs entirely to him. In the reading of
the proof I was assisted by Dr. Fischer. Dr. Rogers, Dr.
Bullot. and Dr. Bancroft. Mr. Rogers made the index for
the first volume. To all these gentlemen my thanks are due.

JACQUES LOEB.

BERKELEY, CALIFORNIA,
October 14, 19U4.

TABLE OF CONTENTS

PART I

I. The Heliotropism of Animals and its Identity with

the Heliotropism of Plants - 1

II. Further Investigations on the Heliotropism of Ani-
mals and its Identity with the Heliotropism of
Plants 89

III. On Instinct and Will in Animals 107

IV. Heteromorphosis 115
V. Geotropism in Animals 17(5

VI. Organization and Growth - 191

VII. Experiments on Cleavage - 253

VIII. The Artificial Transformation of Positively Helio-
tropic Animals into Negatively Heliotropic and
i- ice versa 265

IX. On the Development of Fish Embryos with Sup-

pressed Circulation - 295

X. On a Simple Method of Producing from One Egg
Two or More Embryos Which Are Grown
Together 303

XI. On the Relative Sensitiveness of Fish Embryos in
Various Stages of Development to Lack of Oxygen
and Loss of Water 309

xi

XII. On the Limits of Divisibility of Living Matter

XIII. Remarks on Regeneration

XIV. Contributions to the Brain Physiology of Worms :!ir>

X V. The Physiological Effects of Lack of Oxygen 370

44253

Xll

TABLE OF CONTENTS

PART II

XVI. The Influence of Light on the Development of
XVII.

Organs in Animals 425

Has the Central Nervous System Any Influence
upon the Metamorphosis of Larva .' 43(3

XVIII. On the Theory of Galvanotropism 440

XIX. The Physiological Effects of Ions. I 450

XX. On the Physiological Effects of Electrical Waves 482

XXI. The Physiological Problems of Today 497

XXII. The Physiological Effects of Ions. II- 501

XXIII. Why Is Regeneration of Protoplasmic Fragments

without a Nucleus Difficult or Impossible? 505

XX IV. On the Similarity between the Absorption of Water

by Muscles and by Soaps - 510

XXV. On Ions Which Are Capable of Calling Forth

Rhythmical Contraction- in Skeletal Muscle 518

XXVI. On the Nature of the Process of Fertilization and
the Artificial Production of Normal Larv;e
(Phitei) from the I'n fertilized Eggs of the Sea-
T i chin 539

XXVII. On lou-Proteid Compounds and Their Role in the
Mechanics of Life-Phenomena. The Poison-
ous Character of a Pure NaCl Solution 544

XXVIII. On the Different Effects of Ions upon Myogenic
and Neurogeuic Rhythmical Contractions and
upon Embryonic and Muscular Tissue 559

XXIX. On the Artificial Production of Normal Larva? from
the Unfertilized Eggs of the Sea -Urchin
(Arbacia) 576

XXX. On Artificial Parthenogenesis in Sea-Urchins 624

XXXI. On the Transformation and Regeneration of Organs 627

XXXII. Further Experiments on Artificial Parthenogenesis

and the Nature of the Process of Fertilization - 638

TABLE OF CONTENTS xiii

\\XTII. Experiments on Artificial Parthenogenesis in
Annelids (Chsetopteras) and the Nature of the

Process ot Fertilization 646

XXXIV. On an Apparently Xe\v Form of Muscular Irrita-
bility (Contact-Irritability?) Produced by Solu-
tions of Salts (Preferably Sodium Salts) Whose
Anions Arc Liable to Form Insoluble Calcium
Compounds 692

XXXV. The Toxic and the Antitoxic Effects of Ions as a
Function of Their Valency and Possibly Their
Electrical Charge 708

XXXVI. Maturation, Natural Death, and the Prolongation
of the Life of Unfertilized Starfish Eggs
(Asterias Forbesii) and Their Significance for
the Theory of Fertilization 728

XXXVII. On the Production and Suppression of Muscular
Twitchings and Hypersensitiveness of the Skin
by Electrolytes 748

XXXVIII. On the Methods and Sources of Error in the

Experiments on Artificial Parthenogenesis 766

INDEX 773

PART I

THE HELIOTROPISM OF ANIMALS AND ITS IDENTITY
WITH THE HELIOTROPISM OF PLANTS 1

I. INTRODUCTION

I INTEND to show in the following pages that animal
movements depend upon light in the same way as the move-
ments of plants.

It is a well-known fact that animals, when light falls on
them, move toward the source of light, like the moth, or
move away from it, like the earthworm. It is also well
known that certain plant organs have a tendency to turn
toward or from the source of light when illuminated from
one side only. While the conditions which govern the
behavior of plants toward light have been well analyzed,
especially by Sachs, little has been done to investigate the
conditions upon which depend the movements of animals
toward a source of light. It is the purpose of this paper
to fill this gap, and to enumerate the facts which show that
in reality the animal motions called forth by light depend
upon the same circumstances as the motions which light
produces in plants.

The effects of light which we intend to study are purely
)i/r<-li(uuca1, inasmuch as they consist in changes in position,
MS well as in the direction and the sense of the progressive
movements of living animals. Consequently we shall regard
as essential such circumstances as can help to explain the
mechanical effects of the light. These circumstances, as in
the case of all stimulations, are of a double origin: first.
those belonging to the stimulus in this case the light; and,

1 Pamphlet, Wttrzbur, 1889.

2 STUDIES IN GENERAL PHYSIOLOGY

second, those belonging to the structure of the organism.

So far as the light is concerned, the circumstance which
controls the orientation of the animal and the direction of its
movements is the direction of the rays falling upon the
animal. 1 The condition which is of importance on the part
of the animal is the symmetrical shape of the body.

Sachs discovered that all plant organs which have a
radial structure are orthotropic (this means that they bend,
when light strikes them on one side, until their longitudinal
axes lie in the direction of the rays of light), but that all
dorsiventral structures are plagiotropic, /. e., they place their
surfaces perpendicular to the rays of light. Symmetrically
situated points at the surface possess a quantitatively and
qualitatively e^ual irritability. In this way the organ of a
plant is mechanically forced to orient itself in such a way
that the rays of light strike symmetrical points at equal
angles to the surface. If the plant, as for example the
swarm spore of algse, is capable of a progressive motion, it
must of course, in order to maintain this position, move in
the direction of the rays of light. This is, indeed, found to
be the case.

I shall now show that quite generally in animals the
(Ji red ion of the rays of light controls also the direction of
tliose movements which are caused by light; that, in addi-
tion, quite generally in animals their orientation depends

1 In these experiments it is presumed that the animals move under the influence
of only one source of light. It is explicitly stated in this and the following papers
that if there are several sources of light of unequal intensity, the light with the
strongest intensity determines the orientation and direction of motion of the animal.
Other possible complications are covered by the unequivocal statement, made and
emphasized in this and the following papers on the same subject, that the main
feature in all phenomena of heliotropism is the fact that symmetrical points of the
photosensitive surface of the animal must be struck by the rays of light at the same
angle. It is in full harmony with this fact that if two sources of light of equal
intensity and distance act simultaneously upon a heliotropic animal, the animal
puts its median plane at right angles to the line connecting the two sources of light.
This fact was not only known to me, but had been demonstrated by me on the larvae
of flies as early as 1887, in Wiirzburg, and often enough since. These facts seem to
have escaped several of my critics. [1903]

HELIOTROIMSM OF ANIMALS 3

on the form of ihe body in so far as dorsiventral animals
move with their median planes in the direction of the rai/s
of It'll hi, in which position the rays fall upon symmetrically
situated points of the surface of their bodies at nearly equal
a ii^K-s. In this way the fact that a moth flies into a flame
turns out to be the same mechanical process as that by which
the axis of the stem of a plant puts itself in the direction of
the rays of light. In both cases, however in the fatal
flight of the moth as well as in the orientation of plants -
one point remains unexplained, namely: how can the light
so change the state of the protoplasm as to bring about the
mechanical effects just mentioned? At present we are not
able to form a clear idea of this.

A second condition which has a determining influence
upon the mechanical effects of light on plants is the refran-
gibility of the rays. Sachs has shown that it is chiefly the
more refrangible rays which are able to bring about move-
ments in plant organisms. We sJtall see UK it quite gen-
eral! tj the more refrangible rays are also more effect ire
meeliaideaUij in the animal Icinydom.

Thirdly, we shall prove that the orientation of animals as
well as of plants takes place when the intensity of the light
remains constant. Very often we observe, for example in our
eyes, that a change in the intensity of the light acts as a
stimulus. In addition to these essential considerations of
the effects of light in the animal kingdom, the following
factors play a role, namely:

Fourthly, light causes the orientation of animals (as well
as of plants) only within certain limits of intensity. Fifthly,
temperature influences the movements of orientation in
animals and plants toward light --which is true for all
phenomena of stimulation.

To sum up: The condition* irJiieli control tlie mon-menfx
of animals toward light are idcnli<-al, point for point,

4 STUDIES IN GENERAL PHYSIOLOGY

irifli flioxe irltidi IK ire- been shown to be of paramount
iujlnence in jilanls.

Aside from the problem of proving by suitable experi-
ments the stated propositions, it is also necessary for us to
show what role the orientation toward the light plays in the
economy of life of an animal. I shall therefore first describe
the experimental proofs of the identity of animal lu-liot-
ropism with plant heliotropism, and then show by individual
examples what role heliotropism plays in the economy of life
of animals. To discuss the latter point it will be necessary
also to describe briefly the other forms of irritability pos-
sessed by an animal.

In a short article which appeared in January. 1888, I
described the principal laws upon which depends the orien-
tation of animals to light, and the identity of these laws with
those governing plant heliotropism. 1

II. THE ESSENTIAL PHENOMENA AND LAWS OF HELIOTROPISM

IN PLANTS

Assuming that the reader is acquainted with the orienta-
tion of plants toward a source of light, it will suffice at this
place to call attention briefly to the essential facts which bear
upon our subject. In so doing I shall follow the presenta-
tion given by J. von Sachs in his lectures on plant physi-
ology. 2

Straight stems or roots of growing plants bend when light
falls on them on one side only, or with greater intensity on
one side than on the other, until their tips lie in the direc-
tion of the rays of light. Those organs which turn toward
the source of light are called positively heliotropic; those
which turn from the light, negatively heliotropic.

i " Die Orientierung der Thiere gegen das Licht (thierischer Heliotropismus),"
Sitzunuslierichte tier Wiirzburgt-r physikalisch-medicinischen Gesellschaft, January,
1888.

2 Varies it ngen iiber Pflanzen-Physioloyic, 2d ed. (Leipzig, 1887;.

HELIOTROPISM OF ANIMALS

It was formerly belit-vcd that the bending of the positively
heliotropic parts of plants was due to the fact that the side
which was turned away from the light, gre\v more rapidly,
beeause plants when brought into the dark at first grow more
rapidly than they do in the light. But it was proved in
Sachs's laboratory that iicf/ol /!'<] /j heliotropic organs also
grow more rapidly in the dark. Because of the similarity
of the geotropic and heliotropic movement in plants, Sachs
came to the conclusion that the direction in which the rays
of light penetrate the plant tissue determines the orientation
of the plant toward light. He also proved that not all the
rays of the visible sun spectrum bring about heliotropic
movements, but only, or at least chiefly, the more refrangible
rays. The less refrangible rays, which are of importance in
assimilation, are ineffective heliotropically. If the light be
previously passed through a dark-blue ammoniacal solution
of copper, which absorbs all the red, yellow, and a part of
the green rays, the heliotropic bending occurs in the same
way as in completely white light. If, however, the light
passes through a saturated solution of potassium bichromate,
which lets through only red, yellow, and a part of the green
rays, "the heliotropic shoots remain straight and vertical, no
matter how intense the light is which passes through the
solution." Finally, if the light "is passed through a solu-
tion of quinine sulphate, the fluorescence of which completely
absorbs the ultra-violet rays, the heliotropic curvatures
nevertheless appear a proof that they are caused princi-
pally by the visible blue and violet rays."

The best proof of the theory that the dirrc/ioii of the
ravs of light controls the orientation of plants was found by
studying freely moving plant organs, the swarm-spores of
alg;e. These swarm-spores make progressive movements
like animals, and Strasburger 1 proved that they move in the

i STRASBUEGER, ir/Y/,x/i.</ </<> /./V/i/rx uml </o- \\'iirn- ,//// s,-hn-i'iriii,iporen
(Jena, 1-7- i.

6 STUDIES IN GENERAL PHYSIOLOGY

direction of the rays, to or from the source of light. The
more refrangible rays alone exercise this effect on the swarm-
spores. They behave in the light which has passed through
an ammoniacal solution of copper just as in diffuse daylight.
On the other hand, they are not affected by light which has
passed through a potassium bichromate solution, by light
from a sodium flame, or by the light coming through ruby
glass.

The chlorophyll-bearing protoplasm of cells moves under
the influence of light. 1 The chloroplasts of a thread alga,
Mesocarpus, turn "their broad surfaces toward the sky so that
the rays fall upon them at right angles. If the direction of
the rays is changed, the chloroplasts turn so that their broad
surfaces are again at right angles to the rays. Direct sun-
light, however, causes the chloroplasts to assume another
position they place their surfaces parallel to the rays which
strike them."'

According to modern plant physiology, the whole proto-
plasm of a multicellular plant is to be conceived of as a
continuous mass, as a single protoplasmic body. 2 More
recent investigations have shown that when a plant organ is
illuminated, that side of the organ which becomes concave
from the effect of the light becomes rich in protoplasm, while
the opposite convex side becomes poor. 3 Multicellular organs
behave in this regard like unicellular ones. Thus it appears
that the light forces the protoplasmic mass to move in such a
way that positively heliotropic protoplasm wanders to the
side of the organ which is turned toward the light, while
negatively heliotropic protoplasm wanders to the opposite
side. 4 Should it turn out that this phenomenon really occurs

i STAHL, Botanische Zeitung, 1880. 2 SACHS, loc. cit., p. 94.

3 Wortmann expressed his observations in this way. It is possible that in
reality protoplasm on the concave side is only more opaque than on the opposite
side. This difference in optical appearance may simply be the expression of a
difference in the size of the colloidal particles. [1903]

+ See WORTMAXN, Botanische Zeitung, 1887.

HELIOTBOPISM OF ANIMALS

in all cases, it would prove that the protoplasm of a multi-
cellular plant behaves just like the naked, creeping plas-
niodium. which is also heliotropically irritable.

III. SUMMARY OF THE MECHANICAL EFFECTS OF LIGHT IN THE
ANIMAL KINGDOM WHICH ARE THUS FAR KNOWN

I shall in this chapter summarize briefly the facts and
views in regard to the movements of animals under the in-
fluence of light, so far as they are known up to the present
time. These may be divided into three groups:

1. Casual observations of the older authors (Reaumur,
Trembley ). These are unprejudiced records of simple obser-
vations.

2. Modern investigations on the effects of light from an
anthropomorphic standpoint. The movements of animals
are not attributed to mechanical causes, but to supposed
human sensations of the animals.

3. Investigations according to the method of Sachs, which,
however, have been applied only to Protozoa. The last-
named observations are the most important in these three
groups.

The earliest account of the effects of light on animals
which I have found in the literature is by Reaumur. 1 He
found that moths which are attracted by the candle flame "do
not fly from flower to flower during the day." Since he saw
chiefly the males fly into the flame, he raised the question
as to whether or not the female moths emit light like glow-
worms. "Do not the females of the nocturnal Lepidoptera
emit a light too feeble to make an impression on our eyes,
but sufficiently strong to act on those of their males?" He
had observed, evidently, that the males of the glow-worm
which are attracted by the light to the aboral end of the
females likewise fly into the light. Reaumur was, moreover.

i RE \i MI i . Mt moires pour servir n lliistoire des inscctes, Vol. I, 1, p. 330 (Amster-
dam, 1748).

8 STUDIES IN GENEKAL PHYSIOLOGY

convinced that the glow-worm living in the woods could see.
He made a glass window in a tree in which such worms
lived and noticed that the animals gave a start upon the
approach of a burning candle.

Trembley made far better experiments. 1 He found Ili;it
"water fleas" can be driven around in a circle by a moving
candle :

By the light of a wax taper I observed polyps to which during
the day I had given many water fit-as; in the evening 1 there were
left in the glass some which tha polyps had not consumed. I
noticed that most of them had collected on the side toward the
caudle. I changed the position of the taper, and they followed it.
As I had moved its position repeatedly, and each time had seen
that the water fleas followed it, I moved the taper slowly around
the glass without stopping. They followed, and thus made several
trips around it. I have had the opportunity of repeating this ex-
periment several times.

Trembley's observations on the effect of light on Hydra
were made with great care. After he had repeatedly observed
that the polyps moved to the ''brightest" side of the glass,
he placed "a glass containing many green polyps in a case
which had an opening on one side about opposite the middle of
the glass." He reports as follows concerning their behavior:

When I placed the glass so that the opening in the case was
turned to the light, the polyps always migrated toward that side
of the glass which was opposite this opening, in such a way that
together they made the figure of a gable. I often turned the
glass around, and after several days I observed the polyps again at
the opening arranged as before ( in the form of a gable). To vary
the experiment still further, I fixed the dark case so that the open-
ing was at times straight, at other times inverted, and again the
polyps arranged themselves together.

After he had discovered that polyps which had been cut in
two could "move, eat, and multiply," he tried to see "whether

Abhandlungen zur Geschichte elner Polypenart, transl. by GOTZE
(Quedliuburg, 1791).

HELIOTBOPISM OF .\\I.M\LS 9

these pieces would turn toward the light in the same way as
the undivided polyps." He cut a number of polyps in two:
the anterior halves he placed in one glass, the posterior
halves in another. He found "in oft-repeated experiments
that the animals in both glasses collected in the brightest
regions in the glass."

These are, as far as I know, the only extended observa-
tions to be found in the old physiological literature of the
effects of light upon animals. For a long time no further
study of the effects of light upon animals was made.
Johannes Milller mentions, in the preface to his Physiologic
dcs Cf-esichtssinnes, that he made "investigations on the in-
fluence of colored light on the vital phenomena of plants and
animals," but, as far as I know, the results of his investiga-
tions were never published.

The modern anthropomorphic observations were intro-
duced by Paul Bert. Bert raised the question: Do all
animals see the same rays that we see? 1 He meant to ask
whether all rays of the visible sun spectrum are able to
bring about animal movements. An experiment with Daph-
nia pulex was sufficient for Bert to settle this question. He
projected a spectrum and found that the animals became
restless in all positions of the visible spectrum:

Mes daphnies erraient disperses d'une maniere h pen pres
6gale dans toute l'6tendue du vase obscur, lorsque soudain je lis
tomber sur la fente un rayon color6, un rayon vert. Aussitot elles
s'agit&rent, se grouperent tontes dans la direction de la trainee
lumineuse et un tres-grand noinbre s'eii vint se heurter, montant et
descendant sans relache centre la paroi qni recevait la lumiere.
Or, un semblable r6sultat fnt obteuu pour toutes les regions du
spectre visible. Le rouge, le jaune, le bleu, le violet meme atti-
raii-nt les daphnies. Settlement il fnt facile de remarquer, qu'elles
a -ci niraient beancoup plus rapidenient au jauue on an vert qu'a
tonte ant re conlenr.

1 BERT, .irritii-i:* iir physiologic, 18G9.

10 STUDIES IN GENERAL PHYSIOLOGY

On either side of the spectrum the animals remained at rest.
In addition to this, Bert made another experiment. He
had a spectrum projected on a trough, and observed how
the animals distributed themselves over the different parts
of the spectrum.

L'immense majority se placa dans le jaune, le vert, 1'orange;
une assez grande quantity se voyaient encore daus le rouge, un
certain nombre dans le bleu, quelques-unes de plus en plus rares a
mesure qu'on s'6loignait dans les regions plus r6frangibles du
violet, au dela du rouge, an del de 1'ultra-violet; dans les regions
invisibles, en uu mot, on n'en trouvait que d'isolees en promenade
accidentelle.

From these facts Bert concluded that Daphnia behaves
in the spectrum much as a man would, who, when reading a
book, would move into the brightest part of the spectrum,
into the yellow light.

Lubbock repeated Bert's experiment on Daphnia. 1 One-
half of a dish was covered by a yellow screen; the other
half was left uncovered. In the uncovered half 1,904
animals collected, while 3,096 gathered under the yellow
screen. From this Lubbock concludes that Daphnia has a
'preference" for "yellow." But one would suppose that in
the uncovered part of the dish there was at least as much
yellow light as under the yellow screen; or did the majority
"hate" the blue light?

When Lubbock covered one-half of the trough with
blue glass and left the other uncovered, he found 2,046
animals under the blue glass, and 2,954 in the uncovered
part of the trough. Whether one is to conclude from this
that blue light is in the sense of Lubbock "disagreeable" to
Daphnia is not stated. When half of the trough was
covered with red glass, there collected 1,928 animals under
the red glass, while 3.072 collected in the uncovered por-

i LUBBOCK, " Die Sinne und das geistige Leben der Thiere," Internationale
wissenschaftliche Bibliothek, Vol. LXVII (1889).

HELIOTROPISM OF ANIMALS 11

tions of the dish. When half of the vessel was covered
with an opaque porcelain screen, Lubbock found 2,04S
animals collected under it, and 2,932 animals in the un-
covered half. From these and similar experiments Lubbock
concludes that the animals have a decided preference for
yellow light.

I also have made some experiments on the effects of
rays of different refrangibility on Daphnia, and found that
when the more refrangible rays (blue and violet) fell
upon the animals they hastened to the source of light and
moved up and down on the light side of the vessel. When
I made the same experiment with the less refrangible rays,
the effect was weak or did not take place at all. The result
conforms with other facts which are to be described later.
I shall, therefore, not revert to the Daphnia and their
alleged "preference for yellow."

Lubbock has employed a similar method in his experi-
ments 011 wingless ants; 1 these, however, led to much more
fruitful results than his experiments on Daphnia. In an
experiment in which a vessel was covered with strips of
red, green, yellow, and violet glass he found that 890
animals collected under the red glass, 544 under the green,
41)5 under the yellow, and only 5 under the violet. There
is no doubt in this case that the animals collected under
those glasses where they were struck by the less refrangible
rays. Other experiments showed that red glass acts like an
opaque body.

The observation of Lubbock that ants avoid the ultra-
violet part of the spectrum is also worthy of note. For the
sake of completeness the experiments of Lubbock on bees
and wasps must be mentioned, in which it was found that
under otherwise similar conditions blue objects smeared with
honey were preferred to those of another color.

'LUBBOCK, "Amrisi-u, Hii-ncn mul UV-prii," iliiil., 1883.

12 STUDIES IN GENERAL PHYSIOLOGY

The most extended experiments on the influence of light
on the orientation of animals were made by Graber. 1 His
"comparative studies on light-sensations" (Vergleichende
Liclit-Gefilld-Stinlicn}, as he called his investigations, cover
about fifty species. His method is that followed by Lub-
bock.

The faultiness of this method and the errors of interpre-
tation of the results obtained stand out more clearly in
Graber's writings than in Lubbock's. Graber covers one-
half of a vessel with a partially or completely opaque
screen, and after a time notes how the animals are dis-
tributed in the vessel. If most of the animals are under
the opaque screen, Graber says that they are "fond of the
dark" and ''hate the light,;"' or in the reverse case, that
they are "fond of the light" or of "the white" and "hate the
dark." He therefore uses the conceptions of "white" or
"bright" and "dark, 11 which designate certain effect* of l/n/it
upon a human being for the conceptions of great or small
intensity of the light, and in saying that animals which
"prefer the light'' also "hate the darkness" he makes a
second mistake in that he maintains that strong and weak
light have opposite effects. We shall see, however, that
these effects are similar and differ only in degree. He makes
the same mistake in experimenting on rays of different
refrangibility. The most important among the facts ob-
served by him in this connection is this, that animals which
"prefer the light" with a few exceptions also "prefer" blue,
while those which "hate the light" "prefer" red. His ideas
are expressed in the following remarks, which, however, I
do not fully understand:

The question arises as to the cause of this truly striking 1 rela-
tion between the love for white light and for blue light, on the one
hand, and between the dislike for white light and for blue light,

1 Grundlinien zur Erforschung des Helligkeits- und Farbensinnes der Thiere
(Prag, 18S4).

HELIOTROPISM OF ANIMALS

on the other h;m<l. If the law had reference only to white light,

and ii-il . i No to colored light red, blue, etc. which is, ho\\r\cr,
bv HO means always the ease, one might at first be inclined to be-
lieve that the animals which prefer red avoid mixed light because
it contains many of the hated short waves of the, blue and violet
light; for this very reason it would be more agreeable than dim
light to the animals which prefer blue, for dim mixed light is poor
in all rays, and therefore also in blue. Yet the objection might be
raised against this explanation that mixed light contains as much
red for those animals which prefer red as it contains blue for tho^-
animals which prefer blue. Yet this objection could again be
weakened by the assumption that, since the animals which prefer
red also prefer darkness, they prefer a minus of their chosen color
to a plus of the color they dislike.

Graber finally considers it best "to await further investi-
gations in a field where great darkness still prevails." We
see that Graber in regard to the effects of monochromatic
lifht ao-aiii establishes a contrast in effects where, as we shall

?> o

see, a similarity exists. Graber was prevented from cor-
rectly interpreting his results by attributing the movements
of animals to sensations instead of to physical causes. If
he had given up the anthropomorphic standpoint, he would
soon have discovered that his experiments show that the
more refrangible rays are more effective in causing the
orientation of an animal than the less refrangible ones.

In none of the investigations of Bert, Lubbock, or Graber
has the influence of the direction of the rays on the orienta-
tion been studied. Graber, for example, took it for granted
that an animal moves to the light because, as he expressed
it, "it is fond of the light" or "the white." If it moves
in the opposite direction, it "is fond of the dark." Lubboek
remarks incidentally that "ants do not like light in their
nests, probably because they do not deem it safe."

This sums up the opinions and results of the authors who
sought to explain anthropomorphic-ally the phenomena which
interest us here.

14 STUDIES IN GENERAL PHYSIOLOGY

Finally, I have to mention the heliotropic investigations
on Infusoria which were made along the lines mapped out
by Sachs. To bring these investigations before the reader
I shall describe the more important observations which have
been made on Euglena. Tho influence of the direction of
the rays of light on these Infusoria was first demonstrated
by Stahl: 1

Those individuals which did not swim about freely remained
with their pointed posterior ends attached to the cover-glass or to
other objects, while their free anterior ends were, according- to con-
ditions, either turned toward or away from the source of light. The
loiKjitudinal axes of. both the motile and sessile Euglenaeco/mvW<W
as nearly as possible icith the direction of the rays of liyht. The
motionless ones behaved like the free-swimming ones whenever the
direction or intensity of the light was suddenly changed, except
that they reacted more slowly. If, for example, the glass slip was
suddenly rotated through an angle of 180% the position which the
animals occupied originally with reference to the source of light
was slowly reassunied, while the swimming individuals left their
former path and moved in the original direction toward the light
immediately after a change in its direction.

Engelmarm studied in Euglena the relation between the
effect of the rays of light and their refrangibility. 2 After
he had established the fact that when a drop of Euglena? is
only partially illuminated the animals gradually accumulate
in the lighted area, he brought the animals into a micro-
spectrum. Here they collected on the more refrangible side
of the spectrum. The orientation of Euglena therefore
depends on the direction of the rays, and especially on
that of the more refrangible ones. It must finally be men-
tioned that the anterior ends of the Infusoria are most sen-
sitive to light; yet the pigment spot is not, as might be
supposed, the most sensitive, but the colorless protoplasm in
front of this.

Besides these direct effects of light in phenomena of

1 Botanlsche Zeitung, 1880. 2 p 'fingers Archiv, Vol. XXIX (1882).

11 i: LI OT HOT ISM OF ANIMALS 1 .")

orientation, which alone interest us here, there are also cer-
tain indirect etl'ects on the orientation of low forms of life.
These were also first observed by Engelmann. When the
supply of oxygen is cut off from certain chlorophyll-bearing
organisms, they remain in that part of the spectrum in which
assimilation takes place. In water with its normal amount
of oxygen, as Engelmami found, Stentor viridis, Bursaria,
and the green slipper aniiualcukie do not react to light. 1 If,
however, the supply of oxygen from without is interfered
with, '"the insufficient supply can be compensated for by a
production of oxygen by the chlorophyll granules within the
mesoplasm." Under these conditions the animals return to
the light side of the drop when they accidentally get into the
shady part. When the animals are brought into a micro-
spectrum, they collect in those regions which promote assimi-
lation. The opposite effect takes place, however, when the
supply of oxygen from without exceeds the normal. When
Eugelmanii passed a stream of pure oxygen through the
water, the animals moved from the lighted into the shaded
part of the drop.

Such an indirect orientation toward light as is determined
by assimilation is shown also in the behavior of the purple
bacteria." These, as Engelmami found, collect in those
regions of the spectrum which are most absorbed by the
coloring matter of the bacteria.

These are the most important facts which up to this time
are known concerning the influence of light on the orienta-
tion of animals. Thus far only the observations made on
Infusoria are sufficient to warrant the conclusion that ani-
mal movements depend on light in the same way as the
movements of plants. In the rest of the animal kingdom
either the facts necessary for this conclusion are lacking, or
false statements and conceptions are prevalent. So far as

/.. p. 387. 2 EXGELMAXN, /.'"/" ni.--> /,< /.: ilniKi. 1S88.

16 STUDIES IN GENERAL PHYSIOLOGY

the latter are concerned, it is wrong, as we shall see, to say
that certain animals "are fond of the light" and seek those
in space where light is most intense, while others "are

fond of the dark" and betake themselves to those regions
which are darkest. In contradiction of this idea I shall
prove that the direction of the progressive heliotropic move-
ments of animals is determined solely by the direction of the
rays, 110 matter whether the animals move from regions in
which light is less intense to those in which it is more
intense, or vice versa.

Further than this, it is fundamentally wrong to say that
;ni assumed "preference for color" determines the orientation
of animals toward rays of different refrangibilities; that,
as Graber says, the animals which "are fond of blue" "hate
red," and that those which "are fond of red" "hate blue."
In contradiction of this idea I shall prove that there are no
animals which "are fond of" red or "hate" blue, but only
such as move toward a source of light or away from it ; and
that these movements occur in the same way under the
influence of the more refrangible rays as under that of the
less refrangible rays, only with this purely quantitative
difference, that the more refrangible rays, as in plants, are
much more effective than the less refrangible ones, which
usually have no effect.

I consider it inadvisable to represent the movements ob-
served in animals as the expression of a "color preference,"
or a "color sensation," of a "pleasurable" or "unpleasur-
able sensation," as do most animal physiologists and zoolo-
gists who have studied the effects of light in the animal
kingdom. I do not propose to base an analysis of the
movements of animals on such hypothetical, anthropomorphic
sensations and feelings, but on such conditions as determine

O *

the course of phenomena in inanimate nature as well. Real
natural science began when, instead of fabulizing over the

H i: i.i OT Kim 1 ISM OF A \IM\I.S 17

nature of gravitation, men determined accurately the details
nt' the movement of falling stones, of pendulums, etc., and
described them in the most simple and definite terms. In
biology, especially in regard to the mechanical effects of
light which concern us here, the task of the investigator can
only be to determine and describe the circumstances upon
which depend the movements of animals under the influ-
ence of light.

IV. REMARKS ON THE METHOD OF EXPERIMENTATION. THE

HELIOTROPISM OP AN ANIMAL USUALLY BECOMES EVIDENT

ONLY AT A DEFINITE EPOCH IN ITS EXISTENCE. THE

HELIOTROPISM OF AN ANIMAL CAN EASILY BE OBSCURED
BY A SPECIAL FORM OF CONTACT-IERITABILITY

The facts which I have to prove are so simple that almost
all technical apparatus can be dispensed with. If one
attempts to demonstrate that the orientation of the animals
is controlled by the direction of the rays of light, care must
be taken that light falls upon the animals from only one side.
To accomplish this it is sufficient to carry on the experiments
in a room which is lighted from one side only. Since the
animals with which we are dealing in this discussion are
dorsiventral and place their median planes in the direction
of the rays of light, progressive movements are possible in
only two directions either toward the source of light
(when they will be called positively heliotropic), or away
from the source of light (in which case they will be called
negatively heliotropic). 1

Diffuse daylight was used as the source of light, and only
where specially mentioned was sunlight employed.

1 Some botanists designate the movements of inutile plant organisms toward a
source of litfht as " phototactic," in contract, to the "heliotropic" movements of
>e--ili} plants. Since tin- observations of Sachs, Stahl, and Wortmaim, ho\\i-\-r.
leave no room for doubt that tin- processes are identical in hot h ca.-es, it -rein- to me
that this .separation is nut ju-titird. Otherwise a " photot actic " animal tiucht to
become " heliotropic" when its pr< >i,'re--i vo movements are prevented. For this
n -a -on I use the- same term for similar processes. (See WOETM ANN, i:<itnni.--he Zei-

18 STUDIES IN GENERAL PHYSIOLOGY

There are two methods by which the second fact, that only
the more refrangible rays bring about orientation, can be
proved, namely, by experimenting with prismatic spectra or
with colored screens.

All authors who have studied the behavior of plants
behind colored screens have obtained the same result that
it is only, or more especially, the more refrangible rays which
are heliotropically active. Studies on the behavior of plants
in prismatic spectra have led to harmonious results, in so far
as they confirm the gross results obtained by using colored
screens; yet opinions differ as to the efficacy of the more
limited portions of the spectrum. Since for the present I
wish to show only that the laws governing the orientation
of an animal toward light correspond to the laws governing
the orientation of plants toward the same stimulus, it was
necessary to use as a basis the really established data of plant
phvsioloV, and I therefore shall confine myself to the proof

I &/ " */

of the fact that the more refrangible rays of the spectrum
are exclusively, or almost exclusively, effective. To do this
I proceeded as is usual in plant physiology. In order to
have only the less refrangible rays act on the animals, I
passed the diffuse daylight through a solution of potassium
bichromate or ruby glass ; to study the influence of the more
refrangible rays, I chose cobalt glass or an ammoniacal solu-
tion of copper. The screens were examined spectrosoopically.
The dark-red glass which I used completely absorbed the
more refrangible rays, and let through only the red, yellow,
and a part of the green rays. The dark-blue glass absorbed
the less refrangible red and yellow and a part of the green
rays, with the exception of a small region in the outer red.
Since, however, the heliotropic phenomena appear only
weakly or not at all behind dark-red glass, while they occur
just as in diffuse daylight behind dark -blue glass, the few
red rays which penetrate the dark- blue glass cannot be

HELIOTROI-ISM or ANIMALS I'.l

responsible for the heliotropic phenomena which take place
so energetically behind this screen, but can be due only to
tin- activity of the more refrangible rays.

The other external conditions which must be considered
in heliotropic investigations are so simple that they do not
call for any special explanations. Where they are of impor-
tance they will be self-evident.

It is rerij essential, lioirerer, to realize, that lite Itelio-
tri>i*i of an annual often manifests if self clearly only dnr-
a. ({([finite, often decisive, period of its existence, onl// to
a</ain Of to disappear entirely later. It was only
by observing for weeks and months the animals described in
this treatise, which for the most part I raised myself, that
I have been able to establish this fact.

The caterpillars of Porthesia chrysorrhoea, for example,
are energetically positively heliotropic only during a certain
period of their existence, when they have just left the coc-
coon in which they have wintered, and have not yet taken
food. At this time the entire existence of these animals is
a function of the light. Under natural conditions they
hatch out on a warm spring day. The light compels them
to creep to the tips of the branches, where they find their
first nourishment in the young buds. When fed they are
still positively heliotropic, but very much less so than before.
If anyone should examine them in this condition, he would

i/

scarcely pronounce them heliotropic.

It is not, however, a certain date of the year which gov-
erns this heliotropism ; for whenever I forced the animals to
leave their nest (by raising the temperature), whether at the
beginning of summer or of winter, they were indefatigable
in their attempts at creeping toward the source of light.

\Vinged ants are pronouncedly dependent on light only
at a definite period of their existence at the, time of their
nuptial flight, The same animals which were actively helio-

20 STUDIES IN GENERAL PHYSIOLOGY

tropic at the time of the nuptial flight were practically
indifferent toward the light a few days previously. In the
same way, later on their heliotropisrn was entirely pushed
aside again by another form of irritability, frequently
encountered in the animal kingdom, and to which I shall
soon return.

Fly larvae also possess very different forms of heliotropic
irritability at different epochs in their existence. Negative
heliotropisrn is not very distinct in the newly hatched larvae ;
but the animals turn their ventral surfaces toward a suffi-
ciently intensive source of light without otherwise being
influenced by the direction of the rays of light. Full-grown
larvae, however, place their median planes very sharply in the
direction of the rays of light, provided the light is suffi-
ciently intense. I believe that this periodic appearance of
heliotropic irritability plays a great role in the ecology of
animals. The periodic migrations of many animals, such as
birds of passage, might be explained in this way.

It is a well-known fact that the irritability of an animal
in the larval stage may be entirely opposite in kind to that
of the adult stage. This phenomenon is very common. The
larva of the fly is negatively heliotropic, while the imago is
positively heliotropic; this is also the case with June-bugs
and many other animals. I encountered this inversion of
the sense of heliotropisrn when the animal changed from
the larval stage to the mature state so frequently that
for a time I thought it a universal rule. Such, however, is
not the case. Caterpillars, for example, behave toward light
as does the imago, as I know from my own experience and
from what I can find on the subject in the literature.

The behavior of an animal is determined by the sum
of all the forms of its irritability. The heliotropic irrita-
bility, therefore, may be obscured by a more powerful irrita-
bility of another sort. This is often due to a special kind

HELIOTROPISM OF ANIMALS 21

df contact-irritability, which, so far as I know, lias not \H
been recognized. Man// insects arc co//>e/le</ to />////// Iheir
hodies in contact irilli flu- surfaces of solid hod/ex in a rerij
ilejinUe ii'di/. JNIy attention was called to this phenomenon
in my experiments on animal geotropism, in which I allowed
the animals to move about on geometrically simple bodies
bounded by plane surfaces. I noticed that the animals
ran-ly remained on the plane surfaces, but collected about
the edges, particularly the vertical ones. It is irorfhi/ of note
tlnif certain (inin/ols ultra t/s seek the concavity of the ainjle
Ix'fireen the sides of liollotr cnlx's, trliile others just as con-
stantly more on the conre.r si<lc. The caterpillar of Por-
thesia chrysorrhcea is an example of the latter type. The
other form of this contact-irritability, which leads the ani-
mals to the concavity of the angles, is very common. The
following observations show how this form of irritability
might easily be confused with the irritability toward light,
and so lead to a misconception of the behavior of the animal
toward light.

I studied for several weeks a large number of moths of
the species Amphipyra. The animals are remarkable in
that they are more given to running than to flying. The
rapidity of their running movements calls to mind the lively
movements of cockroaches and ants. While formerly I had
found that all butterflies are positively heliotropic, I observed
that Amphipyne when let loose, did not fly to the window,
but to the nearest wall or to the floor, where they ran about
nimbly and crept under the first suitable object, like cock-
roaches. This looked as though the animals fled from the
source of light. Yet it could be shown that the animals
more toirard a source of li</lit, and that the inclination to
creep intocrevices depends upon the contact-irritability, which
was mentioned before. The following experiments always
succeeded : In the evening, when a lamp was brought into

22 STUDIES IN GENERAL PHYSIOLOGY

the neighborhood of a box containing the animals, those
which reacted at all always flew with great violence to the
side of the vessel which was turned toward the light. In no
case did they fly in the opposite direction. The experiment
was unequivocal and could be interpreted in but one way.
So far as the contact-irritability is concerned, the animals
collected in the four concave vertical edges when kept in a
cubical wooden box, which was covered on top with window
glass. In this position they assumed an indifferent orienta-
tion toward the source of light. To make perfectly sure of
this fact, I employed the following method : I placed a
plate of window glass so close to and parallel with the plane
of the floor of the vessel containing the animals that they
could just wedge themselves in between the floor and the
window glass. The glass plate was entirely exposed to the
light. Those animals which by chance came to the edge of
the glass plate crept under it, and remained in this position
exposed to the light, in contact, however, both above and
below with solid bodies. On the next day all the animals
were under the glass plate. The animals are therefore forced
to bring their bodies in contact with other solid bodies, and
it is this (and not the light) which causes them to creep
under solid bodies. I placed a ball of paper in the vessel
containing the animals ; a part of them crept under the paper
and a part into its folds. In nature these butterflies remain
in the clefts on the bark of trees or on the ground in mead-
ows.

Forficula auricularia are found in great numbers in verti-
cal crevices (such, e. g., as the spaces between gate and gate-
post, in the entrance to gardens). I obtained the animals
for my experiments by hanging a cloth of cotton on the top
of a small grape vine. The animals collected in the folds of
the cloth. These animals in reality move away from the
light ; that is to say, they are negatively heliotropic ; but it

H KLIOTROPISM 01' ANIMALS

would li- wrong to attribute tlicir tendency to creep into the
folds of the cloth to their negative heliotropism. When I
experimented on these animals with the glass plate, I found
that they wedged themselves under it, and remained there
exposed to broad daylight, rather than creep away from it.
Inside of a box the animals collected in the concave edges ;
and it was very noticeable that the animals rarely ran over
the free surfaces, but nearly always along the edges, as if it
were ever necessary for them to have their sides in contact
with solid bodies.

I believe that this form of contact-irritability is identical
with the important phenomenon, observed by J. Dewitz, 1
that spermatozoa are compelled to turn a certain side of their
bodies toward solid bodies. Because of this contact-irrita-
bility a spermatozoon is never able to leave a cover-glass or
a glass slide when once it comes in contact with it. I have
observed the same phenomena in hypotrichal Infusoria.
These always turn one side of their bodies, the ventral,
toward solid bodies. They further resemble the spermatozoa
observed by Dewitz in that they alter the direction of their
ru )vement always in the same sense, so that 011 the cover-
glass of a microscopical preparation are found only Infu-
soria which move in one direction, while on the glass slide
they seem to move in the opposite direction.

In order to distinguish this form of contact-irritability
f -om other forms of contact-irritability (such as the rolling iip
or progressive or retrogressive movements when touched), I
s'lall call the peculiarity, possessed by some animals, of
o/ienting their bodies in a definite way toward the surface
of other xo//W bodies, xlcrrotroiiixni.

The co-operation of other forms of animal irritability with
heliotropism is so simple as to be self-explanatory wherever
we may encounter it in our experiments.

i J. Dr:\vir.. rtt.t;/,-,-* .\n-lih-. V..1. \\XVIII > INN;).

24 STUDIES IN GENERAL PHYSIOLOGY

V. THE POSITIVE HELIOTROPISM OF THE CATERPILLARS OF
PORTHESIA CHRYSORRHCEA

I will enumerate the observations which show the identity
of animal and plant heliotropism in the caterpillars of Por-
thesia chrysorrhoea. I shall mention only such experiments
as in my experience were always successful under the given
conditions, and which may be taken as the prototype of the
experiments made upon, all the animals treated of in this
discussion.

1. Tin- direction of the progressive morement in animals
is determined by the direction of the rays of light. I placed
a large number about a hundred specimens of the small
gregarious caterpillars of Porthesia chrysorrhoea which had
just crept out of the web in which they had passed the win-
ter into a test-tube. They had not fed as yet, and in this
hungry condition they were exposed to the light. The tem-
perature of the room was necessarily more than 12-15 C.,
as otherwise they would have crowded together and fallen
asleep again a state in which they react neither to light
nor to gravity.

Experiment 1. If the test-tube is laid on a dark table,
so that the longitudinal axis of the tube is perpendicular to
the plane of the window, the animals, which are at first scat-
tered about irregularly, all assume the same orientation.
They creep to the upper portion of the test-tube, turn their
heads toward the window, and with their ventral surfaces and
their heads turned toward the light creep in a straight line
toward the window side of the test-tube. The process
requires from one to five minutes, according to the tempera-
ture and the condition of the hibernated animals. AH ivith-
ont exception, provided they are not sickly, move in the
direction of the rays of light to the window side of the test-
tube. If the tube is turned about an angle of 180, the

HELIOTROPISM OF ANIMALS 25

process is repeated, the animals creeping to the window side
of the glass just as before. If, however, the position of the
glass remains unchanged, the animals remain permanently
crowded together on the window side of the test-tube.

E.ri>en'iiteitt <?.--! the test-tube is laid on the table with
the l()ii(/ifii<liii(il r/.r/x /Htrallcl to the
plane of the window, the animals
gradually scatter uniformly over the
whole of the upper part of the tube.
The lower portion of the vessel is in K R

consequence again free from animals.
If the longitudinal axis of the test-
tube lies at even a slight angle with
the plane of the window, the animals
move to the end of the tithe nearest
ihe in'iuloiv, and remain there in their
customary position.

E.r/>cn'ni<'iit 3. --The test-tube is placed perpendicular to
the plane F of the window, and at the beginning of the
experiment the animals are collected at the window side B
of the test-tube (Fig. 1 ). That half of the vessel which lies
nearest the window is now covered with an opaque paste-
board box, K. The following then occurs : The animals
soon appear at A on the room side of the pasteboard box ;
as soon, however, as they emerge from the box K into A,
they turn about, direct their heads toward the window, move
to the edge of the pasteboard, and remain at the boundary
between the covered and the uncovered portions of the tube,
at A and especially at the top of the test-tube. The remark-
able thing is that they are not distributed evenly over the
whole brightly illuminated part of the test-tube. The
explanation is as follows: As soon as the animals near the
window at B are covered by the pasteboard, the weak rays
of light reflected from the walls of the room fall upon them.

26 STUDIES IN GENERAL PHYSIOLOGY

The animals follow the path of these rays and arrive at the
uncovered portion of the tube. As soon, however, as the
strong rays of diffuse light fall upon them at A, they turn
about and direct their heads toward the window, until they
come again under the pasteboard which shuts out the diffuse
light. They are then again attracted by the light of the
room, and so on, until they come to rest at the boundary
between the two regions at A.

At the beginning of the experiment, before the animals
stop moving it can really be seen that they are driven around
in a narrow circle.

If at the beginning of the experiment the animals are
collected, not on the window side, but on the room side of
the test-tube at C, they move toward the window until they
reach the pasteboard at A. If the tube is pulled away from
the window for some distance, while the pasteboard remains
stationary, the animals begin to move, until they reach the
edge of the pasteboard.

If the tube is placed horizontally with the longitudinal
axis parallel to the window, the animals distribute themselves
over the whole length of that portion of the tube which is
not covered by the pasteboard, collecting, however, always
on the window side of the tube.

According to the prevailing views of zoologists and ani-
mal physiologists, the movement of caterpillars toward the
light is determined by the animals' "fondness for light."
They, therefore, move from a region of less intense light to
one of greater intensity. That the essential feature, how-
ever, is the direction of the rays, and not a difference in
their intensity, 1 is evident from the following experiments.

Exjternticnt 4. -The animals are in a glass cylinder a,
some 3cm. in diameter. Light can enter it from all sides
(Fig. 2). The inside of a second test-tube 6, which has the

1 In different parts of the tube. [1903]

HELIOTBOPISM OF ANIMALS 27

same diameter, is covered with dull black paper, except for
a strip about '_hnm. wide. The two test-tubes are placed
together on a table so that their longitudinal axes lie per-
pendicular to the plane ^*'of the window, and the transparent
side cd- of the glass l> is turned up; the animals move along
the illuminated side cd from a to 6, with-
out stopping at the boundary between
them, until they reach the window side c
of the cylinder. The total amount of
light which strikes a caterpillar in the
glass b, however, is less than in the glass a,
since all 'lateral rays are cut off in the
former and the animal is struck by rays
of light only on its ventral side; in test-
tube a light falls upon the animals from
all sides, though the rays from above and
in front are of course the most intense. The animals there-
fore move toward the source of liyht in the direction of the
of //'////, eren if by so doiny to judge from Itiimaii
ions they are led from a ''hriyht" to a "(///,"
place.

In such an experiment 110 animals are found, as a rule,
scattered over the rest of the surface of the glass h. If
both glasses are turned around so that a is nearest the
window side, the animals of course again move from b to a.

The experiments described here were carried on in diffuse
daylight. In sinilia/it, however, the results are the same as
in diffuse daylight. When the glass is placed with the
longitudinal axis in the direction of the rays, the animals
move in the direction of the rays toward the sun and collect
at the end of the glass which is turned toward the sun, even
though in their hungry state they cannot bear the high tem-
perature. When the test-tube is placed with the longitudinal
axis perpendicular to the rays, the animals scatter over the

28

STUDIES IN GENERAL PHYSIOLOGY

whole length of the tube, remaining, however, upon its sunny
side. Orientation takes place more quickly in direct sun-
light than in diffuse daylight.

l\.i-j>erhnent o. A small pencil SS of direct sunlight is
allowed to fall on a table obliquely to the plane of the win-

dow through the window F (Fig.
3). Rays of diffuse daylight fall
upon the remaining portions of
the table. If at the beginning of
this experiment all the animals
are at the end a of the test-tube
-which is so placed on the table
that a is in direct sunlight, while
tin- other half l> is in diffuse day-
light, and is nearer to the plane
of the window than a the fol-
lowing occurs:

The animals move from a
through the pencil of direct sunlight into b, which lies in
the diffuse daylight, where they remain at the cup of the
test-tube. They pass from the direct sunlight into dif-
fuse daylight without even attempting to return into the
sunlight.

This experiment can be explained only by the assumption
tliat ihe orientation of the annual* /s determined by the
direction of the ran*. The animal can and must follow
the rai/s of diffuse liyht which hare the <lirection h-*a.
If, as is customary with zoologists, we believed that these
animals love the light or, more correctly, that they prefer
the more intense light --it would be impossible to see why
they do not remain in the direct sunlight, or at least why
they do not hesitate to go into the diffuse light.

From irhat has been said, no one, I believe, will doubt
that the direction of the progressive movements of the cater-

HELIOTROPISM or ANIMALS 20

<>f l*(trllie*in chrysorrhcea is deli-nnined l>// tin-
direction of I lit- ra/fs of lit/Id, and not by differences in the
intensity of the light in different parts of space. Positive! \
heliotropio animals are compelled to turn their oral pole
toward the source of light and to move in the direction of
the rays toward this source.

-. The dependence of orientation <>n tin- refrangibility
of tlic rat/s. I shall now show that it is the more rcfrain/ilile
rni/* of flic rixilde *i>ech'nm wliicli ore chiefly concerned in
hrint/iin/ a/tout tJie orientation of tlie caterpillars of Por-
tliexiti chrysorrhcea.

Experiment 1. If we place the test-tube on a table and
cover it with a box of dark-blue glass, the animals behave as
if the vessel were uncovered. Without exception, they move
in a strait / lit line to the window side of the vessel and remain
there. If instead of blue glass we use red, which to our
evt-s seerns much brighter than blue glass, no change occurs
in the orientation of the animals at first; after a long time,
however, the animals collect under the red glass on the win-
dow side of the vessel. In direct sunlight, however, orienta-
tion takes place more quickly. Exactly the same phenomena
are observed if an auimoniacal solution of copper is sub-
stituted for the blue glass, or a solution of potassium
bichromate for the ruby glass. This is also true in the
following experiments, where I may not always call special
attention to it. This experiment shows (1) that tJie more
refrain/ihle ratjx IK ire the saute effect as tin'.red i-ai/s, and
('-) find ike less refrangible ra//x Itrhnj al>onf movement* in
tin- wane irai/ as the more rcfrain/ih/e ones, onfij their e( r eet
is less intense. The experiment also proves that it is wrong
to say, as do the anthropomorphists, that the animals "aiv
fond of" blue and "hate" red; for, were this true, the
animals should have been forced to move to the room side
of the test-tube when under the red glass, yet they moved

30 STUDIES IN GENERAL PHYSIOLOGY

toward the window. The animals neither "are fond of"
blue nor "hate" red, but they are like plants, simply
positively heliotropic, and the blue rays are more effective
heliotropically than the red. There is, as I shall state
here once for all, no difference in direction between the
movements called forth by blue light and red light; there
is only a difference in the velocity and precision with which
these heliotropic movements take place.

Experiment 2. The longitudinal axis of the test-tube is
again perpendicular to the plane of the window. The small
caterpillars are at the beginning of the experiment on the
room side of the tube. The window half of the test-tube is
covered with dark-blue glass. The experiment goes on as if
the tube were uncovered; the animals move to the window
side of the test-tube, where they remain under the blue
cover. If the same experiment is repeated, only so that the
blue cover is placed over the room side of the test-tube, the
animals again move to the window, where they remain. The
experiment proves that the more refrangible rays alone have
the same effect as mixed light ; and the fact that the animals
leave the uncovered portions of the test-tube to creep under
the dark-blue cover corroborates what has already been said,
that positively heliotropic animals move in the direction of
the rays of light even when in so doing they pass from a
place of greater intensity of light to one of less intensity.

fc.i-l)eriinent 3. The test-tube again lies horizontally,
with its longitudinal axis perpendicular to the window. At
the beginning of the experiment the animals are on the
window side of the test-tube. If the window half of the
tube is covered with red glass (which may seem much
brighter to us than the blue glass of the previous experi-
ment), immediately after the red glass has been placed over
the animals they appear on the room side of it, and collect
at the boundary between the covered and mn-orcred jxtrts of

H ELIOT ROP ISM OF ANIMALS 31

the tiilir. If at the beginning <>f the experiment (he animals
were mi Ilu> room side of the test-tube, (hey move until they
reach (liis boundary.

IIV therefore </et f/ie name rexitlls !>// usiiit/ red </l(isx
that ire tjot hi/ IIXIIKJ o/>/ni/ie jHixlehoanl in a />rerioiis
e.rjtertiiteiil. Taken together with the preceding ones, this
experiment proves that pre-eminently the more refrangible
ra\ s of mixed day light are heliotropically effective. Although,
as we have just seen, the rays passing through red glass or a
red solution are not absolutely ineffective, yet the weak light
which is reflected from the walls of the room, and which
contains some blue rays, is more effective than the diffused
light reflected from the sky after it is filtered through red
glass. It is for this reason that the animals on the window
side under the red cover migrate to the boundary of the red
screen where they are held by the rays of diffuse daylight.

E.rjieriiiient 4. If, as before, we place the test-tube with
the longitudinal axis perpendicular to the window, and cover
it with red glass on the window side and with blue glass on
the room side, the animals collect under the blue glass at its
boundary with the red glass.

Experiment 5. If we place the test-tube with its longi-
tudinal axis parallel to the window, the animals scatter over
the whole length of that part of the tube which is covered by
blue glass.

From all these experiments it fotloirs flint if /.s eliiejl// flic
more refr<in</i/>/e VY///.S ir/iieli (Iclcriiiiiic Hie orientation of
flic etiler/n'lldrs of Portlicxid eli ri/xorrlnra loiranl ///////.

The only difference between the heliotropism of these
animals and the heliotropism <>f plants is this, that //// /r.vs
i-efrtnii/ililc vf//.s arc not *<> completely ineffective in tin-
ciiftc of flte c(ileri>ill(irs of Porthcxid chrysorrhoea as //n-//
(il>/>(irnilli/ are in IIKIIII/ fildiilx. This point must, however,
be studied more accurately with the aid of a spectrum.

32 STUDIES IN GENERAL PHYSIOLOGY

3. The dependence of the orientation on the intensity of
the rays of light. It is a peculiarity of all animal as well as
plant structures that only external stimuli of a certain inten-
sity can call forth reactions. It can easily be shown that at
the approach of twilight there comes a time when the rays of
diffuse daylight coming through a window no longer attract
caterpillars of Porthesia chrysorrhcea.

If the animals are between two sources of light of differ-
ent intensities, that having the greater intensity is the more
effective. This can easily be shown by bringing the animals
into a room into which light enters from opposite directions.
Other conditions being the samn, the animals move to the
window nearest them. A maximum limit for the intensity
of the light cannot be established, as direct sunlight is in
itself effective. Artificial sources of light above a certain
intensity and containing the more refrangible rays affect the
animals in the same manner as the natural sources of light.
In a dark room caterpillars are attracted by a kerosene flame
as markedly as moths ; the caterpillars, however, are not
burned, because they move so slowly that they have time to
turn back before the zone of fatal temperature is reached.
Such animals as are attracted by direct sunlight may also be
attracted by the candle flame, exactly as is the case in posi-
tively heliotropic plants.

4. At a constant intensify light acts as a continuous
source of stimulation. If the test-tube which is placed with
its longitudinal axis perpendicular to the window is left
undisturbed, the animals remain permanently on the side
nearest the window. Under these conditions we can also
safely open the room side of the vessel without a single
animal changing its position or escaping from, its cage. It is
remarkable, however, that when the test-tube has been left
undisturbed all day, the animals keep their position during
the night. In this way I have kept animals for several days

HKLIOTKOPISM OK ANIMALS

in a test-tube open on the room side; but when I turned tin-
vessel through an angle of ISO' in flic (lii/i/iiiic, hardly two
minutes elapsed before all the animals had moved to the
open end of the vessel which was now turned toward the
window. Under these conditions they of course escaped
from the test-tube. A position which the animals have
assumed under the influence of light is usually not changed
when the light is removed, unless some other stimulus comes

!->

into play.

5. On nci/dh'i-r (/co/ro/iixni (in<l confdct-irrildltilH// in l/n
cdf<Ti>ill<trx of PoHhcxhi cl/ri/^orrltu'd. -The reader may
perhaps have noticed that in all of these experiments on
caterpillars the test-tubes were always placed with their

lomntudinal axes horizontal. This was due to the fact that
o

the animals behave like plant structures, not only in regard
to their heliotropic, but also in regard to their geotropic,
irritability. Just as is frequently the case in positively
heliotropic plants, we find that the caterpillars are also nega-
tively geotropic; that is, they are compelled by gravity to
creep vertically upward until they come to rest in the highest
part of the test-tube. These experiments were made in a
dark room, with the long axis of the test-tube in a verti-
cal direction. If the test-tube is inverted, the animals again
creep to the top ; if left undisturbed, the animals remain in
the uppermost regions of the test-tube. It is necessary in
these experiments, as in those on heliotropism, to have the
temperature of the room at least 15, preferably as high as
LM) -- J . It is simplest to put the test-tube in one's pocket
with its longitudinal axis vertical. In a few minutes the
animals are found at the highest point in the tube. An
increase in temperature increases the geotropic irritability
of the animals.

it must now seem questionable whether in our former
discussion of the heliotropism of these animals we were

34 STUDIES IN GENERAL PHYSIOLOGY

justified in taking as the effect of light the movements of
the animals to the top of the test-tube ; it might, indeed, be
a geotropic phenomenon. To decide this the animals were
placed in a test-tube which was lined with thick black paper
except for a strip 2 mm. wide. The uncovered strip was
turned downward, so that light could enter the vessel only
from below. Diffuse daylight was reflected through the slit
from below by means of a mirror. The animals collected in
the lower, lighted portion of the glass vessel. Their helio-
tropism is therefore more powerful than their geotropism,
even when only weak diffuse daylight is used.

The geotropic experiments succeed only when the animals
have been in the light for some time and have not yet come
to rest. When the animals are kept in the dark for a long
time and the test-tube is not disturbed, they do not crrrp
upward. The orienting effect of the light always exceeds
that of gravity. The effects of gravity, like the effects of
light, usually appear only during certain periods in the life
of the animals; at any rate, they cannot always be demon-
strated with certainty.

The contact-irritability of the caterpillars of Porthesia
chrysorrhcea shows itself by the way in which the animals
remain in the corners and convex sides of solid bodies. I
covered the boxes in which I cultivated my caterpillars with
large, square glass plates. These did not close the box
tightly, so that the animals could creep out and creep upon
the glass plates. Only rarely, however, were they found on
the free surface of the plates. The animals moved along the
rough edges of the plate until they reached the window side
of the dish. I confirmed this observation almost daily for
months. When I placed the animals upon the outside of a
cubical block, they collected by hundreds in one of the
upper corners. Of course, only a few have room in the
corner itself, but, as is generally the case with these ani-

HELIOTROIMSM OF ANIMALS

mals. when a few have collected in a spot thr others on
arriving hold fast to the sides of those already there. An
animal at rest acts upon a creeping one as a convex edge.
On ihc oilier /mm/, / 'mrr 'iicrcr ohxcrrcil Unit I In-, animal*
icilliin flic cubical box collect on concarc r^r/rx. From this
it follows that the friction of gliding over the convex corners
is the source of the stimulation which compels the animal to
come to rest there ; in moving over the concave corners this
friction, of course, does not take place.

These three forms of irritability control mainly the daily
life of the animals. We find them in great numbers in
fruit trees and bushes, where they pass the winter in their
nests; as soon as the warm weather comes, they leave their
nests. Positive heliotropism and negative geotropism com-
pel them to creep upward to the tips of branches, and contact-
irritability holds them fast on the small buds. We can
easily show that neither smell nor a special mystical
"instinct" leads the animals to the buds, as we are able to
compel them by the aid of light to starve in close proximity
to food. The animals move to the window side or to the
top of a test-tube in which they are kept. If then a branch
covered with buds is pushed into the test-tube on the room
side, the animals nevertheless remain where light and gravi-
tation have compelled them to go and are holding them. If,
however, they once are on the buds, the latter act as a
stimulus which may be even stronger than the light. It is
in such a case impossible to draw the animals away from the
food by means of light.

All these forms of irritability can best be demonstrated
on animals which have just left the nest in which they have
spent the winter, and which have not yet eaten anything.
As soon as they have eaten and are about to moult, their
irritability decreases, and at the lime of moulting it is almost
impossible to show any effect of light or gravity upon them.

36 STUDIES IN GENEEAL PHYSIOLOGY

6. The effect of temperature on tJte cater ]>i liars of Por-
thesia chri/sorrhoca. The caterpillars of Porthesia chry-
sorrhoea behave toward a source of heat in a manner opposite
to that in which they behave toward light ; they move away
from the source of heat. If the animals contained in an
opaque vessel are brought in the neighborhood of a hot
stove, they leave the side of the vessel which is nearest the
stove. Yet the heat does not compel the (ini'mal* 1<> move
in a straight line, as tltey do irhen struck by the more
refrangible rays of liyltf. This directing effect of the more
refrangible rays of the visible spectrum is greater than that
of the dark heat rays. In this way it is possible for the
same animal which flees from the source of the dark rays of
heat nevertheless to move in the direction of the sun's rays
to the sunny side of a vessel.

It is a well-known fact that irritability in a tissue is a
function of the temperature. I have already mentioned that
at a temperature of less than 13 C. the animals are no longer
affected by light. It can be shown that heliotropic irrita-
bility increases with an increase in temperature. If the
animals are kept during the day in a room having a tem-
perature of about 18, it is found that they no longer respond
to light when beyond a certain distance from the window.
If, however, the temperature of the test-tube is increased a
few degrees, the animals move the more quickly to the win-
dow side of the tube the higher the temperature. It can
easily be demonstrated that the orientation takes place more
rapidly, and that the direction of the progressive movements
coincides more nearly with the direction of the rays of light,
whenever the temperature is raised. If, however, the tem-
perature is increased to 30 or over, the animals become very
restless; they raise the anterior ends of their bodies higher
than is usual in their movement, and so decrease the velocity
of their progressive movements. The most suitable tem-

11 KM ( >T KOI' ISM 01' A \ I M \ LS

I ir rat u IT fur demonstrating their heliotropic activity lies
between 'JO. and . ! W.

The experiments on the caterpillars of Porthesia chry-
soi-rho3a are typical. I have repeated them on some hundred
species of insects, but I have never found a positively
heliotropic insect whose dependence upon light was of a
different kind from that found in Chrysorrhcea. This
tact has given me the impression that all animal proto-
plasm, as perhaps all plant protoplasm, is heliotropieally
irritable, and that where this is apparently not the case
the heliotropic reaction is inhibited, either temporarily or
permanently, by other causes. For this reason it would
be useless to publish here every single experiment I have
made. This would result in repeating each time the same
phenomena, only under the name of a different insect.
Since there are only negatively and positively heliotropic
animals, it would be of secondary interest to know to which
of the two classes the individual animals belong. But I
believe it necessary to show by concrete examples what part
heliotropism plays in the habits and ecology of animals.

VI. THE POSITIVE HELIOTROPISM AND THE SLEEP OF

BUTTERFLIES

Our knowledge of the behavior of butterflies toward
light has, on the whole, remained at that point which is
marked by the statement of Reaumur that "it is a singular
fact that those butterflies which shun the daylight are pre-
cisely those which fly into lighted chambers." The paradox
has not yet been explained why those butterflies which are
ii"t to be seen by day fly into the flame at night, while the
day butterflies apparently do not possess the tragic "inst inct "
of the night Lepidoptera. There is 110 lack of conjecture
on this point. Romanes believes that the lamp is a "strange
object'' to the moths, and that "the desire to examine this

38 STUDIES IN GENERAL PHYSIOLOGY

strange object" drives the moths into the flame. We find,
however, that the caterpillars of Forth esia chrysorrhcea
creep as well toward the sun as toward a lamp. Yet,
according to Romanes, the sun ought to be a familiar ob-
ject to these animals. Such anthropomorphic opinions as
those of Romanes are evidently as useless in the analysis
of life-phenomena as the speculations of metaphysicians -
e. g., Hegel's on physical phenomena. A scientific analysis
of the behavior of moths toward light leads to a very simple
explanation of the paradox.

Experiment 1. Specimens of Sphinx euphorbias, Bom-
byx lanestris, and other moths are kept in a large glass box.
The box is placed in a room into which only daylight and no
artificial light enters. As soon as the animals begin to
fly, at the approach of twilight or later, they collect at the
window side of their boxes. Whenever the box is reversed
the animals fly back to the window side. This experiment
is rendered more complete by the following observations:

I kept the pupae of moths in an open box. Most of
the moths hatched at night. On the following morning I
always found them collected at the closed window of the
room. Here they remained all day exposed to the light.
Finally, when I caused the moths to fly by day, I noticed
that they flew to the window as do all other positively
heliotropic insects. These experiments show that the
animals are attracted, not only by a lamp, but also by
diffuse dauli(jht. They also show that Reaumur's idea that
moths shun daylight is wrong. The experiments indicate
that the animals are positively heliotropic toward diffuse
daylight, although, as we shall soon see, this positive helio-
tropism may during the daytime be obscured by another
form of irritability.

ti

Experiment 2. I brought some specimens of Sphinx
euphorbias into a room which had a window only on one

HELIOTROPISM OF ANIMALS H'.l

On the wall of the room opposite the window I placed
a kerosene lamp. At the approach of twilight, when the
animals began to fly about, I brought them into the middle
of the room, so that they were equidistant from the lamp
and the window, and left them alone. They flew to the
window. Yet, when I brought them into the immediate
neighborhood (within about a meter) of the lamp, they flew
into the flame. I repeated this experiment arid convinced
myself that they always flew to one of the two sources of
light, either the window or the lamp ; to the latter, however,
only when they were in its immediate neighborhood.

This experiment shows that the animals do not even pre-
fer artificial to the natural light, but that the artificial light
attracts them only when its intensity is greater than that of
the diffuse daylight, which is the case at night when the
animals are within a certain distance of the lamp, varying
with the intensity of the flame. The heliotropic sphere of
attraction of an electric arc light is therefore larger than
that of a candle flame, and the number of nioths attracted by
it correspondingly greater.

Experiment 3. It must yet be proved that it is chiefly
only the more refrangible rays of light which determine the
movements of the moths. I studied the behavior of Sphinx
euphorbia?, which began to fly at about 9 o'clock in the

evening.

The animals were contained in a large box, 40 cm. long,
the upper wall of which was of glass. Whenever I turned
the box the animals at once flew to the window side and
crowded against the upper glass wall through which the light
came. When I placed a red glass over the window side of
the l)ox, the animals at once flew to the room side. Thev
collected at the edge of the red glass, but on the room side
of it, where they were not covered by it. Hero tliev
attempted to fly upward. When I used blue glass instead of

40 STUDIES IN GENERAL PHYSIOLOGY

red, they flew under it to the window side of the box. At
fifteen minutes past 9 o'clock they came to rest and no longer
reacted to light. When exposed to daylight on the follow-
ing day, they did not stir, and made no attempt to creep away
from the light, although sufficient opportunity was offered.

I repeatedly established the fact that the movements of
night butterflies are determined by the more refrangible rays
of the spectrum on other specimens of Sphinx euphorbia?.
It was therefore not to be expected that in lamplight any
other than the more refrangible rays would bring about
movements. I have convinced myself that the moths of
Geometra piniaria are readily attracted by the light of a
lamp when behind blue glass, but not when behind red glass.

The night butterflies, therefore, shun neither diffuse nor
intense light, nor do they prefer artificial light to diffuse
daylight ; the correct expression of the facts is rather this,
that most species react fo light only <it nit/lit, when they are
positively heliotropic like the day Lepidoptera. We find in
butterflies jtcriodic rariaiioii in irritdlnlity (as in iiidiiij
j>ldiifx), <ni<l tlu'xc rdriatioiis correspond io Hte cliauacs of
da>l inl ni</lif. As certain flowers open their calices only by
night, while others open theirs by day, so certain butterflies
fly only by day, while others fly only by night. Both classes
of butterflies, however, are positively heliotropic ; and it
seems as if the irritability of the night butterflies toward
light is not less, but even greater, than that of the day but-
terflies ; for the intensity of the light which causes heliotropic
phenomena in moths is apparently much less than the mini-
mal intensity which stimulates day butterflies to heliotropic
movements.

The phenomena of sleep in butterflies are perhaps more
complex than the corresponding phenomena in plants. One
thing is, however, certain that the periodicity of the noc-
turnal movements of butterflies does not change during the

HELIOTKOIMSM OF ANIMM.S II

tii>l two or three days if the animals are kept in the dark.-
I'lider these circumstances tin- moths become restless at the
u>iial time. Reaumur showed that moths begin to fly in the
evening when kept in a box. I must leave it undecided for
the present, whether this periodicity finally disappears if the
animals are kept still longer in the dark. I have tried
repeatedly to cause Sphinx euphorbias to fly in the daytime
li\ a sinltlcii (liiiiiiiiifion in the intensity of the Hi/lit. When
1 protected the animals from all jarring / iicrcr succeeded
lichrccn 'tin I /? o'clock in the mornii/ij. Yet I was easily
successful iii the afternoon, long before the beginning <>('
twilight. I will cite here several of my experiments. One
morning I placed a Sphinx euphorbias, which had begun to
tiv at ( .' o'clock on the previous evening, on the window cur-
lain, where it remained quietly. At 2:45 I returned it to its
^lass box, which stood in a dark corner and into which light
fell only through a narrow slit. An hour went by, but the
animal did not leave its place. It then moved to the light
side of the box, without flying. I carried the animal back to
the window, where it remained quietly. After twenty min-
utes I returned it again to the dark box. Half an hour later,
at half-past 4, it finally began to fly.

The next day I allowed it to remain at rest near the win-
dow, and it did not begin to fly until 9 P. M. at well-advanced
twilight. On the following day I kept it in the dark box,
and at half-past 3 in the afternoon it had already begun to
fly. At noon on the succeeding day a heavy storm came up
and it grew quite dark. The moth, which until then had
remained quietly at the window, began to fly. I have had
the same experience with other examples of this specie-;.
These facts seem to indicate that it is possible to influence
the time of waking of Sphinx euphorbise by diminishing the
intensity of the light, but only when they would soon wake
up without artificial interference.

42 STUDIES IN GENERAL PHYSIOLOGY

The day butterflies are positively heliotropic like the night
butterflies. The only striking feature is that in certain day
butterflies the intensity of the light must be very great to
bring about heliotropic movements. Specimens of Papilio
machaon (which I had raised) remained at rest during the
day at a window where they were exposed to the diffuse day-
light and could be carried around on the finger; as soon,
however, as they were brought into direct sunlight, they flew
toward the window in the direction of the rays of light,
and this with such force that they dropped down as if stunned.
In direct sunlight they pressed themselves closely against
the window pane. In diffuse daylight the animals, if they
moved at all, crept toward the source of light; but indirect
sunlight they flew toward it. My attempts to attract Papilio
machaon by the weak light of a kerosene lamp were unsuc-
cessful.

I will add at this point my general observations on the
caterpillars of butterflies. I have not found these periodic
variations in heliotropic irritability in most caterpillars, not
even those of Sphinx euphorbiae. The caterpillars which
I studied reacted to light at all times of the day and night.
The eati'rpt'Uafs ayree, however, with the day and tifyht
butterflies in so far as they are all, without exception,
positively heliotropic.

This positive heliotropism is most marked in the cater-
pillar of the willow-borer, which lives in the stems of the
willow where it is not at all exposed to light. Such cases
are also known in plants. Roots, for instance, are helio-
tropically irritable, and yet, as Sachs points out, under nor-
mal conditions their heliotropism is of no use to them.
They can certainly not have acquired it through natural
selection. According to the Darwinian theory, we would
expect that the caterpillars of willow-borers should be nega-
tively heliotropic, or at least indifferent to light. But the

HELIOTROPISM or AXIM\I,S (.',

liehavior of an animal is merely the rcxnlttiiil of all i/s
forms of irri/<i/>ili///. and so it may happen that an animal
is positively heliotropie even when it has no opportunity to
make use of it. The Iarvo3 of many saw-flies behave just as
the caterpillars of Lepidoptera. I have made observations
mi the larviB of Nematus ventricosus, which are exactly like
those on Porthesia chrysoirhoea, which have been described.

I have not yet succeeded in demonstrating a heliotropie
reaction to diffuse light in the indigenous pupae. Wilhehn
Miiller, however, has observed effects of light in South
American species. 1 The pupa? can move at three joints.
( )nlv a lateral movement to the right and left is possible in
some of the species; in other species only a dorsal move-
ment of the body is possible; in a third species of pupa3 a
combination of both kinds of movements is possible. Miiller
observed that all three classes of movements can be brought
about under the influence of light. He found that some
pupa3 turned not only away from the light, but also toward
it. He also found that when the animals had been exposed
to the dark for some time, they "needed some time to become
susceptible again to the influence of light.'' In interpreting
the phenomena Miiller follows the Darwinian idea, so that
the thought never occurs to him that he might be dealing
with phenomena similar to the heliotropie phenomena of
plants.

Tin- iK'tjdtirc </<'ofroi>ism of lite Lepidoptera. -The
movements of very young or recently hatched animals have
for the most part been misunderstood, because they have
always been considered a function of mysterious "instincts"'
of the animals, while the direction of their motions is in
reality determined by definite external forces. The same
cause which prescribes the course of a falling stone or deter-
mines the orbits of planets, namely gravitation, determines

i Mf'LLER, Zooloyische JnltrliH,-ln:>; Vol. I ISM; . H>. "''>* II.

44 STUDIES IN GENERAL PHYSIOLOGY

also the path which a butterfly follows that has just emerged
from the pupa case. The geotropic irritability is at that time
especially strong ; the newly hatched animals remain restless,
and are compelled to run about until they come to a vertical
wall, on which they can put the longitudinal axes of their
bodies vertically, with their heads upward. Here they remain
quietly until their wings are unfolded. The powerful mani-
festation of negative geotropism at the time of hatching is
no isolated phenomenon in insects. In summer we find
great numbers of the ecdyses of the larvae of Ephemeridaa
on the banks of streams. They are found on blades of grass
or steep banks, with their longitudinal axes usually vertical
and the head upward. That gravity, and not light alone,
plays the chief role here is shown by the fact that I have
found the ecdyses in the same position under bridges where
no light could strike them from above.

This observation on the larvaa of Ephemeridaa makes it
impossible for us to accept the idea that the "purpose" of
the orientation of the freshly hatched imago of a butterfly
is that the wings may unfold ; for negative geotropism
appears in the larvae of Ephemeridas at a time when 110
wings are present. The caterpillars of butterflies are also
negatively geotropic like the freshly hatched moths, even
though not so markedly. Immediately after hatching geo-
tropism is much stronger in the imago of the butterfly than
heliotropism a phenomenon rarely observed in the animal
kingdom. If a freshly hatched imago is on a vertical wall,
it does not change its orientation toward the center of
gravity even when the direction, refrangibility, or intensity of
the light is changed.

What is true of the heliotropism of. Lepidoptera, that it
is most marked during certain periods of their existence,
holds good also for their geotropism. Amphipyra is ener-
getically negatively geotropic immediately after moulting.

II i: i.i T KO IMSM OF ANIMALS

Several days later the animals assume every possible position
with re t' -re i ice to the vertical. They prefer to remain on
vertical walls, yet they will creep just as readily into hori-

/.ontal folds and crevices.

VII. THE POSITIVE HELIOTROPISM OF PLANT LICE

Anyone closely studying a rose covered with wingless
plant lice will notice that they are arranged in a definite
way on the plant. On a vertical stem they rest with the
head downward; on the leaves they are usually found on
the underside, mostly on the principal veins. Here one also
notices a certain regularity in their orientation, in so far as
the animals on the principal vein turn their oral poles toward
the stem, and their aboral poles toward the point of the
leaf. The orientation of the animals seems therefore to be
controlled by the structure of the plant, and not directly by
external forces.

But the plant lice do not behave on all plants as on the
rose. On a palm, for example, I found 110 such definite
orientation of the animals toward the plant, even though iu
this case also they show a preference for the lower surfaces
of the leaves.

Yet it might seem reasonable to suppose that light or
gravity compels the plant lice to seek the lower surfaces of
the leaves. I twisted several leaves of Cineraria, the dorsal
sides of which were covered with plant lice, so that the
dorsal sides were directed upward and toward the window,
and fixed the leaves in this position. I watched the animals
for two days and found by actual count that the animals
remained at rest. I repeated the same experiment on the
plant lice of palm leaves, but also with negative results.

M v experiments on the orientation of ncic-horn wingless
plant lice were practically negative when I removed them
from the plant and placed them in a glass vessel. Yet in

46 STUDIES IN GENERAL PHYSIOLOGY

the older wingless animals I could notice an inclination to
move toward the source of light. Wlien their wiiu/s had
sprouted, however, the orientation of the jrfant lice teas
extraordinarily definite. In this state they are perhaps
the most suitable animals we have for demonstrating the
phenomena of heliotropism. Not all species are equally
irritable ; Cineraria afforded me the best specimens. I have
never found a species of plant louse which was not definitely
positively heliotropic. I kept the plants near a closed
window. The animals were attracted by the sun to the
window, where they crept upward. When the animals are
lightly touched with the point of a pen, they fall down a
second or two later. If a glass vessel is held under them, a
large number of these animals can be collected in an unin-
jured condition in a short time. I found it much better to
work with such animals as have already flown from the
plant, than to collect the winged animals from the plant
itself. To obtain the winged plant lice in great numbers it
is necessary only to allow a plant which is covered with them
to dry out gradually. Under such conditions the wings
grow out very rapidly.

All the experiments irldch were made irifli Portltesin
chrysorrhcea can be repeated with exactly similar results
on irimjed plant lice contained in a test-tube.

As in the heliotropism of caterpillars, the heliotropism of
plant lice is determined chiefly by the more refrangible rays,
which compel the animals to move in the direction of the
rays toward the source of light. If we place the test-tube
containing the animals on a horizontal table, they always
move toward the source of light, whether this be lamplight,
diffuse daylight, or direct sunlight. The orientation occurs
the more rapidly the more intense the light. If the intensity
of the light is constant, the plant lice, like the caterpillars of
Porthesia chrysorrhoaa, are compelled to remain perma-

HELIOTRUI-ISM OF ANIMALS 47

nentlv mi tin- side of (lie test-tube which is turned toward
ilu 1 source of light.

If direct sunlight comes through the window, and the
tube containing the animals is so placed that one-half lies in
the direct sunlight, while the other half is in diffuse day-
li'dit, and if the latter half is nearer the plane of the win-

O L

dow than the former, the animals will move to the window
side of the vessel, like the caterpillars of Porthesia chry-
sorrhoea; they -leave the direct sunlight and move into dif-
fuse daylight in order to follow as nearly as possible the
direction of the rays. The result is the same when the dif-
fuse daylight first passes through dark-blue glass. The
animals are compelled to go to the window side of the tube
under all conditions, 110 matter whether the test-tube is
covered entirely or only in part by the blue glass, or
whether the blue glass is placed over the window or the
room side of the tube. The less refrangible rays which
pass through deep-red glass are not very effective. In con-
sequence, if the test-tube is entirely covered with red glass,
the animals, if not very sensitive, distribute themselves
evenly over the whole test-tube, just as in the dark; or, if
more sensitive, they collect after a long time on the window
side of the test-tube. But even then they do not quite
behave as under blue glass. While under blue glass they
collect in a very small area on the window side of the tube,
under red glass they occupy a much larger area. If only a
part of the test-tube is covered with red glass, the animals
collect at the window side of the uncovered portion of the
test-tube, as the less refrangible rays have only a minimal
effect. When I placed a test-tube containing highly sen-
sitive plant lice on a horizontal table perpendicular to the
plane of the window, and covered the window side of the
test-tube with a l>rit/lif-rc<l glass, the animals collected at the
boundary between the uncovered and the covered part of the

48 STUDIES IN GENERAL PHYSIOLOGY

tube when diffuse daylight entered through the window.
The more refrangible rays reflected from the walls of the
room were more effective than the rays from the window
which had passed through the light-red glass. This pre-
vented the animals from going under the red glass. But
when I used direct snnliyht, tlte animals moved under t he-
red glass to the 'icindoir xi<le of Hie vessel and remained
there. When the animals were collected at the room side of
the test-tube lying horizontally on a table and with its lon-
gitudinal axis perpendicular to the plane of the window, and
an opaque cover was placed over the room side of the tube,
while a dark-red glass was placed over the rest of the tube,
the animals went under the red glass and gradually collected
there on the window side of the tube. But when I placed
the opaque cover over the window side and the red glass over
the room side of the tube, and the animals were under the
opaque cover at the beginning of the experiment, they did
not collect under the red glass. The rays reflected from the
wall of the room had lost their directing power in filtering
through the red glass. The experiments were made in the
diffuse light of a dark day. On a bright day the animals
moved to the room side of the tube under the red glass.

The rays which pass through red glass have therefore the
same effect, only they are weaker than the rays which pass
through blue glass.

I have already mentioned the fact that the day Lepi-
doptera begin to fly as soon as direct sunlight falls upon
them, while in diffuse light their heliotropic movements con-
sist chiefly in creeping. The same difference in the effects
of different intensities of light can be easily demonstrated
in winged plant lice. In diffuse light of low intensity they
move forward by cree^iinj; when brought into the sun they

fly-

To obtain a measure of the difference in the activity of

HELIOTRUI-ISM OF ANIMALS 1 ( .)

rays of different intensities and refrangibilities, I measured
tin- time it required for the animals in a test-tube to pass a
line scratched in the glass, when moving under the influence
of light from the room side of the tube to the window.
In these experiments I used some sluggish winged plant
lice which I had taken from a plum tree. As a rule, the
heliotropic movements of plant lice took place much more
quickly than in the experiment to be described here. The
experiment was made in diffuse light on August 8, 1888.
At the beginning of the experiment all of the animals were
on the room side.

The animals passed the marks as follows:

In the first minute 11 animals

In the second minute 17

In the third minute - 19

In the fourth minute 21

In the fifth minute 10 "

In the sixth minute 12

In the seventh minute - 13

Only three animals had at this time not yet crossed the
line. Several minutes later, at 9:20 o'clock, when the sun
was coming through a fleecy white cloud, I made the following
experiment in direct sunlight with the same animals. At
the beginning of the experiment the animals were again on
the room side of the tube.

The animals this time passed the mark as follows :

In the first minute 31 animals

In the second minute 36

In the third minute - 23

Half a minute later the last sixteen animals had also passed
the mark. The velocity of the movement was twice as great
in direct sunlight as in ordinary daylight. These experi-
ments were repeated and gave practically the same results.
At 10:17 I placed the animals under a dark-blue glass.

50 STUDIES IN GENERAL PHYSIOLOGY

This time it took ten minutes for the animals to orient them-
selves a longer time, therefore, than in white light.

At 10:29 I covered the test-tube with red glass, and since
I knew that in diffuse light the heliotropic movements take
place only very slowly under red glass, I brought the animals
at once into direct sunlight. It required seventeen minutes
before the majority of the animals had passed to the window
side of the mark. In diffuse light it required an hour for
orientation to take place under red glass; in a new experiment
it required twelve minutes under blue glass.

I noticed no periodic change in irritability in plant lice-
such as that observed in Lepidoptera, but I did notice a
decrease in heliotropic irritability, a kind of rigor when the
animals have been left undisturbed in the dark for some
time.

If the test-tube remained undisturbed, the animals
remained permanently on the side nearest the window.
When I very carefully turned the test-tube through an angle
of 180 in the daytime, the animals again moved toward the
window, even when they had been left undisturbed for
hours. When, however, I kept the animals quietly in the
dark, and after some hours carefully placed the tube near a
lamp, the animals did not move from the position which they
had maintained through the day. They seemed to be asleep.
But when I shook the tube so that the animals began to
move, they promptly oriented themselves toward the light as
often as I turned the tube around.

I found that winged plant lice are negatively geotropic
as well as positively heliotropic, as is the case in the larvre
of Chrysorrhoea. If the animals in the test-tube were very
vigorous, a change in the position of the tube with reference
to the vertical brought about a change in the orientation of
the animals toward the center of the earth; they traveled
upward at as small an angle as possible with the vertical, and

HELIOTBOPISM OF ANIMALS ~>\

collected ;it the highest point, in the test-tube. This experi-
ment must, of course, be made in a dark room. When the
animals are first brought into the dark, the experiment can
be repeated many times with exactly the same result; every
change in the position of the test-tube with reference to the
vertical compels the animals to creep upward and to collect
at the highest point in the tube. When, however, the ani-
mals were kept permanently in the dark, the reaction ceased
soon, and the animals remained motionless, no matter how
often the position of the test-tube was reversed. The ani-
mals were in a sort of rigor. When they were placed on an
inclined or vertical plane, they moved upward. G-eotropic
orientation occurred as soon as the plane made an angle of
30 with the horizontal; the geotropic movements were the
more certain and precise the nearer the plane approached the
vertical. When light fell on the animals at the same time,
their orientation was determined by the resultant of the
direction of the rays of light and gravitation, in which, how-
ever, the light was the stronger force even at a great distance
from the window.

The winged animals behave toward a source of heat in the
same manner as the caterpillars of Porthesia chrysorrhcea.
When I brought the animals in an opaque vessel into a room
having a temperature of 18 and placed them near a stove,
they left the side of the vessel which was turned toward the
stove, as soon as its temperature increased a few degrees. At
a temperature of 9 the animals were so sluggish that a
definite reaction to light or gravity did not take place. A
temperature of 20-24 is the most suitable for the experi-
ments. When I surrounded one-half of the vessel with a
water-bag having a temperature of I'll , the other half with
one having a temperature of ll)..~, the animals moved, under
the influence of light, from the warmer into the cooler area.
But thev did not move far into the latter, as their movements

52 STUDIES IN GENERAL PHYSIOLOGY

soon ceased. Under the influence of light, the animals also
moved from a region having a temperature of 12 to one
having a temperature of 24.

VIII. THE CONNECTION BETWEEN HELIOTROPISM AND SEXU-
ALITY IN ANTS

At the time of sexual maturity the male and female ants
fly from the nest on a warm day to pair in the air. This
"nuptial flight" is, as shown by the following observations,
determined by a very pronounced positive heliotropixm,
which appears especially at the, period of sexual maturity.

I discovered a nest of brown garden ants in the wall of a
house which was struck late in the afternoon by direct sun-
light. In August, 1888, I observed that on warm days in
the afternoon, as soon as the sun struck the wall, at about 5
o'clock, the winged ants came out in swarms and then flew
away in the direction of the rays of sunlight. I procured a
large number of iriin/ed- ants from such a swarm and studied
their behavior toward light. These animals were energeti-
cally positively heliofropic, and behaved in all respects like
the caterpillars of Porthesia chrysorrhcea.

When I put the winged ants into a test-tube and placed
this with the longitudinal axis perpendicular to the plane of
the window, the animals moved to the window side as often
as the tube was turned around. The velocity of the helio-
tropic movements was greater in these animals than in any
others that I have studied. When the tube was not disturbed
the animals remained on the window side nearest the win-
dow. When the longitudinal axis of the test-tube lay par-
allel to the plane of the window, the animals distributed
themselves evenly over the whole length of the tube. When
one-half of the tube was in direct sunlight, while the other
half was in diffuse daylight, but nearer the window, the ani-
mals collected in the window side of the tube, they went from

HELIOTROPISM OF AXT.MAI.S

the direct sunlight into the shade. Tin- direction of tin- rai/x,
mill not tin- distribution of tin- inleiixitij of the litflil, fit, the
text-tube, therefore, determine* the direction of Hie />ro-
(/rexxire moremeiifx.

The bine rai/x irrre, pre-eminently effect ire. When the
test-tube was covered with blue glass, either entirely or in
part, the orientation was changed in no way. When the
tube was entirely covered with red glass, the movements
occurred more-slowly. The animals finally collected on the
window side, but it took a long time. When the tube lay
with the longitudinal axis perpendicular to the window, and
the portion nearest the window was covered with red glass,
the animals collected at the boundary between the uncovered
and covered parts. Diffuse daylight affected the animals
just like sunlight. These facts may suffice to show that at the
time of the nuptial flight the winged ants are energetically
positively heliotropic.

Yet I found that up to tlie time of the nuptial flight,
li< flit IK id practically no effect on winged ants wJiicJi were
taken from the sctmc. next.

Animals which I collected after the nuptial flight also did
not react very distinctly to light. If heliotropisni was still
present at all, it was obscured by other forms of irritability,
particularly stereotropism.

The nuptial flight of the ants of this nest always took
place at about 5 o'clock in the afternoon, when the sun's
rays fell upon the nest. That it was the latter condition,
and not the time of day, which determined the period of
flight is shown by the fact that in other nests, which were
reached by the sunlight earlier in the day, the flights took
place earlier. Usually the flight occurs at about noon, when
the sun's rays strike the earth perpendicularly and the tem-
perature is relatively high. Both the males and the females
which I collected from the swarm which had left the nest

54 STUDIES IN GENERAL PHYSIOLOGY

late in the afternoon escaped through the window on the next
day at any time that I freed them. The scent of the females
therefore does not determine the nuptial flight of the males,
and vice versa; after sunset the ants no longer flew away
when liberated.

I have already shown that direct sunlight or intense dif-
fuse daylight calls forth flight movements in plant lice and
day Lepidoptera. This also occurs in winged ants. In dif-
fuse daylight the male and female ants move toward the
source of light only by using their legs; in direct sunlight,
however, they fly.

Sunlight, therefore, causes flight movements in ants at the
time of sexual maturity, and this fact determines the nuptial
flight. Immediately after copulation another form of irrita-
bility becomes more prominent 1 which compels the ants to
to crowd into crevices (to "found a new nest").

The connection between sexuality and heliotropism in ants
is shown still further by the fact that at the time of the nup-
tial flight no heliotropism can be demonstrated in the workers.
Workers taken from the same nest as the other ants when
placed in a test-tube moved about irregularly in it, and finally
came to rest on the stopper, no matter in what position I
placed the tube with reference to the window. I then placed
several winged ants which reacted energetically toward light
in the same tube with the workers. The workers apparently
became now also positively heliotropic, that is to say, they
moved with the winged ants to the window side of the tube
whenever it was reversed. This lasted, however, only some
ten minutes, when the workers settled again permanently on
the stopper and were no longer affected by the light while
the winged ants reacted to the light just as before.

The observations of Lubbock seem to indicate that helio-
tropism may be present also in the workers at certain periods

i Stereotropism. [1903]

HELIOTROPISM OF ANIMALS 55

in their existence. In the experiments of Lubbock the
workers confaiiicil in tin- nest not only collected under red
glass, but also carried their Iarvo3 there. The animals are
therefore negatively heliotropic. 1

All these facts, however, do not yet exhaust the connec-
tion between sexuality and heliotropic irritability. The
heliotropism of the male and female ants is also different,
inasmuch as it requires more intense light to cause helio-
tropic movements in females than in males. In isolating
the males and females of the same swarm I noticed that the
females had ceased to execute heliotropic movements before
it seemed as if twilight had really begun. The males how-
ever still collected on the window side of the tube long after

O

sunset. Experiments with colored glasses succeeded in males
when the light was so faint that I had difficulty in dis-
tinguishing the color of the glasses. On dark, cloudy days
females showed no heliotropic reactions toward the window,
while the males did. It harmonizes with this observation
that on cloudy afternoons I saw occasionally winged males
leave the nest, but no females.

As soon as the intensity of the light had become so small
that heliotropic phenomena were no longer produced, another
form of irritability appeared in the winged ants, especially
in the females, namely, stereotropism. The animals then
crowded into all crevices. I placed the animals in a dark
box, and laid a small, folded piece of velvet into one corner
of it. After a few moments they had crept into the folds of
the velvet. With the males it took a much longer time than
with the females. This irritability, however, did not appear
as long as the light was sufficiently- intense to call forth

*/

heliotropic phenomena. When exposed to light, the animals

crept neither under the piece of velvet nor into crevices. It

is very probable that a similar difference in heliotropic irri-

i The <>b-<Tvnt ions recorded iu LubborU - paper admit another possibility, f 1003]

5t> STUDIES IN GENERAL PHYSIOLOGY

lability exists in the two sexes of the Lepidoptera. Reaumur
states that in the main only males fly into the candle flame.
From this fact, which is correct, it follows that it must
require a more intense light to cause the females to execute
heliotropic movements than is necessary for the males. Both
male and female moths are attracted by sources of light
which are stronger than the candle flame, for instance, the
electric arc light. It is a well-known fact that the females
fly less than the males. It is imaginable that this is due to
the fact that the females are less irritable toward the light
than the males.

The difference in the irritability of male and female ants
toward light brings up the question as to whether the differ-
ence in the development of the sense organs, particularly
the eyes, which is often observed in males and females of
the same species, is connected with this difference in irrita-
bility. The males of ants have larger eyes than the females.
But the cause of the difference in sensitiveness may lie
deeper, as is, for example, indicated by the following obser-
vation made by Semper : "In all species of the cave beetle
Machaerites only the females are blind, while the males have
well-developed eyes ; notwithstanding this fact they always
live together." 1 Eyes therefore develop more easily in males
than in females even in the dark. It might be worth while
to determine whether in these cave-dwellers the males are
also heliotropically more sensitive than the females.

IX. THE NEGATIVE HELIOTROPISM AND OTHER FORMS OF
IRRITABILITY OF THE LARVJE OF MUSCA VOMITORIA

The phenomena of irritability in negatively heliotropic
animals obey the same laws as those in positively heliotropic
animals ; with this difference, however, that negatively helio-
tropic animals turn their aboral poles toward the source of
light instead of their oral poles, and that in consequence the

1SEMPEK, Die natiirlichen Existenzbedingungen der Thiere, Vol. I, p. 101.

HELIOTROPISM OF ANIMALS 57

direction of their progressive movements under the influence
of the rays of light is away from the source of light. My
description of negative heliotropism need therefore be I ml
brief. I .have chosen as an example of negatively heliotropic
animals the lar\;e of Musca vomitoria, which are addi-
tionally interesting in that they are completely blind, Helio-
troj)isiu in animals is therefore, a characteristic of their
protoplasm, ami i/t <i X/MT///C characteristic of their eyes;
just as in plants, which have no eyes.

In order to study the negative heliotropism of Musca
larvie it is best to take the almost fully (jro/rn larcce fresh
from the cadaver on which they were reared. When the
light, which may be either diffuse daylight or direct sun-
li<>'ht according to the sensitiveness of the animals, is of the

O O

proper intensity, the directing influence of the rays of
light can be demonstrated more beautifully in the larvsB of
the fly than in any other animal. I placed a number of
these animals on a horizontal board and exposed them to
sunlight. This was at about 4 o'clock in the afternoon,
when the rays of light fell obliquely through the window.
I shut out that part of the rays which came through the
window from above by means of blinds. As soon as the
animals came into the sunlight, they were oriented with
their oral poles toward the room, and their aboral poles
toward the window. They crept with mathematical precision
in the direction of the rays. When a shadow was thrown on
the board by a penholder, it could be noticed that the
animals moved away from the light in a direction exactly
parallel to the edge of the shadow. The directing force of the
rays was so strong that the animals crept closely along the edge
of the shadow without crossing it. They acted as though
they were impaled on the ray of light which passed through
their median plane. When I turned the board around, the
animals immediately turned about also, and again placed

58 STUDIES IN GENERAL PHYSIOLOGY

their median planes in the direction of the rays. (That this
was due to the effect of the light, and not a compensatory
movement that might have been produced by the rapid
turning of the board is shown by the fact that compensatory
movements do not exist in Musca larvae.)

I was able to show that fly larvae are compelled to move
from less intense light into more intense light under the
influence of the rays of light, just as it could be shown that
positively heliotropic animals do not go from dark places to
light ones, but follow the direction of the rays, even when by
so doing they move from a region of greater intensity of light
to one of less. I put the almost fully grown larvae into a test-
tube and placed it horizontally on the table, with its longitu-
dinal axis perpendicular to the plane of the window. The
sun's rays made a small angle with the window. By means
of a screen I arranged the test-tube so that only diffuse
light fell through the window upon the half turned toward
the window, while direct sunlight fell upon the half turned
toward the room. At the beginning of the experiment the
animals were all on the window side of the test-tube. They
immediately moved from the shaded part into the direct
sunlight on the room side, and remained there.

Incidentally I was able to observe that the light stimuli
which strike the oral pole of these completely blind animals
are most important in the orientation of the animals toward
light. When the animals crossed the boundary from diffuse
light into direct sunlight, the reaction caused by the increase
in the intensity of the light did not take place until a half
or a third of the body of the animal was in the sunlight
(because in all phenomena of stimulation some time elapses
between the application of the stimulus and the reaction to
it). The animal checked its movement and turned its head
through an angle of 90-130 from side to side. If in so
doing the head again came into the shade, the animal

HELIOTKOI-ISM OF ANIMALS r> ( .)

returned into the shade: lint if this did not happen, as was
more usually the ease, the animal continued its movement
into the sunlight. The animals did not always check their
movements in passing from a shaded area into the sunlight.
Often they moved without delay from the shade into the
sunlight. The following observation shows that the rays of
light which strike the head mainly determine the orientation :
When I placed a fully grown animal on a board, and pushed
the board from the shade into the sun, so that only the head
of the animal was struck by sunlight, the larva immediately
placed its median plane in the direction of the sun's rays.
When, however, 1 put only the aboral pole into the sunlight,
this orientation did not occur. Animals from which the
first few segments of the oral pole had been amputated no
longer oriented themselves toward the -light. Yet little weight
is to be given to vivisection experiments, which are followed
only by an inhibition of the effects of a stimulus.

When I allowed the sun's rays to fall on the plane of the
board perpendicularly, the animals moved over it in all
directions. As in this case the animals could not follow the
direction of the rays of light, it had 110 other influence upon
them than to increase their restlessness, and no uniform
orientation resulted.

It could be shown very beautifully in these full-grown
larva: 1 that essentially only the more refrangible rays are con-
cerned in exercising a directing influence upon these animals.
I placed a large number of fully grown larvae on the middle
of a horizontal board in a darkened room, and exposed them
to the sun's ravs which made but a small angle with the
horizon. Within ten to twenty seconds every animal had
I .laced its median plane in the direction of the rays of light,
and moved exactly parallel to the shadow of a vertical object
which had been thrown upon the board for comparison. I
treated a new lot of animals in exactly the same way, but

60 STUDIES IN GENERAL PHYSIOLOGY

before exposing them to the sunlight I covered them with a
box of dark-blue glass. Within ten to twenty seconds these
animals had also placed their median planes sharply and
precisely in the direction of the rays of light, in which
direction they moved toward the room side. When I took a
third lot of fresh animals and covered them with red glass,
the orientation of the animals into the direction of the rays
did not occur. They crept to the right and to the left, occa-
sionally moving a short distance toward the source of light ;
but even after minutes under the red glass the precise orien-
tation of the animals, which followed under the blue glass in
a few seconds, did not occur. Under red glass the animals
behaved toward direct sunlight just as they did under blue
glass toward very weak <l(tijlight. That the rays which pass
through red glass are not absolutely without effect seems to
be shown by the fact that the animals avoided going to the
window side, and that they finally collected at the room side
of the board. The directing force of the red rays seems
therefore to be limited to this, that the animals will not
move for long distances toward the source of light. In con-
sequence, the animals must collect ultimately on the room
side of the vessel.

In all the previous experiments the animals were on a
plane board. When at the beginning of an experiment the
animals were collected 011 the window side of a test-tube
which lay horizontal and perpendicular to the plane of the
window, in direct sunlight and under blue glass all the ani-
mals turned their oral poles within ten seconds toward the
room side of the tube. In about twenty seconds they
migrated to the room end of the tube. When the same ani-
mals were exposed in the same way to direct sunlight, but
under red glass, they neither oriented themselves nor moved
toward the room side of the tube during the next four
minutes, even though they were very restless.

HELIOTROPISM OF ANIMALS HI

In this ease, therefore, just as in tin- cast- of Hit- positively
heliotropie animals, it is chiefly the more refrangible rays
\vh icli effect the orientation of the animals. The helio-
tropic influence of the less refrangible rays, however, is
much less in the eyeless fly larvae than in any other ani-
mals that I have studied. The animals moved in the direc-
tion of the rays, even in diffuse light, at a distance of one
meter from the window, but the less the intensity of the
lii^ht, the more easily did other stimuli (such as contact
stimuli) cause a deviation of the animals from the straight
line. I have often repeated these experiments in the course
of the last two years, and have each time obtained essentially
similar results. The irritability of the animal is, however,
not always the same; especially does its irritability vary
during different periods of its life. I have, however, con-
vinced myself that the Iarva3 are negatively heliotropic even
immediately after being hatched, although they do not move
as precisely in the direction of the rays as the fully grown
larva-.

I placed some fly eggs on smoked glass plates and allowed
them to hatch. As the larvae removed the soot in their
path, they thus registered graphically the paths they took
from the eggs. The glass plates lay on a horizontal table in
a room lighted from one side only. The paths followed by
the larvse ran, almost without exception, toward the room
side of the plates. In the few exceptions the path usually
ran first toward the window, then bent, and went toward the
room side of the plate. It was neither a mysterious force of
nature nor an obscure "inherited instinct' 1 which dictated
the direction of the movements of these animals, but only
the direction of the rays of light, just as gravity determines
the orientation of the Lepidoptera when they emerge from
the chrysalis.

When the diffuse daylight which struck the lar\;e came

62 STUDIES IN GENERAL PHYSIOLOGY

from two windows the planes of which were at an angle of
90 with each other, the paths taken by the larvse lay diago-
nally between the two planes. 1 When I placed the plate with
the eggs in an absolutely dark room, the paths followed by
the larva? ran concentrically around the nest; the animals
had scattered equally in all directions over the plate, but,
contrary to the behavior of the animals in the light, which
always moved as far as possible toward the room side, the
circle in which the animals moved in the dark was very nar-
row. They did not leave the glass plate. The constant
intensity of the light acts, as in the case of the positively
heliotropic animals, as a constant stimulus which causes the
animals to move in one definite direction (either toward or
away from the source of the light), until some other stimulus
intervenes, w r hich modifies or abolishes the effect of the light.
In my preliminary communication on animal heliotropisrn
I mentioned an effect of light on fly larvae which I called a
kind of anisotropy, and which I am at a loss how to in-
clude under the other phenomena of heliotropism. The
phenomenon under discussion appears only in intense light
and in iieirly hatched or very //<>i<it<{ larvce. The phenomenon
consists in this, that the animals 2 turn their ventral surfaces
toward the source of light without placing their median
plane in the direction of the rays. I have never seen this
orientation in adult larvaB. When I put the animals into a
test-tube placed with its longitudinal axis perpendicular to
the window, and exposed them to the direct rays of sunlight
or diffuse light close to the window, the animals left the
lower side of the tube and moved to the upper. In this the
animals, therefore, resembled positively heliotropic animals,
and I might have believed that I was dealing with one of

1 This experiment was recently published by an American physiologist as a new
discovery to prove that I had overlooked the importance of the intensity of light!
I 1903 J

2 When kept in a test-tube. [1903]

II i: I.IOTKOIMSM OF ANIMALS ('.:?

those cases occasionally observed in plants when- in light of
great intensitv (he heliotropism of an organ is <lie opposite
of that in light of less intensity. A closer examination,
however, showed this not to be the case. \Vlien the test-
tube lav perpendicular to the plane of the window, positively
heliotropic animals contained in it moved, as we have seen,
not only to the upper, but also to the window side of the test-
tube. This was not the case with the newly hatched fly
larvae. They all turned their ventral surfaces toward the
source of light, but otherwise moved about irregularly. I
placed the animals in a test-tube which was covered with
black paper, except for a small slit, and let direct sunlight
enter the tube only through the slit. The animals which were
on the lower side of the tube left it as soon as the light
struck their backs, and crept upward; but no animal which
was sheltered from the light was attracted to the upper,
lighted side of the tube, as was the case under similar con-
ditions in the positively heliotropic caterpillars of Chrysor-
rhcea. When I held the glass vertically, more animals
collected 011 the window side, but they did not all creep up-
ward, as did the positively heliotropic animals. When I
placed the animals on the outside of a test-tube, they did
not move upward, but collected for the most part on the
under side of the tube. This experiment was not very
decisive, however, as the animals easily fell off the tube.
These facts can be interpreted in no other way than that
the intense light compels the fly larvae to turn their rail nil
Surfaces toward the xonr<-<' of lit/lit, in which condition
they are indifferent to the orientation of their median planes
toward the ravs of light. The ventral position is assumed
only when the animals are exposed to light. With this,
however, the striking features of the movements of orien-
1;ition in tlv larvae are by no means exhausted. While the
movements of orientation in all the other animals go on

64 STUDIES IN GENERAL PHYSIOLOGY

under blue glass just as rapidly and in the same way as
in mixed daylight since in mixed daylight it is chiefly
the more refrangible rays which are heliotropically effect-
ive the ventral orientation of fly larvae which has just
been described occurs neither under blue nor under red
</l<t*s. In direct sunlight it took one to one and a half
minutes before the animals were densely gathered on the
upper side of the horizontally lying test-tube. Not one of
these animals moved to the upper side of the tube in less
than twenty-five minutes under red glass, or in less than
five minutes under blue glass.

The ventral orientation of the Musca larvae toward a
source of light can be observed most distinctly in freshly
hatched larvae. As the animal grows larger, the phenomenon
becomes less marked. The lump of eggs laid by a fly was
distributed among three tubes. In all three tubes the animals
immediately after hatching oriented themselves ventrally
toward the diffuse light. I then fed meat to the animals
in one tube and left the animals in the other two tubes
unfed. On the next day the unfed animals were oriented
ventrally toward the daylight, while this was not the case in
the rapidly growing Iarva3 which had been fed. I have ob-
tained the same result by feeding the larvae of one lot of
eggs with fat, while another lot was given lean meat. The
latter grew more rapidly than the former. While those fed
on fat were oriented ventrally in diffuse daylight, direct sun-
light was necessary to bring about this effect in those fed on
meat.

I might have doubted that this was the effect of light,
had I not been able to prove that with a decrease in the in-
tensity of the light the phenomenon becomes less distinct,
and finally disappears entirely. I do not think that the
ventral orientation could have been the effect of heat, as the
animals move away from a non-luminous source of heat, as

H ELIOT KOI'ISM OK A N I M \ I.S (').")

we shall srr presentlv. Because of the preponderance of
this \viitrul orientation, it is no easy matter to demonstrate
the negative heliot ropism. of the young larvae in a test- till >e;
they assume the ventral orientation, and no longer trouble
themselves about the direction of the rays of light.

The ofirn/df/on offl/e larvce of Miisca to/i-anl a source of
licaf. If a Mus.ca larva in its movements comes to a spot where
the temperature is only one degree higher than in the sur-
rounding area, it stops and turns its head laterally. If in
so doing its head encounters a spot with a lower temperature,
it turns thither and continues to move in this direction.
One can easily convince oneself of this by laying the tip of a
linger on a spot on the outside of a test-tube containing the
larv,-r. The increase of temperature of the spot touched can
be ascertained by a sensitive and finely graduated thermom-
eter. As soon as the animal comes to the spot touched by
the finger, it turns its head. If it does not turn far enough
to touch a cooler spot, it continues in the old direction to the
region of higher temperature. According to this, the stimuli
which reach the oral pole determine the orientation of the
animal toward a source of heat also, just as in the case of
light. If the experimenter puts a test-tube containing a
1 irge number of Musca larvre into his pocket, where no light
reaches them, the animals collect in a few minutes densely
on that side of the tube which is turned away from his body.
The same thing happens when the tube is exposed to the
rays of a non-luminous source of heat.

If one-half of a tube is surrounded by a water jacket of a
higher temperature, and the other half by a water jacket of
room temperature, the animals in the warmer part become
restless or perish ; thru nrr mil orienlc<l, lioirrrrr. <iu<l conse-
quently run no/ sttrr f/iriiixrlrrx It// nior/ni/ info that /mr/ion

of tin' Inlir having lower temperature.

In these experiments the animals were contained in a

66 STUDIES IN GENERAL PHYSIOLOGY

long test-tube a (Fig. 4). The upper half was surrounded
by a wide hollow cylinder &, the bottom of which was com-
posed of the cork stopper c, into which a fitted water-tight.
The lower half of a extended into the hollow cylinder d. b
and (/ were filled with water to a given height. When the
temperature of the water in the cylinder b was
34, while that of the water in (/ was 18, the
animals in the upper part of the tube became
very restless, but did not creep into the cooler
part of the tube. Nor were the animals oriented
when the temperature in the lower part of the
cylinder was higher than in the upper. Such

FIG. 4 an orienting effect of temperature was observed
only when an animal came to the boundary between the
warmer and the cooler zones at c. In such a case the animal
moved into the zone having the lower temperature, but not
into the other.

By means of diffuse daylight, however, it was an easy
matter to drive the animals from a place of lower tempera-
ture to one of higher. This is possible because the light
orients the animals and dictates to them sharply the direc-
tion of their progressive movement, w T hile the same is done
by a source of heat only to a slight extent. It was therefore
possible to drive the animals from diffuse light into direct
sunlight, notwithstanding the difference in temperature.

At low temperatures, even -|-10 , it is scarcely possible
to demonstrate the heliotropism of fly larvae. Heliotropic
experiments in these animals succeeded best at a tempera-
ture of 20-25 .

The orientation of Mnsca larvae toward chemical stimuli.
If on a summer day a piece of meat is set in the open,
blow flies collect on it in great numbers and deposit their
eo-o-g. There can be no doubt that a chemical stimulus

O&

attracts the animals and causes them to lay their eggs.

HELIOTROPISM OF ANIMALS t>7

IVcaving meat lias the same attractive effect on the larvre of
tlies. It' such animals are in a test-tube containing decaying
11 H -at, and the tightly fitting cork is loosened a little, the
animals which were crawling between the meat and the open
end of the tube turn and go back toward the meat. I mois-
tened a small area on a plate by rubbing it with decaying meat.
I placed some half-grown larva 1 which I had taken from the
meat in the middle of this moist surface. They at first crept
toward the room side of the plate, but turned when they
came to the boundary of the surface smeared with the putrid
meat, and remained within it. Not until half an hour later,
when the spot had dried completely, did they leave it.
When I merely moistened a spot on the plate with pure
water, the larvje did not remain on it.

When I removed the animals from a cadaver and placed
them on a glass plate, and brought a piece of decaying meat
into their neighborhood, the animals crept toward it, even if
in so doing they were obliged to move toward the window ;
this occurred, however, only w T hen the animals w r ere in the
immediate neighborhood of the meat. When they were
more than a centimeter and a half away from the meat, they
were no longer attracted by it ; they then followed the direc-
tion of the rays of light and starved in the neighborhood of
food. Animals which had not yet tasted food were also
attracted by the decaying meat. Fat, even when foul,
attracted the animals only slightly or riot at all ; this is very
remarkable, as the female flies are also more readily attracted
by meat than by fat. I often placed a piece of horse flesh
and a piece of horse fat side by side in the sun. At a time
when the flesh was covered with eggs, the fat was almost free
from them. It seems, therefore, as though flic same chemical
sthnulus which attracts the, lame- ra^.sv.s the, jlics to deposit
(heir ('(/(js. Decaying cheese also attracted the larvre, but
ammonia and assat'trlida were without effect. Some volatile

68 STUDIES IN GENERAL PHYSIOLOGY

substance must be formed in the decomposition of the pro-
teids which attracts the Musca larvae even from a distance.

The contact-irritability of Musca larva'. It is a well-
known fact that Musca larvae are inclined to crowd into
cracks and crevices in the earth, and it is astonishing
through what small cracks the adult larvae can slip. This
irritability might impress a Darwinian as though the ani-
mals wished to protect themselves from the light. That this
contact-irritability is entirely independent of their helio-
tropism is shown by the fact that these animals crowd them-
selves under a completely transparent glass plate, even if by
so doing they have to move toward the light.

The animals retain this form of irritability even when put
into a vessel of water, in which they soon die. I noticed
this phenomenon in feeding tritons with fly larvae. Small
stones lay on the bottom of the vessel, and the larvae crowded
themselves under them as eagerly and as skilfully as if
they had always lived under them. The perniciousness of
this irritability in the case in question is apparent when we
remember that it keeps the animals from reaching the sur-
face of the water again, so that they are drowned.

In these experiments I was struck by the fact that the
animals, when placed under the surface of the water, do not
swim upward and so avoid death, but swim downward. I
cannot explain this fact. Under other conditions positive
geotropism cannot be demonstrated in these animals.

The posit ire heliotropism of flics at the time of sexual
maturity. The fly, which as a larva is negatively helio-
tropic, is positively heliotropic in the state of sexual
maturity. This reversal in the sense of heliotropism in
changing to the adult state is not uncommon. Yet it is a
striking fact that, while heliotropism is reversed, the orienta-
tion toward chemical substances is the same in the female
flies at sexual maturity as in the larval state.

HELIOTROI-ISM or ANIMALS

Positive heliotropism can be demonstrated in flics by the
same experiments as in plant lice; only it must be noticed
that flies are provided with several more kinds of irrita-
bility than plant lice, and that in consequence heliotropism
may be obscured when other stimuli besides light come into
play. In one experiment, for example, I observed
that light, gravity, and heat were without effect
on the flies, because the animals always remained
on the cork stopper in the test-tube. Some sub-
stance was probably on the stopper that attracted
the flies; for when I put the animals into a flask
with a clean glass stopper, they reacted to light.

I am indebted to Professor Ernst Mach for a beautiful
observation on the influence of light on the orientation of
the house fly :

Several years ago I accidentally made an observation which I
have never been able to follow further. While adjusting 1 my rotat-
ing polarization apparatus in a dark room, by the help of sunlight,
whereby a bright quadratic picture a some 16 cm. across (Fig. 5)
\\;is rotated three or four times per second in a circle of a radius of
30 cm., a fly F (Fig. 6) happened to enter
the bundle of rays LL, went through the
whole rotation as though stunned, and fell
upon the table. I was able to repeat this L
\periment twice. The fly, which was appa r-
ently sound, escaped while I was giving niy
attention to something else. FIG. c

In this case, then, the same effect was produced by rotat-
ing the rays of light as by revolving the fly on a centrifugal
machine. 1

1 R&dl has recently runic to tin- conclu-ion that tin? reactions of insects on a
centrifugal machine are indeed caused by the li^'lil. Jf this is correct, they are
identical with Maeh's ob^-rvat ion. [r.in:;]

70 STUDIES IN GENERAL PHYSIOLOGY

X. THE NEGATIVE HELIOTROPISM OF THE LARV.E OF TENEBRIO

MOLITOR

The larvae of a beetle Tenebrio rnolitor, which can easily
be collected in large quantities, are also suitable animals
upon which to demonstrate negative heliotropism. When
such animals are placed on a plate, they creep to the room
side of it; if the intensity of the light is sufficiently great,
they remain there. If the plate be covered with dark-blue
glass, the result of the experiment is the same. If the plate
be covered with red glass, the animals move in the concave
edge of the plate both toward the window and away from it ;
a definite orientation does not occur. Under red glass they
behave just as in the dark ; under blue glass, just as in the
light.

I covered one-half of a plate with blue glass and one-half
with red glass, so that the boundary between them lay in the
direction of the rays. The animals were distributed equally
over the plate at the beginning of the experiment. All the
animals in the blue half moved to the room side of the plate,
but none in the opposite direction; in the red half they
moved in all directions. The animals moved from the blue
into the red, but never from the red into the blue. When I
covered one-half of the plate with opaque cardboard, the
other half with red glass, so that the boundary between them
again coincided with the direction of the rays of light, the
animals scattered in all directions in the two halves of the
plate. After a long time, however, the greater number col-
lected under the cardboard.

The experiments which have been described were made in
direct sunlight. If on a dark day the plate is some distance
from the window and the light is not very intense, the ani-
mals, which at the beginning of the experiment were in the
middle of the plate, will gradually creep toward the room
side; when, however, they reach the shallow groove in the

HELIOTROPISM OF ANIMALS 71

plate, they do not again leave it, and now creep toward the
window also. The animals are forced to bring 1 the surfaces
of their bodies as much as possible in contact with other solid
bodies.

These phenomena are not altered when the plate is cov-
ered with blue glass. If, however, it is covered with red
glass, the animals, even when in the middle of the plate,
move as frequently toward the window as toward the room
side. So far as the stereotropisui of these animals is con-
cerned, it must be added that the animals collect in the con-
cave edges of dark boxes.

It might be supposed that the function of stereotropism
is to protect the bodies of the animals from evaporation as
far as possible. I covered one-half of the bottom of a box
with a moist cloth and the other with a dry one, and, after
putting fifty animals in each half of the box, I placed it in
the dark. After two hours not a single animal was found
in the moist half of the box. The animals flee from moisture
and seek dry spots. Contact-irritability and negative heliot-
ropisin determine the habits of these animals, which live
protected from the light, in flour.

Tlic n<'(/(ifirc Itrlioh'opism of f lie larvae of June bugs.
The behavior of the larvae of Melolontha vulgaris is quite
similar to that of Tenebrio molitor. As they move for the
most part while lying on their sides, their orientation takes
place rather slowly ; nor do they follow in. the direction of the
r;i\s of light as sharply as do the animals which have
been described above. They flee from the light and move
from the window to the room side of a vessel.

The following experiment, which also serves to give an
idea of the time required for experiments on these animals,
shows that only the more refrangible rays are of chief
importance in bringing about the heliotropic phenomena :
At 10:40 o'clock I placed twenty-three larva* in the middle

72 STUDIES IN GENERAL PHYSIOLOGY

of a round plate covered with blue glass. The animals
moved to the room side of the plate, tried to creep over the
edge, and at 10:45 came to rest on the room side. I waited
five minutes, and at 10:50 substituted red glass for the blue.
The animals scattered equally over the whole plate, and at
11 nine animals were on the window side, the rest about uni-
formly scattered over the whole plate. I then substituted
blue glass for the red. At 11:07 all the animals were col-
lected on the room side of the plate. At 11:10 I again
covered them with red glass. The animals immediately
began to creep over the plate in all directions. At 11:20
twelve animals were collected near the window, six in the
middle, two on the side, and three on the room side of the
plate. I kept the plate covered with red glass, and watched
to see whether after a time the rays going through the r<>d
glass would not also bring about an orientation. No change
occurred in the course of the next hour. Gradually, how-
ever, more and more animals moved to the room side of the
plate, and at 3:30 all but five animals were collected here.
The animals, therefore, finally show a negative heliotropisin
under red glass also. The rays passing through red glass
are therefore similar in their effects to those which go through
blue glass, only they are not so effective. In this respect the
behavior of these animals corresponds with that of plants.

The larvpe burrow into the ground. Negative heliotro-
pisin may co-operate here, but stereotropism is without doubt
the chief factor concerned.

The question arises whether it is not geotropism which
causes the animals to bore into the ground, as in the case of
roots. In order to determine this I made the following
experiment : I filled a hollow cardboard cylinder, some 5 cm.
in diameter, with earth. The cylinder was about 20 cm.
high. I fastened the cylinder on a stand, with its longitudinal
axis vertical, and brought it so near to a table that it

HELIOTROPISM OF ANIMALS , ::

just touched two larva- lying upon it. I also placed two
larva> on top of I he cylinder. If the animals were negative! v
geotropic, the upper animals should have buried themselves
more quickly than the lower. But the opposite was the
case. After forty-five minutes the lower animals had
burrowed upward so that they were completely out of
sight : the upper were not buried until an hour later. There-
fore, even though they may be negatively geotropic, for
which I have as yet no proof, the contact-irritability of these
animals determines that they shall burrow into the ground.

XI. THE DISTRIBUTION OF HELIOTROPIC PHENOMENA IN THE

ANIMAL KINGDOM

The experiments which have thus far been described
were carried out on insects.

So far as experiments on representatives of the other
divisions of the animal kingdom are concerned, I have con-
firmed the identity of animal with plant heliotropism on
crabs (Gammarus locusta, Cuma Rathkii), naked snails and
worms (leeches, planarians, earth-worms and others). Experi-
ments on infusoria are already sufficiently complete to show
that Sachs's laws of heliotropism also hold good for them. 1

Investigations have not yet been made on Ccelenterates
and Echinoderms; Trembley's experiments on Hydra, how-
ever, show that in their case also the relation is the same; at
least it seems to me that Trembley's experiments cannot be
interpreted unless we assume that the progressive movements
of Hydra are determined by the direction of the rays of
light.

I used the following method with aquatic animals: To
prove that the direction of the rays determines the direction
of the progressive movement. [ used a long, four-cornered
glass box, one wall of which was made of a watch-glass. The

i Sec tlie papers of Strasburger, Engelmann, and Stahl cited in the introduction.

74 STUDIES IN GENERAL PHYSIOLOGY

convex side of the watch-glass was turned outward. When
direct sunlight fell upon the glass, the rays were focused a
few centimeters behind the glass wall. Notwithstanding
this fact, the positively heliotropic animals moved in the
direction of the rays from the room side of the glass box
through the focal point to the front of the box, although the
intensity of the light was the greatest in the focus. This
could be shown very beautifully in some tiny, positively
hcliotropic worms I found in the brackish water at Kiel, but
whose identity unfortunately I failed to determine.

Positive heliotropism is encountered more often in the
plant kingdom than negative heliotropism. It is worth while
to mention the fact that positive heliotropism appears to
exist in more species in the animal kingdom also than does
negative heliotropism.

AH ((ifcrjitllurs <ni<l Lci>i<!(>i>tera, whether they fly by
day or night, can, according to my observations, be con-
sidered positively heliotropic. Thus far I have tried in vain
to find negatively heliotropic Lepidoptera or caterpillars.
The great majority of the other winged insects are also
positively heliotropic.

We also encounter positive heliotropism in animals which
live in water, and even in mud, and which therefore can
never profit by light. I was much interested in some obser-
vations I made in this direction on a small Crustacean (Cuma
Rathkii) which lives on the bottom of the bay of Kiel. The
animal can be fished out of the mud in which it buries itself
only with a dragnet. Notwithstanding this fact, the animal
is strongly positively heliotropic. When I kept these small
crabs in a glass vessel and allowed light to fall upon them
from one side only, the moving animals collected at the side
of the vessel nearest the light. The resting animals were
oriented, and turned their oral poles toward the source of
light and their median planes in the direction of the rays.

HELIOTROPISM OF ANIMALS 7.~>

"When the source of light or the vessel was carefully turned
about, the animals changed their orientation until their
median planes were again in the direction of the rays of
liyht. The directing force of the light exhibited itself here
in the same manner as in the Euglenre in the experiments of
Stahl.

I next placed a small glass box filled with mud in the
same vessel with the crabs. The animals did not scent the
mud; at least not one of them moved into the box containing
the mud. When I disturbed the animals (by touching them
with a pencil), they first swam upward and then, if I did not
disturb them further, slowly fell to the bottom. If an
animal happened to fall upon the mud, it immediately became
lively as soon as it touched the mud. It burrowed into it
eagerly, after which it was impossible to get the animals to
react to light. The other animals which fell to the glass
bottom of the vessel remained inactive.

Thus we see that contact with the mud had a greater
effect than light; contact-irritability is more intense than
heliotropisni. It is in this way that it happens that the
animal, besides being a poor swimmer, lives away from the
light in spite of its positive heliotropisni.

XII. THE DEPENDENCE OF THE ORIENTATION UPON THE

FORM OF THE BODY

In the introduction I have called attention to the fact that
the orientation of an animal toward light, like every phe-
nomenon of irritability, is determined by two factors: first,
external causes in this case the light and, second, inter-
nal causes, namely, the structure of the animal.

So far as the structure of the animals is concerned, we
are dealing in this paper exclusively with animals whose
bodies consist of two morphologically symmetrical halves,
and which have morphologically dillVri'nt ventral and dorsal

76 STUDIES IN GENERAL PHYSIOLOGY

surfaces and morphologically different oral and aboral poles.
We disregard all other detailed morphological peculiarities,
because those mentioned suffice to explain the orientin^

*- ZD

movements of an animal, as they do for the movements of
plants. The distribution of irritability on the surface of
an animal corresponds to the above-mentioned morphologi-
cal relations. Elements at the surface of the body sym-
metrically situated with reference to the median plane have
equal irritabilities. This condition compels the animal to
orient itself toward a source of light in such a way that the
rays of light strike the symmetrical points in the body at
equal angles; this is the case when the animal places its
median plane in the direction of the rays of light. Points
on the dorsal or ventral surface equidistant from the
median plane have unequal irritabilities, the irritability
being in general the greater the nearer the points are to the
oral pole. In the same way, the irritability of a dorsal ele-
ment is different from the irritability of the opposite ventral
clement. If these assumptions regarding the connection
between irritability and the main structure of an animal are
correct, it follows, without further discussion, that an animal
with bilateral symmetry is compelled to place its median
plane in the direction of the rays of light and to move in
this direction either toward or away from the source of
light. We must therefore prove that the described distribu-
tion of irritability on the surface of an animal is not fiction,
but reality.

1. The oral pole of an animal is more irritable lieliotropi-
cally than Hie aboral pole, no matter whether the animal
has eyes or not.

I have already mentioned that the blind adult Musca
larva immediately places the entire median plane of its body
in the direction of the rays of light when sunlight strikes only
its oral pole. When, however, the oral pole remains in the

HELIOTROPISM OF ANIMALS 77

shade, while the aboral pole is exposed to the sun, the (in/-
iinil f/ors m>/ hr///// //s median }>/<ni<" into lite direction of the
rai/x of lit/ltf and </Ws not more- in ////s direction, although it
may become very restless. Light therefore prescribes the
direction of the progressive movements in these animals
when it strikes, the oral pole, but it does not have this effect
when the light falls only upon the aboral pole.

I have already called attention to Engelmann's observa-
tion that Eugleiia viridis reacts clearly only when the light
falls upon the oral pole. Euglena viridis possesses a pig-
ment spot at this pole which is called the "eye-spot."
Physiologically this expression is a very unhappy one, as we
do not know that this spot has anything in common with the
functions of our eye. Engelinann, however, expressly states
that the most sensitive spot of the Euglena is not the pig-
ment spot, but the colorless protoplasm lying just in front of

it.

The earthworm has no eyes. Hoffmeister found that the
animals recede from the light when the anterior end of the
body is illuminated. If, however, the oral pole is shaded
and the rest of the body is illuminated, no effect is produced.
Darwin repeated and corroborated this experiment. 1

Fresh-water Planarians have eyes and are negatively helio-
tropic, but their sensitiveness to light is not very great. I
cut Plauarians in two in the middle. The oral piece reacted
to light soon after the operation, just as the whole animal.
Not once, however, did I see indications of a heliotropic
movement in the aboral piece before regeneration had
set in" (which is very complete in these animals), even
though the aboral piece was by no means inactive, but crept
around very energetically in the glass and reacted promptly
when touched. When I placed both pieces on the window

iD\i;\\i\. Die Bildung der Ackererde diirch </<< Tli<'iti</k<-H // M"i<;-mo-. trans.
by Cuii's, isvj, ,,,,. H IT.

2 1 observed different facts in an American freh-\vatiT I'l.ui.iri.m. [1903J

78 STUDIES IN GENERAL PHYSIOLOGY

side of the vessel, the oral piece moved toward the room
side, but not the aboral piece. When the oral piece moved
from the room side toward the window, it soon turned about.
Under similar conditions the aboral piece continued to creep
until it reached the window. When the vessel containing
the animals was carefully reversed, the aboral animal was
not affected, but the oral animal immediately moved toward
the room side.

It can easily be shown that in leeches the head, which
contains tli- ryes, reacts more energetically toward light
than the aboral pole. If some small stones are lying on the
bottom of a beaker which contains such animals, and the
vessel is suddenly illuminated, the animals push their heads
under the stones, while the aboral pole remains at rest even
though exposed to the light. It is astonishing to notice
how long after the illumination the reaction appears. It is
not unusual for thirty to seventy seconds to elapse between
the illumination and the beginning of the movement. Hoff-
meister observed a still longer latent period in the case of
the earthworm. It would be unnecessary to show that in
animals which possess eyes the oral pole is more sensitive
toward light than the aboral. We may therefore accept it
as certain that the oral pole of an animal is more sensitive
toward light than the aboral, whether the animal does or
does not possess eyes.

In consequence of this fact, it is difficult for an animal to
move perpendicularly or obliquely to rays of light emanating
from a sufficiently intense source, for, as the oral pole is
more sensitive than the aboral, the former must turn more
energetically toward or away from the source of light
(depending upon whether the animal is positively or nega-
tively heliotropic) than the aboral.

2. The lieUotropic irritability is also different on the
ventral and dorsal surfaces of a dorsiventral animal. To

HELIOTROPISM OF ANIMALS 71)

my knowledge, no ol)S6rvationS on this subject have as \ et
been made on animals.

Planarians and leeches afford an example of the ditl'er-
enee between dorsal and ventral irritability. In leeches the
veniral surface, which has no eyes, is more sensitive to light
than the dorsal surface. It has already been said that this
animal leaves the dorsal side of its aboral pole exposed to the
light, if only its head is protected from the light. Such
animals stick to the side of a beaker, so that their ventral
surfaces, which carry the suckers, are directed outward. If
diffuse light of a sufficient intensity falls upon the ventral
surfaces of the animals, most of them leave the window and
move to the room side. The animals then turn their dorsal
surfaces to the light.

In this case, as in all the others, only the more refrangible
rays are chiefly active. When the animals are covered with
red glass, orientation does not follow, or only after some time.
If blue glass is held over them, the orientation takes place
just as in diffuse daylight.

The difference between the irritability of the ventral and
the dorsal surfaces of dorsiventral animals is therefore com-
parable with that between the oral and the aboral poles.

3. The dependence of the irritability of a dorsiventral
animal on the symmetry of its body must yet be discussed.
Those elements of the body of a dorsiventral animal which
occupy symmetrical positions with reference to the median
plane have equal irritabilities.

The facts which prove this are to such an extent objects
of daily experience that a brief allusion to them will suffice.
If a touch on one side of the animal calls forth a movement
to the left, then the same stimulus applied to the opposite
symmetrical spot on the body will cause the same amount of
movement to the right. An object appearing in the right
field of vision causes the same amount of movement as one

80 STUDIES IN GENERAL PHYSIOLOGY

appearing in the left at the same distance from the median
plane. In short, we can say that tln> s//ii/iiicfn'c(il jtlune of
an (ininnil from iiun'itltolotjirdl st<tii<li>oint is also l/i<>

symmetrical plane from />////s/V>/w//a// xlumli><>int.

This distribution of irritability on the surface of an
animal determines the orientation of dorsiventral animals
toward a source of light. If the median plane lies in the
direction of tin- rays of light, the symmetrical points of the
surface of the animal are struck by the rays at an equal
angle. The effects of the stimuli on the right and left halves
of the body annihilate each other, since they are equal in
intensity and opposite in direction. The light can therefore
produce no tendency to turn to the right or the left. When,
however, tin- median plane is oriented obliquely toward the
source of light, unequal forces act upon symmetrical ele-
ments, and a tendency to turn must arise which continues
until the median plain- coincides with the direction of the
rays of light.

This dependence of irritability on the form of the body
causes Musca larvae to move away from the source of light
precisely in the direction of the rays, and plant lice to move
just as precisely in the direction of the rays toward the
source of light. The heliotropic movements of an animal
are therefore dependent on the symmetrical relations of its
body, in the same manner as was shown by Sachs to be the
case in plants.

To show how far these conceptions of heliotropic phenom-
ena in animals differ from the prevailing notions on the
subject, especially those of the Darwinians, I shall give the
views of Romanes on this subject. Romanes mentions the
well-known facts that insects of all kinds fly into the flame,
that many birds are attracted by the light of lighthouses,
and fishes by the lanterns. He explains the phenomenon as
follows: "The habit must be attributed to mere curiosity, or

HELIOTROPISM OF ANIMALS 81

tlrsirc to c.i'd n/i in' lite i/cir <u/<l striking objectj" and then
quotes some remarks which he found in the manuscripts of
Darwin: "Query: Why do moths and certain gnats fly into
(.mdlrs. Imt not into the moon when the same is at the hori-
zon ? I noticed, long ago that they fly much less frequently
into candles on- a moonlight night. When a cloud passes
over the moon, they are again attracted by the candle."
Romanes believes that: " The answer must be Unit f//<> moon
is a finniJiar object, which insects consider as a matter of
course, and so have no desire to examine //."

As we have seen, it is not the "new and striking" object
and "the desire to examine it" which drive the insects to
the lamp, for they are attracted, as I have shown, also by
the natural source of light, the sun. No reason seems to
exist to my mind for believing that the moon is a more
familiar object to the insects than the sun. I cannot well
see, however, how Romanes harmonizes the phenomena of
negative heliotropism in animals with "the desire to examine
unfamiliar objects." The history of science has taught us
that confusion always reigns when anthropomorphic motives
are brought into scientific research. Before the time of
Galileo a body sinking in a fluid "sought its place." ' Galileo
and his followers put an end to the sovereignty of this
psychology, at least in inanimate nature. Mankind has had
no reason to regret this revolution. In biology, however,
cvt'ii at this date, protoplasmic substances still move toward
the source of light "because of curiosity."

XIII. SUMMARY OF RESULTS

I shall conclude by summarizing the more important
results of my investigation:

I. The dependence of animal movements on light is in
i -very point the same as the dependence of plant movements
on the same source of stimulation.

' See MAI M, <; -xrhichte der .l/o-/i-/;i//.-, 1st cd., p. 117.

82 STUDIES IN GENERAL PHYSIOLOGY

1. The direction of the median plane, or the direction of
the progressive movements of an animal, coincides with the
direction of the rays of light. 1

2. The more refrangible rays of the visible spectrum are
exclusively or more effective, than the less refrangible
rays, in causing the orientation of the animals, as is also
the case in plants.

3. Light of a constant intensity acts as a constant stimu-
lus in animals as well as in plants.

4. The intensity of the light is of importance in animal
heliotropism, in so far as only light of a certain intensity can
cause heliotropic movements, and in so far as with an increase
in the intensity the orientation of the animals toward the
source of light becomes more exact. Direct sunlight causes
winged insects (ants, Lepidoptera, plant lice, etc.) to fly,
while diffuse light usually causes them only to creep. Posi-
tively heliotropic animals will move toward the source of
light, even if in so doing they go from places of greater
intensity of light to places of less intensity ; negatively helio-
tropic animals move away from the source of light, even if
in so doing they pass from regions of less intense light to
regions of greater intensity.

">. Heliotropic movements occur only between certain
limits of temperature. An optimum temperature lies beween
these two limits at which the heliotropic movements occur
most rapidly and precisely. This holds true also in plants.

II. The orientation of an animal toward a source of light
depends on the form of the body, just as the orientation of a
plant to light depends on the form of the plant.

1. Symmetrical points on the surface of the body of dor-
siventral animals possess equal irritabilities. 1 '

1 If there is only a single source of light. If there are two sources of light of
different intensities, the animal is oriented by the stronger of the two lights. If their
intensities be equal, the animal is oriented in such a way as to have symmetrical
points of its body struck by the rays at the same angle. [1903]

2 Equal in magnitude, not in direction. [1903]

HELIOTROPISM OF ANIMALS 83

The heliotropic irritability of the oral pole of an ani-
mal is different from the irritability of the aboral pole, and
is generally greater than the heliotropic irritability of the
aboral pole.

3. The irritability of the ventral surface is different from
the irritability of the dorsal surface.

These three conditions taken together cause dorsiventral
animals to place their median planes in the direction of the
rays, and to move toward or away from the source of light in
this direction.

4. Eyeless animals (such as the lame of Musca vomitoria)
behave in this respect like animals having eyes.

III. The heliotropic irritability of an animal manifests
itself frequently only at certain epochs of its existence.

1. In winged ants this epoch is the time of the nuptial
flight.

2. In plant lice it is the time when wings are present.

3. In the larvse of Musca vomitoria negative heliotropism
is most prominent when they are fully grown.

4. In a large number of animals the sense of heliotropism
is of the opposite kind in the larval and the adult states.

5. Both the night and day Lepidoptera are positively
heliotropic, and their heliotropism is similar to that of every
other positively heliotropic animal. The period of sleep of
the night Lepidoptera, however, falls in the daytime, and
only for this reason is their heliotropism manifested exclu-
sivelv at night.

/ O

IV. In many animals heliotropic irritability is connected
with sexuality. Aside from the nuptial flight of ants, the
fact must be mentioned here that in ants and Lepidoptera
the males are heliotropically more sensitive than the females.

V. The behavior of an animal depends on the sum total
of its different forms of irritability. In this way it may
happen that Cuma Rathkii and the caterpillars of the willow-

^4- STUDIES IN GENERAL PHYSIOLOGY

borer, which live in the dark, are positively heliotropic with-
out deriving any benefit from this form of irritability.

VI. There is one form of irritability widely distributed
throughout the animal kingdom, which has been studied but
little, and which can easily be confounded with negative
heliotropism. It consists in many animals being compelled
to orient their bodies against the surfaces of other solid
bodies in a certain way, or bringing their bodies in contact
with other solid bodies on as many sides as possible (stere-
otropism). Certain animals seek only the concave corners
and edges of boxes (Forficula auricularia, ants, Auiphipyra,
the larvae of Musca vomitoria, etc.) ; while others fasten
themselves only to the convex edges and corners (caterpillars
of Porthesia chrysorrheea).

VII. A non-luminous source of heat may influence the
orientation, but generally it is not able to prescribe the direc-
tion of the progressive movements of animals. In this way
it happens that animals which move away from a source of
heat may be forced by the light to move from diffuse light
into sunlight, and to remain exposed to the high temperature
of the sunlight, even though this may cause their death.
The influence of a non-luminous source of heat can best be
compared to the influence of a weak source of light, which is
just sufficient to hinder a negatively heliotropic animal from
going toward the source of light, but is not sufficient to force
the animal to move accurately in the direction of the rays.

We have yet to draw a conclusion from the results of these
experiments, which could not be formulated until now. We
have seen that the heliotropic movements of animals possess-
ing a nervous system are determined in all respects by the
same external conditions and depend in the same way on the
external form of the body as do the heliotropic movements
of plants, which have no nervous system. These heliotropic
phenomena cannot therefore defend upon specific character-
istics of ilif central uerrons system.

HELIOTROPISM OF ANIMALS 85

xiv. ADDENDUM: SOME FURTHER EXPERIMENTS ON THE

GEOTROPISM OF INSECTS

As I have several times had occasion in this volume to
mention the influence of gravity on the orientation of ani-
mals a subject which in this form has not yet been discussed
in physiological literature it may perhaps be desirable to
add a few further facts on animal geotropism. I must say
beforehand, however, that my experiments in this field are
not yet completed, and that I intend to return to this sub-
ject.

1. I have found that caterpillars (for example, Bombyx
neustria) when placed in a hollow vessel creep vertically
upward. When ire irixh to pour such caterpillars out of a
/v.s-.sv/, ire- employ a method opposite to that used in pouring
out a h'<pdd ; we must hold the mouth- of the vessel njurard.
When such caterpillars are contained in a glass vessel, the
diffuse daylight entering from above in itself would bring
about this effect ; I therefore made this experiment in
wooden vessels. When the opening of the vessel was
directed downward the animals crept upward, and not an
animal escaped from the vessel. Geotropism, however, like
heliotropism, is especially evident only at certain epochs in
the life of the animal; for the geotropic experiments were
not at all times successful even in the same animals.

1 2. Small beetles, particularly Coccinella, which can always
be procured with ease in great numbers, were placed in a
wooden box, and to protect them from the effects of light I
put the box in a dark closet. The animals, which were at
first scattered over the whole box, were found the next day
collected at the highest point in the box, on the upper side,
where they remained. When I turned the box about, they
changed their orientation and moved again to the top. The
behavior of Coccinellidse and other beetles (particularly leaf

86 STUDIES IN GENERAL PHYSIOLOGY

beetles) is somewhat remarkable, as many other animals, such
as caterpillars, plant lice, etc., do not react geotropically
after they have been kept in the dark for some time.

3. Another phenomenon can be observed in Coccinellid<,
to which I have already referred in my former paper on the
"Orientation of Animals toward the Center of Gravity of the
Earth." l Cockroaches, for example, usually assume a position
on the vertical walls of boxes ; they never remain for any length
of time on the horizontal bottom, where their ventral surfaces
are turned toward the center of gravity. If, for example,
the roaches are scattered about on the four vertical walls of
a box, and the box is carefully and slowly turned through an
angle of 90 on a horizontal axis, only those animals whose
ventral surfaces are turned toward the center of gravity in
the new position begin to move. They leave the plane on
which they were up to the time the box was turned and again
seek a vertical plane from which to suspend themselves. Yet
the animals on the other three vertical walls remain at rest
while the box is turned. This seems to indicate that the
animals cannot turn their ventral surfaces toward the center
of gravity for a long time. Whether this means that the
animals' extremities can bear the pull of the load of the body
better than its pressure, I cannot say. The same phenome-
non is also observed in CocciiiellidaB. If the box is placed
obliquely, the two oblique planes on which the animals turn
their ventral surfaces toward the center of gravity are first
abandoned.

4. Aside from the peculiarity which has been mentioned,
the Coccinellidse, like roaches, are found hanging on walls
in all possible orientations toward the center of gravity of
the earth. Great differences are therefore found in this
regard also, for there are animals (such as the newly hatched
Lepidoptera) trliirh /iitf t/tc median plane of their bodies in

iSitzunysberichte der Wiirzburyer physikalisch-medicinischen Gesellschaft, 1888.

HELIOTROPISM OF ANIMALS 87

Ilic direction of the rerfieal and turn their head*
There are, Itoicerer, also animals which orient ///r///.sv//v.s in
e.niefl// {lie rererxe manner anil turn their head* doirnirard.
To this class belongs the garden spider, which we find hang-
ing in this position in the center of its web for hours. I
found the same behavior in one of the Diptera, which I have
not yet classified.

5. If a beetle or a house fly (from which the wings have
been removed) is placed on the disk of a centrifugal machine,
which is rotated slowly, the insect turns its body around the
same axis, but in an opposite direction to that of the revolv-
ing plate.

<>If the velocity of the machine is increased, these compen-
satory movements cease. These animals therefore behave in
this respect exactly as Mach claims vertebrates behave which
possess a labyrinth. 1 But while the movements of vertebrates
continue for some time after the movement of the centrifugal
machine has ceased, but in a sense opposite to those occur-
ring during: rotation, I have never been able to bring about

O O O

these compensatory after-movements in insects. 2

r. When one hemisphere of the brain of a house fly is
removed the same disturbances in orientation appear as after
the same operation on a rabbit. The fly from which the left
hemisphere has been removed moves continually toward the
rii/lif in its progressive movements. These deviations are
gn-ater or less according to the success of the operation. I
showed in an earlier paper that the turn-table reactions of
dogs and rabbits deprived of a hemisphere might be unsyni-
niftrical. If a fly which has lost the left half of its brain is
placed on a rotating disk, we find that on turning the disk
in the direction of the hands of a watch as seen from above,

1 MACH, Cniii'll/iiii-ii (/</ l.clin- run </// l{fii-c</i(ti<ixciiii>Jiii<htiiii<-n I Leipzig, IST.'i).

2 The observations of Lyon and of R&dl suggest tin- po--ii>ilit.\ ih.u thr-c> piic-
IH) mi 'ii. -i < I- ! H -i ic I upon t hi- ey- of thc->c- animals. \Vlirn the eye-; arc' removed or black-
enc-il tin- reaei i'.n- o-a-i'. | I'."!.')]

88 STUDIES IN GENERAL PHYSIOLOGY

the fly makes slight or only weak compensatory movements
Yet when the disk is turned in the opposite direction the fly
reacts very promptly (apparently even better than before the
operation). After destroying both hemispheres or ampu-
tating the head I no longer obtained compensatory move-
ments in the fly. The experiments of Mach show that the
compensatory movements of vertebrates emanate from the
head ; according to Mach, the labyrinth is to be considered
the essential organ. Such an organ, however, does not
exist in the head of a fly.

7. If the halteres are removed from a fly, it can no longer
fly upward ; in the attempt to fly it immediately falls to the
ground, where it frequently tumbles about. Gleichen-
Russwurm established this fact during the last century. I
found that such a fly reacts normally on the centrifugal
machine. The destruction of the halteres does not there-
fore have the same effect as the destruction of the labyrinth
in frogs, birds, or mammals, in which, according to the
experiments of Hogies and Schrader, compensatory move-
ments cease when the labyrinth is destroyed. The conjec-
ture expressed by others, and by me in my first publication,
that the direction of sound has an influence on orientation
has thus far led me to no new facts.

8. I have not yet been able to demonstrate compensatory
movements on the centrifugal machine in caterpillars, Musca
Iarva3, and snails.

II

FURTHER INVESTIGATIONS ON THE HELIOTROPISM
OF ANIMALS AND ITS IDENTITY WITH THE HELI-
OTROPISM OF PLANTS'

IN a former paper I showed that the dependence of
animal movement upon light is identical with that of plants
on the same source of stimulation. 2 I showed that the law
put forward by Sachs for the heliotropism of plants,
namely, that the direction of the rays of light deter-
mines the orientation, holds good also for animals. Free-
moving animals are compelled to execute their progressive
movements in the direction of the rays of light, as is the case
with the swarm-spores of certain Alga3. It was further
proved that the more refrangible rays of the visible spectrum
are the rays that are solely, or at least chiefly, effective in
bringing about the movements of these animals; as is the
case in the heliotropic movement of plants. After I had
proved the identity of this relationship point for point, I
believed it permissible to designate these reactions of ani-
mals by the same term as that used for the identical reactions
of plants, namely, "heliotropism."

At that time, however, I had proved this identity only in
the case of free-moving animals. The task still remained to
ascertain and investigate the influence of light upon the
orientation of sesxile animals, and to decide whether the
influence of the light is in this case also similar to that upon
sessile plants. It is known that in plants the direction of the

Archiv, Vol. XLVII (1890), p. 391.
"Parti, p. 1. See also LOEB, Sitzuii<jx>n-i-i<'ittc der WUrzburger phynkalisch-

medicinischen Gesellschaft (1888) ; GROOM TJND LOEB, Biologisches Ceniralhlutt, Vol. X
(1890;.

89

90 STUDIES IN GENERAL PHYSIOLOGY

light rays determines the orientation of the organs of the
plant. It is characteristic of the organs of sessile plants that
Jtt'liotropic cu features are produced when the plant is illu-
minated from but one side. A growing stem continues to
bend when illuminated from one side only until the growing
tip lies in the direction of the rays of light. Progressive
movement in the direction of the rays of light, which is the
rule for free-moving animals and plants, is of course impos-
sible for sessile organisms. Everyone who has cultivated
flowers in a room has no doubt observed the heliotropic
beiidings in the plants. The question now arises whether
t/u'xe heliotropic curvatures can also be produced in sessile
animals when illuminated from one side only. I shall show
in the following pages that this is, indeed, the case.

1. The experiments described here were made on the large
marine Annelid, Spirographis Spallanzanii. It lives in a tube
which is quite flexible, yet sufficiently rigid to keep the ani-
nial in a definite position. The tube is formed from the
secretions of the animal. The aboral end of the tube is
fastened (by a secretion) to stones or other solid objects.
The gills of the animal, which are arranged at its anterior
end in several spiral turns radial to the longitudinal axis of
the animal, are usually found unfolded and projecting beyond
the open end of the tube. As the tube is almost impervious
to light, the latter will act chiefly upon the gills. So far as
we know at present, the animal has no eyes.

The animal can move freely inside the tube, the inner
surface of which is perfectly smooth, and can be removed
from it without the slightest injury by cutting open the tube.
I have occasionally seen the worm leave the tube of its own
accord, when the water in the aquarium became bad.

The layman seeing these animals in the tubes with their

FURTHKR I \ VKSTIC.ATIONS ON H EL IOT ROPI s M ( .>l

nils t'nlh unfolded takes them at first for plants hearing a

palm-like crown (the gills) upon a long naked stein (the
tube). A slight jar, however, causes the animals to draw
hack their gills rapidly into the tubes.

When the animal is taken from the sea and kept in an
aquarium, it is at first indifferent to the light. This con-
tinues until the animal has attached itself by its foot to the
bottom of the aquarium a period often of several days. As
soon as this has taken place, however, the orienting influence
of the light begins to be noticeable. If light falls upon the
animal from one side only, heliotropic curvatures make their
appearance in flic tnlte. The animal turns its oral pole
toirard the source of liyht and bends its tube until the axis
of its radially e.rpamled (jills lies in the direction of the raijs
of lit/lit. The animal maintains this orientation as long as
the direction of the rays of light remains unaltered.

To test more accurately to ichat extent the direction of
the rays of light determines the orientation of the animals, I
put them into an aquarium which stood at the window, and
which could be completely screened from the light by a zinc
box. The outlines of the aquarium are indicated in the
drawings (Figs. 7 and 8) by black lines, the outlines of the
zinc box by dotted lines. The wall abed of the zinc box
could be moved vertically upward, so that the amount of
light entering the aquarium could be regulated. The zinc
box, the walls of which were painted black on the inside,
was so placed over the aquarium that the movable wall was
on the window side of the aquarium. If this wall was
raised only slightly, as shown in Fig. 8, the rays entered
the aquarium almost horizontally. When it was drawn
farther up, as in Fig. 7, rays entered from above in addition
to the horizontal rays. These were more intense than the
rays entering horizontally.

On December 14, ISV.l, I put nine vigorous specimens of

92

STUDIES IN GENERAL PHYSIOLOGY

Spirographis Spallanzanii, each about 15 era. long, on the
bottom of the aquarium, with the longitudinal axes of their
tubes perpendicular to the plane of the window. Eight of
them lay with their oral poles toward the room side efgli
(Fig. 7) of the aquarium; one with its oral pole toward the
window side. The first two days passed without any change

FIG. 7

in the orientation ; the animals first attached the aboral ends
of their tubes to the floor of the aquarium. In the course of
the third day the tubes of six of the animals, which were
placed with their oral pole toward the room side, began to
bend in an almost horizontal plane, the concavity of the curv-
ature beiny directed toward the window. The other two
animals, which had likewise been placed with their heads
toward the room side, first elevated the head end and then
curved the tube concavely toward the irindoie. Finally, the
ninth animal, which I had placed in the aquarium with its
head toward the window,, raised its head a little.

FURTHER INVESTIGATIONS ON HELIOTROPISM '.:*

Within the next few days the six first-incut ioncd animals
further elevated their heads, so that the animals on Decem-
ber "2'2 eight days after being placed in the aquarium
were all similarly oriented toward the light. The IK-IK! inix
diri'ctcd fatrnnl the irindoir, and flic a.rix of xi/ni me/ >>/ of
the /////s irhicli ircrc c.rjHmcd fo flic Injht lot/ in flic direction
of the more iiifciixc /v///.s- of <l<ii/li<i/tf irhich eidered from

FIG. 8

trUhonl (tin/ ahore. I waited to discover whether this orien-
tation would last. The aquarium remained undisturbed
until February 1<>, ISllQ; that is, for more than two months.
The animals also did not change their positions, as indicated

in Fig.

.

3. On the afternoon of February 17, 1890, the aquarium
was turned 180 about its vertical axis, and the zinc box
was again inverted over the aquarium so that the movable
end was directed toward the window. BV turninc- the

J

aquarium around in this way, the heads of the animals,
which had been until then directed toward the source of
light, were suddenly turned toward the room side of the

94: STUDIES IN GENERAL PHYSIOLOGY

aquarium. My object in turning the aquarium around was
to see whether a change in the direction of the rays of light
would cause the animals to reverse their heliotropic curva-
tures and to turn their heads again toward the source of light.
There was no change during the course of the afternoon and

night. But toward noon of
the following day I found two
animals, which in the morning
had still been in the position
AB (Fig. 9), in the position
ABi ; F indicates the plane of
the window. The portion DB
of the tube had described the
surface DBB l about the point
D as center. A similar change

in the orientation of all the
FIG. 9 ... , ,

remaining animals took place

during that and the following day. In this experiment the
direction of the ravs of light was modified somewhat ; the

t/

wall aln-d was left quite low, so that almost not hi in/ but
horizontal rays entered the aquarium (Fig. 8). I wished to
determine whether the animals would continue to follow the
direction of the rays and so assume an almost horizontal
position. This did, indeed, occur. On February 22, 1890,
five days after reversing the aquarium, the orientation was
accomplished, as indicated in Fig. 8. The animals had
turned their heads toward the source of light, find the axes
of their gills lay almost horizon! <dly in the direction of the
rays of li</ht. I left the conditions of the experiment
unchanged until toward the end of March, and during all
that time the animals maintained their orientation.

4. If the rays of light fall vertically from above into the
aquarium, Spirographis directs its tube vertically upward,
exactly as a stem grows vertically upward into the open air.

FURTHER INVESTIGATIONS ON HELIOTROPISM ( .5

Tlii< experiment was made in another aquarium, in which
the light rays entered chiefly from above. The animals in
the large aquarium of the zoological station at Naples are
iisually found mainly in this position; the light enters this
aquarium chiefly from above. Here, however, where free-
swimming forms easily disturb the orientation of Spiro-
iTraphis, it is not always so perfect as when all possible dis-
turbing causes are avoided, as in an aquarium used only for
such experiments.

"). It follows from these experiments that gravitation
exerts only a slight effect, if any, upon animals which are
subjected simultaneously to the effects of light and gravity.
It was, however, necessary to discover whether a geotropic
erection of the animals would not occur under the influence
of gravity alone in a completely darkened room.

On March 21, 1890, I placed a large number of Spiro-
graphis in a horizontal position upon the floor of an aquarium
in the dark room. On March 24 most of the animals had
attached themselves by their aboral ends to the bottom of
the aquarium. The oral ends of the tubes were then elevated
until the gills no longer touched the bottom of the aquarium.
The axis of the spiral did not stand vertically (as was the
case when light fell vertically into the aquarium, or as should
have been the case had the animals been geotropically irri-
table), but only at a slight angle from the horizontal. The
animals remained in this position until the end of the experi-
ment, which was interrupted in the middle of April. Gravity
therefore has no important influence upon the orientation of
Spirographis Spallanzami.

(>. The contact-irritability of the gills is manifested by
the fact that flic// hcinl airatj from solid surfaces. This
form of irritability can modify the result of the heliotropic
experiments upon the animals. I placed several of the ani-
mals upon the floor of an aquarium which was so shallow

96 STUDIES IN GENERAL PHYSIOLOGY

that the animals could not erect themselves. They were so
placed in the aquarium that their longitudinal axes lay per-
pendicular to the side ab (Fig. 10) of the aquarium, and their
pedal extremities M touched the glass wall ab. The side a
faced and was parallel to the plane of the window. The
animals fastened

a ^L 3 themselves to the wall

ab, and then began
to react, in their char-
acteristic way, to the
light, by which the
head was turned and
the tube became con-
cave toward the
FIG. 10 source of light. The

tube MX assumed the position MX^ As soon, however,
as the tentacles touched the glass wall ab, the tip N turned
away from the glass wall. The heliotropic bending gradually
affected all the elements of the tube MX, so that the
Spirographis finally reached the position MX 2 , in which it
remained throughout the period of observation four
months.

I repeated this experiment a number of times, always
with the same result.

7. The heliotropic phenomena of Spirographis took place
both in direct sunlight and in diffuse daylight. The minimum
light intensity just sufficient to bring about these phenomena
is very small. I have not yet studied the effect of rays of
different refrangibility in producing these phenomena. Since
thus far the more refrangible rays have proved to be the
most effective heliotropically both in plants and animals, it
is to be suspected that Spirographis also will prove no excep-
tion.

8. As is well known, Sachs has formulated the law that

Kl ICTIir.U I XVI' STH1ATIOXS ON H E L IOT RO P I S M 17

radial plants are orthotropic; i.e., they place Ilicir longi-
tudinal axes in the direction of the rays of light, or of
gravity. It will have occurred to tin* reader Ilia! Spiro-
graphis, the body of which, like that of all Annelids, is built
on the dorsiventral and not on the radial plan, reacts toward
the light as a radial plant organ. I have, however, already
einphasi/.ed the fact that only the radially arranged gills of
the animal are exposed to the light, while the remainder of
the animal is inclosed in the tube. These observations,
therefore, show that a radial annual or</an also olx-i/s Hie
lair <>f orientation esfa/ilislied hi/ Saelis for jthuits (even
though Spirographls possesses a central nervous system,
which the plant does not).

It is also of physiological interest that the respiratory
organ of Spirographis is so highly sensitive to light that
the orientation of the whole animal in space depends
essentially upon this sensitiveness. This fact may perhaps
explain why Branchionima, a Serpnlida quite similar in
structure to Spirographis, has well-developed eyes upon its
gills.

'>. If Spirographis is carefully removed from its tube, it
is not able to raise its body from the floor. In such a con-
dition it creeps about like an earthworm, only much more
slowly. I have occasionally seen such animals creep to the
window side of the aquarium. They appeared, however, to
suffer from contact stimuli, to which they were constantly
exposed in this condition ; they all died within a few days.

1U. I am not in a position to make a definite statement
concerning the mechanics by which the heliotropic curvature
of the tube is brought about in these animals. The wall of
the tube of an adult animal is 1. '_'""> \.~> mm. or more thick.
It is very flexible and elastic. ////"' animal is taken out of
the Inlie after the /alter has lieen lienl I// roni/li the liel iof ropic
reactions of l/ie animal, the lithe nevertheless maintain* its

98 STUDIES IN GENERAL PHYSIOLOGY

. The wall on the outer concave side of the tube is
therefore permanently shortened. It might seem that the
limit of elasticity of the tube is so low that it retains, like a
piece of lead, a curvature imparted to it through the muscular
force of the animal. But this is not the case. I put a thick rod
of lead into the straight tube of a Spirographis and bent it till
the tube was strongly curved. The lead rod was allowed to
remain in the tube. When a week afterward I withdrew the
rod from the tube, it retained only a trace of the curve
impressed upon it. Similar failure followed my attempt to
straighten by the same method, a heliotropically curved
tube. Yet, as I have already shown, Spirographis is able to
straighten its curved tube within a few hours after a change
in the direction of the rays of light, and, what is more, the
tube remains straight. The tube retains its curvature even
after it has been split open. The animal has, however,
besides pressure and pull, another means at its disposal to
change permanently the orientation of the tube, namely, the
production of a secretion and the formation of a new layer
within the tube. The idea that permanence in the curvature
is attained in this way is supported by the fact that the
inner layer of the tube is much more elastic than the outer
layers, so that the formation of a new inner layer on one
side of the tube might curve it permanently. The following
fact supports this view : If a tube is cut open lengthwise,
the cut margins roll inward. If the individual layers are
separated, as can be done easily, it is seen that the tendency
of the inner layers to curl up is greater than that of the
outer layers, and that of the innermost, newest layer is the
greatest of all. The formation of a new inner layer on one
side of the tube would, therefore, be sufficient to maintain
the curvature of the tube permanently. The formation of a
new layer cannot be observed directly. One is also disap-
pointed in the hope of finding one side of the wall of the

FURTHER INVESTIGATIONS ON HELIOTROPISM '.'.

tube thicker than the other, for the thickness of the wall of
perfectly straight tul>es varies greatly in different places of the
same cross-section. The thickness of the wall is therefore no
criterion in answering our question. I can therefore formu-
late the following theory of the origin of the heliotropic

curvature in the tube, only
by reserving the right to
lest, and perhaps modify
it later. I believe that,
when illuminated from one
side only, the animal
strives at first to bring the
axis of its mils as nearlv

o

as possible in the direction
of the rays of light. In do-
ing so the animal perhaps

bends the tube by aid of its muscular force. Since the tube,
however, tends to resume its original position because of its
elasticity, the body of the Spirographis must rub more
strongly against the concave wall of the tube than against the
other. This increased friction brings about a great activity
of the skin glands, whose secretion forms the material of the
tube. That friction indeed leads to secretion, and with it to
the formation of a tube, I have been able to prove directly
in the case of the Actinian, Cerianthus membranaceus. I
have been able to establish the following facts regarding
Spirographis which seem to indicate a similar behavior. I
cut small pieces from the tube. The animal was in conse-
quence obliged to rub against the cut margins during its
movements ; and a copious secretion was indeed formed in a
short time, which soon closed the opening with a new mem-
Itrane. There is, moreover, always more or I'-ss friction on
the anterior margin of the tube when the animal stretches
out its head. In fact, the tube grows constantly from this

100 STUDIES IN GENERAL PHYSIOLOGY

end, as illustrated in Fig. 11. In this experiment I had cut
a long broad piece aa l out of the tube at a, so that the
anterior piece of the tube a l b remained attached to the rest of
the tube only by a thin piece p. After the operation the
animal showed its gills at a, and no longer used the piece j6
of the tube. New material was deposited at a within a few
days, and in the course of three weeks the new piece c was
formed. Its light color readily characterized it as new.
I had at the same time cut away the aboral end of the
tube completely. Before my very eyes the movement of the
aboral end upon the sand caused the secretion of a sticky
mass, to which particles of sand became attached. In this
manner the new piece of tube <lc was built, consisting of
grains of sand cemented together by the glandular secretion
from the tube. The newly formed piece was perfectly
smooth on the inside. The secretion from the skin glands
continues as long as there is any noticeable amount of fric-
tion. When I removed Spirographis from its tube and
placed it in a smooth test-tube, practically no secretion
occurred. Secretion occurred only from the parapodia in
the form of long, fine threads, similar to those produced by
the spinning glands of spiders. If, however, the naked
Spirographis was laid upon the sand, the aboral end was
soon covered by a shell of sand kernels. I have never,
however, seen the animals form a complete tube when
removed from their old ones ; for in their exposed condition
they soon die.

II

Spirographis Spallanzanii attains its heliotropic orienta-
tion when illuminated from one side by curving its flexible
tube ; new growth of the tube is not necessary. There are
other Serpulidse, however, .the calcareous tubes of which are
stiff and inflexible. These Serpulidas, like Spirographis,
expose their gills to the light, and these, too, react according

I-YRTHER INVESTIGATIONS ON HELIOTROIM s \i lot

to their structure as radial organs. Such a Serpulida, if
heliotropically irritable, must place the longitudinal axis of
its cylindrical tube in the direction of the rays of light. If
the calcareous tube is brought into any other position with
reference to tho source of light, the animal must make use
of one of two possibilities in order to regain its proper orien-
tation: either it must lengthen its calcareous tube, and bend
the iicicli/ (jrotriinj \><ir\ until the axis of the tube again

FIG. 12

lies in the direction of the rays of light, or else leave its
tube entirely and build a new one having the proper
orientation. The animal makes use of the first of these
possibilities. I experimented with Serpula uncinata. These
Annelids inhabit calcareous tubes and are gregarious.
Large white blocks are found in the Gulf of Naples which
consist entirely of the tubes of countless numbers of such
Annelids massed together. I noticed that the individual
tubes in such a mass all had the same orientation, and in
those cases in which the blocks showed the base upon which
they had rested 011 the horizontal bottom of the ocean it was
plainly visible that the tubes must have stood in the water
with their longitudinal axes vertical. Serpula can, like
Spirographis. move about freely within its tube. I laid a
largo block of innumerable annelid tubes, each of which

102 STUDIES IN GENERAL PHYSIOLOGY

stood almost mathematically straight and parallel, upon the
floor of the aquarium so that the longitudinal axes <tl> of the
tubes, which had previously been vertical, now had a hori-
zontal position (Fig. 12). The liglit fell info the aquarium
from above. I noticed that in the course of the next day
the Serpulidse, which like Spirographis presented only their
radially arranged gills to the light, bent them strongly
upward. Individual tnlx'x 1lien liegan to grow, and in such
a way that the neirly formed portions of the tubes all bent
upward until the free tip of the tube lay in the direction of
the rays of light (which in this case was identical with the
direction of gravity), after which the tubes continued to grow
in the direction of the rays of light (a ml of ' g rarity). Within
six weeks the entire block was covered with tubes which
curved upward; not a single individual had continued to
grow in the original direction ah. The figure shows the
Serpulidse curving upward at the free edge of the block. The
final effect in this case therefore again corresponds to the
theory of geotropism and heliotropism as presented by Sachs :
the axis of the gills which react as a radial organ lies finally
in the direction of the rays of light (and of gravity). While
in the case of Spirographis, however (the tube of which is
flexible), this effect was brought about through a change in
the orientation of the old tube, the same effect was attained
in the case of Serpula (the tube of which is inflexible) only
through the hcliotropic curvature of that portion of the tube
which iras in the process of growth.

In the above-mentioned experiment the direction of the
light rays was identical with the direction of gravity. I
have not yet been able to decide whether light alone deter-
mines the orientation of the tube, or whether gravity also
plays a role. I hope later to make a series of experiments
regarding this point.

FURTHER INVESTIGATIONS ON HELIOTROPISM

m

1. I have endeavored to find other animals in which helio-
tropic curvatures, arc formed only in the growing parts.
These efforts have been successful in the Hydroids. Stems of
Sertularia fpolyzonias ?) were cut off near the root and fixed
in the sand in an inverted position, so that the cut end was
directed upward. The stems were placed near a window
through which the light fell obliquely and from above. The
animals began to regenerate; new polyp-bearing stems grew
from the cut end as well as new roots; 1 hut while, the, new
steins grew upward and toward the irindoir, the roots grc/r
iloiniirard and toirard the room side. The polyp-bearing
shoots are positively i the roofs 'itegatirclt/, heliotropic. That
the negatively heliotropic elements were true roots was
proved by the fact that when brought in contact with a solid
body they attached themselves to it and continued to grow
over its surface in close contact with it. They could be
loosened from their attachments only by force. The polyp-
bearing stems do not possess this kind of contact-irritability.
The heliotropic phenomena will be readily understood by the
aid of Figs. 18, 1-t, and 15 : ab is the old stem, 6 the cut
end; the stem is fixed in the sand to the point ac. From the
cut end b arise newly formed roots TF 15 which bend down-
ward away from the light and toward the room side of the
aquarium. The new polyp-bearing shoots S grow upward
and toward the window. The arrow marks the direction of
the rays of diffuse daylight in this experiment.

'2. In these experiments new growths occasionally sprang
from the middle of the old stem, which, so far as their con-
tact irritability was concerned, reacted as roots. Those
tendrils which attached themselves to solid bodies ircrc
/js negatively heliotropic. They grew downward and

1 Wliii-h i- <>f important:.- iu tin- tln'ury <>f organization.

104

STUDIES IN GENERAL PHYSIOLOGY

toward the room side, and remained free of Hydraiitbs. (See
Fig. 13, W a .)

On the other hand, I saw also new polyp-bearing stems
arise from the old stems, although much less frequently;
these grew in the opposite direction, namely upward.

3. That in the case of Sertularia it is, indeed, only the
(jroirii/i/ parts which produce the heliotropic curvatures is

FIG. 13

FIG. 14

FIG. 15

shown by the following experiment. The growing tips were
cut off a large number of Sertularia stems. The stems began
to grow, and in the course of a few days sent out new sprouts.
The new growth is strikingly different in color from the old
stem ; while the latter is rather brown (from having been
covered by Algae ?), the color of the new growths is a light
yellow. The growing elements curved thcmselres until the
growing points lay in the direction of the rays of light and
then continued to grow in this direction. During all this
time no change in the orientation of tJte old stem occurred,
nor did any take place in other uninjured stems, in which no
linear growth occurred during this time.

How far gravity played a role in these experiments I was

FURTHER INVESTIGATIONS ON HELIOTKOI-ISM 105

unable ID determine accurately. The Sertularia cullival.'d
in a dark rcx>m ceased ID grow, though I question \vhet her
this was entirely duo to lack of light.

4. Light (and perhaps gravity) influences not, only the
orientation, l>ut also the />ox/7/Vm of the newly formed organs.
L have observed, and not in the case of Sertularia only, that
the ne\v i>ol//ji-ln'(irin</ branches (tlirays arise from the upper
surface of the stem. In Fig. 15, a new stem iS springs from
the upper side (the side directed toward the source of light)
of the stolon W t . I do not desire to discuss these points
more minutely here, as they will form the basis of a paper
which is to appear soon, on the form of animals.

The experiments on Sertularia described here serve only
to complete the general consideration of animal heliotropisni
and to show more fully the identity of animal and plant
heliotropism. The special investigation of the heliotropic
behavior of Hydroids is to be the subject of future study.
That this is both an interesting and a fruitful h'eld
is shown by the beautiful work of Hans Driesch, which has
just appeared, on the ''Heliotropism of Hydroids." Driesch
arrives at the following result :

The stolons which are produced instead of polyps under
unfavorable conditions iu Sertularella polyzonias, are with the
exception of the first, which is turned away from the light from the
very beginning, all positively heliotropic at first, becoming 1 nega-
tively heliotropic after the growth of the daughter-stolons. They
arise from the side of the mother-stolon, which is turned town id
the light. (P. 152.;

This observation of Driesch agrees very well with mine. I
shall return to them ill my "Physiological ^Morphology of
Animals."

The results of this studv may be summarized as follows :

1. Certain sessile animals (Serpulidse, Hydroids) which
are compelled to react to light and gravity as radial organ-

l Zoologische Jahrbllcher, Vol. V.

106 STUDIES IN GENERAL PHYSIOLOGY

isms always place the axes of their radial organs in the
direction of the light rays, as do the radial organs of sessile
plants.

2. The fact that sessile animals, such as the Serpulidre,
have a central nervous system, while plants have not, does
not bring about any difference in the heliotropic effect.

3. If the light enters from one side, there are produced in
the above-mentioned animals hcliofro)>ie eiirrati/res which
correspond to those obtained in sessile plant organs under
similar conditions.

4. There are sessile animals which attain these helio-
tropic curvatures only during the period of growth, as is the
case with certain plants. Sertularia and Eudeudrium, among
others, belong in this group, in which only flic t/rotriity parts
are able to beml heliotropically ; Serpula uncinata, which is
able to change the orientation of its otherwise stiff tube only
irheii the latter /s </r<>/n'i/</, also belongs in this group.

5. Spirographis Spallanzanii, the tube of which is flexible,
is capable of heliotropic curvatures without accompanying
phenomena of growth, as are also certain jointed plant organs
which attain their heliotropic orientation without phenomena
of growth.

Although I do not consider my study of animal heli-
otropism ended with this paper, yet I think I have shown
that the heliotropism of sessile animals is essentially identi-
cal with the heliotropism of sessile plants.

ITI
ON INSTINCT AND WILL IN ANIMALS 1

1. IN the biological literature one still finds authors who
treat the "instinct 1 ' or the "will" of animals as a circum-
stance which determines motions, so that the scientist who
enters the region of animated nature encounters an entirely
nr\\- category of causes, such as are said continually to pro-
duce before our eyes great effects, without it being possible-
for an engineer ever to make use of these causes in the physi-
cal world. "Instinct" and "will" in animals, as causes
which determine movements, stand upon the same plane as
the supernatural powers of theologians, which are also said
to determine motions, but upon which an engineer could not
well rely.

My investigations on the heliotropism of animals led me
to analyze in a few cases the conditions which determine the
apparently accidental direction of animal movements which,
according to traditional notions, are called voluntary or
instinctive. Wherever I have thus far investigated the
cause of such "voluntary" or "instinctive" movements in
animals, I have without exception discovered such circum-
stances at work as are known in inanimate nature as deter-
mining movements. By the help of these causes it is pos-
sible to control the "voluntary" movements of a living ani-
mal just as securely and unequivocally as the engineer has
been able to control the movements in inanimate nature.
What /m.s been token for 11m effect of'-iciU" or "instinct"
is in realilij Ilic effect of U</ht, of yrtu ////, of friction, of
chenn'col forces, etc. The following may be added by way
of fuller explanation:

lPflatjersAr<.-ht'i; V.,1. XLVII (1S9J>. p. 107.

107

108 STUDIES IN GENERAL PHYSIOLOGY

The position which the tube of Spirographis Spallanzanii
assumes in space is such, as we have seen, that the animal
turns its oral pole toward the light, and puts the axis of its
radial gills into the direction of the rays of light. The- direc-
tion of ih^ r<(ys oflitjlit is the condition ichicJi determines
the orientation of these aninioh unequivocally. If the ques-
tion should arise as to how to hold a great number of living
Spirographes continually and rulnnlorilij in a definite
position in space, this could be done, as our investigations
have shown, by simply allowing the rays of light to fall upon
'the animals in the direction which we wisli the animals to
assume and hold. If anyone endeavors to compel Spiro-
graphis to assume a definite spatial orientation either through
instinct'' 1 or "will." he will be obliged to seek the aid of
the rays of light in order to obtain the desired result, even if
he afterward believes that, beside, before, behind, after, or
between the light rays the "instinct" 1 or "will" of the ani-
mal co-operated with the light to bring about the move-
ment. He will further be able to convince himself that the
direction of the light, if sufficiently intense, is alone- and
unequivocally able to determine the orientation.

The direction of the "voluntary"' movements of the
winged plant lice is determined by the direction of the rays
of light. The animals are forced to turn their oral poles
toward the light and to move in the direction of the rays of
light. If the animals are introduced into a transparent
vessel, they live and die on the side of the vessel which is
turned toward the light. If anyone should wish to fcrce
these animals to move in a fixed direction toward a definite
point "voluntarily," he knows now how this may be accom-
plished. He need only allow sufficiently intense light to fall
upon the animals in the direction in which it is wished that
they should go.

As is well known, the direction of the rays of light, par-

<>\ INSTINCT AND WILL IN ANIMALS 109

tieularly that of the more refrangiUe ones, determines also
the orientation of the organs of a plant. By the help of
light the botanist controls the orientation of a plant at will.
Whv should lu> maintain that the "will" or the "instinct"'

tt

of the plant co-operates with the rays of light when the
orientation is determined so/r/// and unequivocally by the
latter V The movements of an animal toward the light are,
however, as I have shown, identical point for point with
the movement of a plant toward the light. Wherever the
orientation of plants has been satisfactorily controlled experi-
mentally, light has, indeed, been considered the sole deter-
mining factor ; but in the case of animals, in which in similar
experiments light is without doubt also the sole determining
factor, "instinct" and "free will 1 ' have still been considered
to play a role.

Just as the direction of the rays of light (particularly that
of the more refrangible ones) is the essential factor in the ex-
am pies described above, and in many others given in my
papers on heliotropism, so in other cases it is gravity, in others
again contact with solid bodies, in still others chemical forces,
etc., which determine the movements of the animals.

2. In order to state the cause which determines in each
instance the '"voluntary" movements of an animal, I desig-
nated the movements by their external cause. I spoke
therefore, as has long been the custom in plant physiology,
of heliotropism when the direction of the rays of light
determines the direction of the movements of an animal or
its orientation ; of geotropism, when gravity, or of sfcrcof-
ri>/>/x>i/, when contact with solid bodies, determines the
orientation or the movements ; etc.

A zoologist asked me reproachfully what had been gained
by designating as "stereotropism" what had been designated
as "instinct." I was discussing the fact that certain ani-
mals creep into the crevices of solid bodies, and the /oologist

110 STUDIES IN GENERAL PHYSIOLOGY

was of the opinion that the animal behaves thus through
"instinct." If a physicist finds that liquids rise in a capil-
lary, or that one liquid forms a convex while another a concave,
meniscus in a glass tube, he will be less easily satisfied than the
zoologist, according to whom everything is done through
"instinct." The physicist will endeavor to discover more
precisely what conditions underlie the phenomenon. This,
it seems to me, is also the problem of the biologist a prob-
lem which is not even recognized, much less solved, by
saijiny the canse of such < such a motion is (in "instinct."
From a biological standpoint one would at first take it for
granted that light causes animals to creep into crevices.
But I was able to show that the animals creep into the
crevices between solid bodies even ir/ieii (lie sot id bodies are
2>erfectlij transparent ami are e.r/tosed to a strong light;
secondly, that the animals behave in a similar way when
put in a ]>erfecf/t/ dark room. Light is not, therefore, the
physical cause which determines this phenomenon. I proved
this for Forficula, ants, the larvse of Musca vomitoria, etc.
Plateau had previously established this fact by a similar
experiment upon Cryptops, with which I was not familiar at
that time, however. The animals creep into narrow crevices,
therefore, not because of the light, but because they are
forced to bring as much of their bodies as possible in contact
with solid bodies. The friction and the pressure produced
by the solid bodies are therefore the determining cause. This
view, that light has nothing to do with the phenomenon, but
that it is the friction produced by contact with solid bodies,
has this advantage over the traditional phrase " It is instinct,"
that pressure and friction are physical agencies which, like
light, can be controlled quantitatively and qualitatively, and
by which we can prescribe unequivocally the "voluntary"
movements and the ''voluntary" orientation of an animal.
I will here add that, while there are a large number of

ON INSTINCT AND WILL IN ANIMALS 111

animals which are forced to bring llit'ir bodies in contact
with solid objects on all sides as far as possible, there
are others which show exactly the opposite form of irrita-
bility and immediately draw themselves away from a solid
body with which they chance to have come in contact. To these
belong the Nauplii of Balanus perforatus, the tiny Myside;e
of the Bay of Naples, the gills of Spirographis Spallan-
zanii, etc. That that form of irritability which I have
called "stereotropism" plays a prominent role in life-
phenomena, however, follows from the fact that the entrance
of the spermatozoon into the egg (as shown by the investi-
gations of Dewitz 1 ) is governed by this form of irritability,
and that the migration of leucocytes is likewise determined
largely by contact-irritability. I have, moreover, inciden-
tally found, in my investigations on the influence of external
stimuli upon the form of the body, that stereotropism influ-
ences not only the shape, but also the size and velocity, of
the growth of certain organs. These investigations were
made upon Hydroids. I succeeded in producing stcrcotropic
citri-ftfiircs (away from solid bodies) in certain organs with
the same certainty that I produced heliotropic curvatures.
Certain organs, when not in contact with solid bodies,
attain, within the same period of time and under otherwise
similar conditions, only one-tenth the length which they attain
when in contact on one side with a solid body. It is for
these reasons that I have made no mistake and performed no
useless task in calling attention to the importance of this
contact-irritability in the animal kingdom, to which I have
found it necessary to give a special name.

;}. I have thus far given only examples in which a single
source of stimulation determines the ''voluntary" movements
of animals. But in a large number of cases the movements nf
animals are not dependent upon one cause of stimulation

i DI:\VIT/, PtlHf/f-m Archil-. Vol. \\XVII. See also MASSAKT, Bulletin de

r.lr,i,i,'ini, royalt <i< /.v/, //,,/,, liruxrllr.-.

112 STUDIES IN GENERAL PHYSIOLOGY

alone; more frequently several causes co-operate, and the
movements which are produced in this way, the cause of which
is again sought by many in the "will" of the animal, are
only the resultant of various causes operating at the same
time. In very intense light the full-grown larvre of Musca
vornitoria move away from the light in the direction of the
light rays: they pass by a piece of meat lying in their way.
If the light is sufficiently weak, however, the chemical influ-
ence of the volatile substances arising from the meat ex-
ceeds the orienting influences of the light and the larvse crawl
to the meat. With other animals which are still more sus-
ceptible to chemical stimuli as, for example, the male
Lepidoptera, which, as is well known, are attracted to the
female from great distances entirely through the effect of
chemical stimuli heliotropisni may be entirely masked by
these chemical stimuli. It is not always an easy matter to
say, from the movements which an animal always executes,
what are the conditions determining these movements.

4. Another complicating circumstance is still to be added.
Life-phenomena are phenomena of irritability ; /. <'., they are
not dependent solely upon the external causes acting upon
the organism at a given moment, but upon these and the
conditions present within the organism taken together; and
the latter conditions are in themselves variable. The study

J

of animal heliotropisni revealed the fact that one and the
same animal may react differently toward the light during
different periods of its existence. The caterpillars of Por-
thesia chrysorrhoea after having fasted through the winter
are energetically positively heliotropic. After the animals
have eaten, heliotropisni may still exist, but intense light,
which formerly determined their movements with definiteness,
has no more effect upon them than did quite weak light
previously. Plant lice become sensitive to light that is to
say, positively heliotropic only after they have fed; the

ON INSTINCT AND WILL IN ANIMALS 113

larvae of Musca vomitoria are energetically negatively helio-
tropic only when fully grown, etc. In ants sensitiveness to
light is, as 1 have shown, connected with sexuality. The
males are more sensitive than the females; at the time of the
nuptial flight the males and females become energetically
hrliotropir, while the so-called workers remain practically
uninfluenced by the light. There must also be mentioned
the change which occurs in the sense of heliotropism of many
animals in different stages of their development. The full-
grown larva of Musca vomitoria is negatively heliotropic;
yet the sexually mature insect is positively heliotropic.
Such a behavior is quite widely distributed.

Finally, it is not infrequently possible to change at will,
through the influence of light, positively heliotropic animals
to negatively heliotropic animals, and the reverse. The
larvae of Balanus perforates, the larvse of certain worms, and
indeed a large number of other animals, become positively
heliotropic when they are left in the dark for a long time.
If they are brought into light of sufficient intensity, they
become negatively heliotropic after a time, and this the more
quickly the more intense the light.

We do not, therefore, always meet with simple conditions
in analyzing the causes which determine the "voluntary"
movements of an animal; but, however complicated they
may be, the ''voluntary 1 ' movements of animals are never-
theless, as our experience indicates, always unequivocally
determined only by such circumstances as determine also
the movements of bodies in inanimate nature.

o. To be sure, many of the authors who oppose my con-
clusions would protest if it were said of them that they hold
the "will" to be something which cannot be explained on
physical or chemical grounds. But if some physical agency is
pointed out which prescribes unequivocally the orientation
of ananiin;il bodv or the direction of its movements, which

114 STUDIES IN GENERAL PHYSIOLOGY

were formerly believed to be determined by the "will" of
the animal, the authors are still dissatisfied. They did not
doubt that ultimately a physical solution of the question
would be found, but they expected something more sublime,
something which is more closely related to the mysticism of
the ganglion cells. Of course, our knowledge of the process
is not exhausted when it is proved that the direction of the
rays of light prescribes the direction of the progressive move-
ments of ELeinatococcus swarm-spores or the Nauplii of
Balanns; just as little as the knowledge of the chemical
effects of light is today exhausted. Yet no one will say
that "instinct" is the determining circumstance in these
physical phenomena.

6. Just as the past generation of physiologists felt it
to be a handicap that instead of looking for the causes of
life-phenomena, investigators were satisfied with the phrase,
"The vital force is tin- cause," so it is a handicap to us that
within the more limited sphere of the so-called psychic life-
phenomena the influence of this scholastic method of think-
ing has survived to the present time. The handicap lies in
the fact, that if one says that "instinct" or "will" deter-
mines a motion, the true problem involved is ignored or
concealed. This true problem is the analysis of the circum-
stances which in each case determine unequivocally the
"voluntary" movements of an animal. It was the object of
this paper to point out that we must endeavor to solve this
problem with as little concern for "instinct" and "will" as
for "vital force."

IV

HETEROMORPHOSIS '

I. INTRODUCTION

IT is well known that a number of animals possess the
power of forming a new organ in the place of an organ which
has been lost. It has always been taken as a matter of course
in animal physiology that the regenerated organ is neces-
sarily identical in form and function with the one which has
been lost. The experience of the botanists, however, shows
that this does not hold true in the case of plants, and a few
sporadic observations upon animals which, however, have
not been taken into consideration for this problem seemed
to suggest that similar conditions might be found in animals.
/ Jut re undertaken the task of finding out whether <ni<l l>/j
irliat means it is possible in (ininicils to prod nee. <tt trill in
the jtldce of a lost organ a typically different one differ-
ent not only inform, l>nt Ixo in function. It is my purpose
to report the results of these experiments in the following
pages.

The organs which I tried to substitute for each other in
these experiments are the oral and aboral poles (head and
foot). I have succeeded in finding animals in which it is
possible to produce at desire a head in place of a foot at the
aboral end, without injuring the vitality of the animal. The
animal shown in Fig. lt>, a Tubularian, has by artificial means
been so altered that it terminates in a head at both its oral
and aboral ends. If, for any reason, it were necessary to
create any number of such bioral Tubularians, this deinnnd
could be satisfied. In another Hydroid, Aglaophenia phmia,

1 \Vilrzburg, 1891. Th<- |.;im;ih!ot is <l:it ! 1 vM, :ilt .Imti-li il app'an'.l in isii.

115

116 STUDIES IN GENERAL PHYSIOLOGY

it is possible so to change the form of the animal that it ter-
minates at both ends either in oral (Fig. 17) or aboral poles,
and yet continues to live. On the other hand, I have found
animals in which all attempts at the transformation of organs
have thus far been unsuccessful. To this group belong
Cerianthus uiembraiiaceus and many other Actinians. I
succeeded, however, in bringing about a permanent change
of form in one of these animals (Cerianthus menibranaceus),
in which I was able to cause the growth of any number
(within certain limits) of mouths, one above the other, in one
and the same animal.

The regeneration of lost organs in animals has often been
made the subject of study, usually, however, only to see
which organs can be regenerated, and further to study more
closely the anatomical or histological details of the process
of regeneration. But it has rarely been considered that
these phenomena can give us an insight into the conditions
that control the morphogenesis of animals. Where this has
been done, it has almost always been with the intention of

/

showing that under all conditions only one and the same
organ grows from any definite point on the animal.

Allinan 1 was perhaps the first to define this sharply as a
law of the formation of organs. From the well-known
experiments of Trembley, 2 Dalyell, 3 and from his own obser-
vations, he formulated the theory of the "polarity" of the
animal body. Allman cut pieces from the stem of Tubu-
larians and marked the end which had been directed toward
the head of the animal. Even though this cut end was mor-
phologically entirely similar to the other, yet a head was
formed only at this oral end, while no head was formed at

1 GEORGE J. ALLMAX, Report of the British Association for the Advancement
of Science, 1864.

2 A. TREMBLET, Mfmoires pour serrir a 1'histoire d'lm genre de polypes d'eau
douce a bras en forme de cones (Leide, 1744).

3 J. G. DALIELL, Rare and Remarkable Animals of Scotland (London, 1&47).

H ETEROMORPHOSTS 117

the opposite end. That Allman chose the name "polarity'
for this behavior suggests the possibility that he may have
thought of the analogy of this fact to the behavior of a niag-
ni't ; for a fragment of a broken magnet always has a north
pole at that end which in the original magnet was directed
toward the North Pole. If however, the book of Dalyell is
subjected to a close scrutiny, it is found that this author
occasionally (at two or three places in the book) mentions
observations which do not harmonize with the theory of
polarity. In these cases, however, Dalyell believed that he
was dealing with accidental monstrosities which this careful
and patient observer did not consider of sufficient impor-
tance to follow out experimentally, or to take into considera-
tion for a theory of organization.

W. Marshall 1 builds on Allrnan's theory of polarity in
his experiments upon Hydra. When Hydra vulgaris is cut
into pieces, "one is struck most forcibly with the extraordi-
nary polarity of the animal, in consequence of which new
tentacles and a new mouth are always formed at the oral
edge of the cut piece" (p. 698).

A further expression of this idea is found in Nussbaum's
papers on "The Divisibility of Living Matter." Nussbauin
found that when a piece is cut from an Infusorian, new cilia
develop from the edge of the wound in the same number
and in the same position that they occupied before the
injury. He goes even farther than Allinan and concludes that

Every minute particle of living protoplasm is oriented; otherwise
we could not understand the regular appearance of new cilia at
definite points when the infusorian has been divided. Just as we
e;m distinguish in an infusorian between the anterior and the pos-
terior, right and left, and dorsal and ventral surfaces, so each
minute particle of protoplasm must likewise be oriented according
to the three axes in space.

1W. MARSHALL, Zeitsrhrift filr wissenschnftlichc Znl<rii<; V.>1. XKXVIF (1882).
2 M. NubSBAUii, Archivfiir mikroKOpische Anatomie, Vols. XXVI and XXIX.

118 STUDIES IN GENERAL PHYSIOLOGY

Almost all the numerous other authors who have worked
upon the regeneration of organs in animals also regard it as
self-evident that the regenerated organ must be identical
with the lost organ in form and function. The facts which
I shall bring forward in the following pages will show, how-
ever, that this theory is certainly too narrow. For I suc-
ceeded in doing away with "polarity" first of all in that
very animal upon which Allman based his theory of
"polarity" -namely, in Tubularia.

One of the first authors who concerned himself with
the study of the phenomena of regeneration, Charles Bonnet,
looked upon them in a less biased way than did Allman.
Bonnet, to whom Trembley had very early communicated
the fact of the phenomenal regenerating power in Hydra,
attempted to convince himself of the truth of Trembley's
statements; since, however, he was unable to obtain Hydra,
he tried whether similar results could not be obtained upon
worms. Bonnet used two species of worms in his experi-
ments. In the first species, which he designates as vers
roiKjedtrcs, he found the conditions which are typical for
Hydra, and which correspond to the theory of '"polarity."
If the head of such a worm was cut off, a new head was
formed at the cut end; when the tail was cut off, a new tail
was formed at the point of section. If the head and tail
were both cut off, a head was formed at the oral end, and a
tail at the aboral end. In a second species, the vcrs
blanchdtres, the results were not so regular. When only
the head or tail was cut off, the lost part was always
regenerated. If, however, a piece was cut out of the middle
of the worm, it happened that such a piece formed a tail at the
oral end, instead of a head. Bonnet observed this three times. 1

I have found no reference in the literature which would
indicate that these observations of Bonnet have ever been

i CH. BOXNET, CEuvres d'histoire naturelle et de philosophic (Neuchatel, 1779),
Vol. I (Trait6 d'insectologie).

HETEROMORPHOSIS IK)

touted and confirmed. I am not in a position to state
whether they are correct or not.

The theory given by Bonnet is in some points similar to
a theory brought forward by Diihamel in his Phij^ujnc, ties
<irl>rcs, and to which Sachs goes back in his papers on
"Stoff und Form der Pflanzenorgane."

Bonnet believes that just as there are specific germs for
the development of the entire animal, there are also special
germs for the development of the various organs ; he assumes
the existence of certain head germs and certain tail germs.
In order, however, that these germs may develop, they must
be particularly well nourished. Their nutrition is accom-
plished, as in plants (according to Duhamel), by various
kinds of saps, one of which serves for the nutrition of the
head, while the other nourishes the tail. The latter flows
from head to tail, the former in the reverse direction. If,
now, the head is cut off, the saps which heretofore served to
nourish the head, can now be utilized for the nutrition of
the head germs, and the latter begin to grow out at the cut
oral end into a new head. In a similar way the tail germs
may begin to grow when the tail is cut off. It is assumed
that the tail germs and the head germs are distributed
evenly throughout the body of the vers romjcdfrcs; for this
reason a head must always grow from the oral end of a
fragment cut from any portion of the animal, while the
aboral end must always give rise to a tail. Upon the other
hand, in the vers blanchdtres the head germs are found
only in the neighborhood of the head, while the tail germs
are distributed through the entire body. For this reason
the worm regenerates a new head when the head is cut off,
while a new tail is formed at either end when a piece is cut
out of the middle of the worm.'

iAr/jcil(iL ilex liiitunisrhen Institu/s in \VUrzburg, herausgegeben von SACHS,
Vol. II U^ s -', PI-- I.VJ;imlJ89.

2f'H. pjONM.r. I'liiisiili'-nition sur ICK cor/is organist's. Art. L'.V.t IT.; tfcurrcs (Neu-
chftti-l, ITT'.i .., Vol. VI, i.p. 48 ff.

120 STUDIES IN GENERAL PHYSIOLOGY

I shall not discuss the importance of the theory of Bonnet.
I only mention it here because it takes into consideration
the fact that sometimes a tail may be formed instead of a
head, which is not done in Allman's theory of polarity. I
shall avoid all theoretical discussions in this paper, and con
fine myself to the task of showing whether and how it is
possible to cause with certainty in an animal the growth of an
aboral pole in the place of an oral one, and vice versa, at will.

For the formation of an organ which in form and function
is different from that which has been lost I shall use the
term hcleromorphosis. By the term reyenrrafion I under-
stand the replacement of a lost organ by one which is
identical with that which has been lost.

II. HETEROMORPHOSIS IN TUBULARIA MESEMBRYANTHEMUM

A layman would be in doubt as to whether he should call
a specimen of Tubularia mesembryanthemtim a plant or an
animal. From a much-branched system of roots (or stolons),
which are attached to a solid substratum, arise numerous
delicate unbranched stems, several centimeters high, which
end in polyps that are usually red and look very much like
flowers. These polyps take up and digest the food for the
animal. The animals belong to the class of Hydroids and
are found in great numbers in the Bay of Naples.

The zoologists have developed a very complicated ter-
minology for the individual organs of the Hydroids, which
may be very useful in purely descriptive morphology, but
does not take into consideration the forms of irritability of
the various organs. Causal morphology, which attempts to
discover the circumstances that determine form, has to con-
sider first of all the irritabilities of the individual organs.
For the purposes of the physiologist it is therefore necessary
to take these into account in describing and naming the
various organs.

HETEROMORPHOSTS 121

I distinguish in Tutmlnria, according to the dillVn-nces
in irritability, between the stems and tlie root. By the root
is understood that part of the Tuhularian which is endowed
with a special contact-irritability (stereotropism), by virtue
of which it attaches itself to solid bodies and keeps the
animal in a fixed position. By the stem is understood that
part of the animal which bears the sexual elements and the
polyps, and which is endowed with the opposite irritability,
in consequence of which it grows away from the substratum
to which the animal is attached. This simple terminology,
which is based upon the irritability of the organs, will
suffice for our purposes. Of the entire animal only the
polyps can move spontaneously; the stem is immovable. If
we cut a piece out of a stem, we must discriminate between
its oral and aboral ends, according to the orientation of the
piece in the original uninjured animal. The oral end is
that which was originally directed toward the polyps, the
aboral end, that which was directed toward the root. I
shall now describe the main experiments individually.

1. I cut off the roots and polyps of a series of stems,
and put these mutilated stems with their aboral ends ver-
tically into the sand sufficiently deep to keep them in a ver-
tical position. At the free oral ends, which were surrounded
on all sides by sea- water, new polyps were formed in a short
time at the proper temperature and with favorable speci-
mens within two days. These corresponded in form with
the old polyps. No growth took place at the ends which
were buried in the sand, no matter how long the observations
were carried on (in some instances for several months).

When I put stems with their oral ends in the sand, a
j>"/'fp Wdx formed tit the free, <ihor<il fiolc. In favorable
cases this was formed in a few days. Neither a polyp nor a
root was formed at the oral end, which had been covered by
sand, no matter how long I continued my observations.

122

STUDIES IN GENERAL PHYSIOLOGY

Contrary io the theory of the "polarity" of the animal
body, therefore, fragments of Tnhnlaria mesembryanthemum
are able to form polyps eren at their aboral endn.

2. I supported pieces cut from the stem of Tubularia
niesembryantheuiuui in such a way that both cut ends were

surrounded by water. To do this I sup-
ported them in the meshes of a long wire
net, or in the holes of a metal plate set up
in the aquarium for this purpose. Polyps
were formed at both the oral and the aboral
ends of tJie frat/nients, HO that the stem
terminated in a head at each end. Fig.
K) represents such an animal sketched from
life and enlarged twice. fib is the piece
removed from the old Tubularian. Polyps
were formed at both ends, and the stem
then grew in length from both ends, ac
and bd are the new pieces that grew after
the formation of the polyps.

I have in this way been able to produce
at any time any number of animals which
terminate in an oral pole at each of the two
ends of their bodv. I shall hereafter

/

designate animals which terminate in a head

at each end bioral animals.
I would particularly emphasize the fact that such an
animal remains bioral for the rest of its life. It is a
well-known fact that in a normal animal the oral polyp is
lost spontaneously after some time, and that a new one is
formed sooner or later in its place. In the case of the bioral
animals a constant blooming, shedding, and reappearance of
the polyps occurs, not only at the oral end, but also at the
aboral end. during the entire duration of their life.

3. I was able, therefore, not only to cause the develop-

FIG. 16

HETEROMORPHOSIS

ment of a head at both ends of the fragment of a stem, but
also to prevent the formation of a head at will, by simply
putting this pole into the sand. When both poles are put in
the sand, no head is formed at either end. If one of the
poles which has been in the sand for some time, and on
which the formation of a head has been prevented in this
way, is pulled out of the sand so that it is again surrounded
on all sides by water, a head may form at this end. If the
animal is covered only by an exceedingly thin layer of sand,
a polyp will still be formed which makes its appearance
between the grains of sand, much as the stem of a ger-
minating seed may grow through a thin layer of earth.

One of the poles of a piece of a stem was pushed between
two slides laid upon each other and held together by thin
rubber bands. Needles were placed between the two slides,
and one end of the stem of the Tubularian was laid in the
wedge-shaped space thus formed. In this way the end was
subjected to slight pressure. No polyp was formed at the
end subjected to this slight pressure, no matter how long I
waited; while at the other end, which was not pressed upon
and was surrounded by water, a polyp was formed in the
usual time. When the piece was removed from between the
slides, a new polyp frequently developed at the end that had
been subjected to the pressure. That light is not necessary
to the formation of a polyp was proved by the fact that
pieces of Tubularian stem will grow new polyps in a dark-
ened vessel. The experiments described were made in well-
aerated aquaria.

4. When the polyps and the roots are cut off from long
stems of Tubularia mesembryanthemuui, it is found Ihat the
new polyps are always formed one, two, or three days earlier
at the oral than at the aboral end. I believe that the cause
of this phenomenon, which may be considered MS an intima-
tion of "polarity," lies in the fact that when long pieces are

124 STUDIES IN GENERAL PHYSIOLOGY

cut from the stem, the lumen at the cut oral end is usually
wider than that at the aboral end; for when I cut from the
middle of the stem shorter pieces, which showed no differ-
ence in the diameter of the luinina, a polyp often formed
earlier at the aboral end than at the oral.

5. The size of the newly formed polyp also depends to a
certain extent upon the diameter of the stem at the cut end.
When the diameter was very small, the polyp was also very
small; when the diameter was large, the polyp was also
larger.

6. It might still be imagined that, besides the mechani-
cal factors thus far considered, a physiological factor might
also play a role. It might be thought that the substance of
which the polyp is formed is present in a larger amount at
the oral than at the aboral pole. To test this point I chose
a large number of very long Tubulariau stems that had been
cut off close to the roots, and at the cut ends of which polyps
had been grown. I bisected these stems transversely, and
kept the oral and aboral halves in separate beakers. If the
substance required for the formation of the polyps were
unequally distributed in the stem, then the one series of
fragments should have formed polyps sooner than the other
series. This was never the case; but as was again noted
erery fragment formed a polyp sooner at its oral than at
its aboral end, even though the difference in time often
amounted to only one half -day or less.

7. While I hare always succeeded with suitable mate-
rial, and with the experiment under the proper external con-
ditions in making a head grow at the aboral end of the
stem, I have thus far not yet succeeded in making a root
grow at the oral end of a stem. When I cut off the stems
close to the substratum to which the roots were attached
and brought the aboral ends in contact with tJte walls of the
aquarium, the end, when it grew at all, attached itself to the

HETEROMORPHOSIS 125

solid l>ody and became a root; however, irlicn contact iril/i
flic trail of flic (KI mi riii in irax broken so that water sur-
rounded the root on all sides, a polyp was formed also at the
end of the root, In my further experiments I shall try to
tii id conditions under which the animal will form roots at
both poles with just as great certainty as it now forms heads.
From the experiments thus far discussed, I can only con-
elude that the formation of polyps in Tubularia mesembrv-
anthemum can be brought about much more easily than the
formation of roots.

III. THE LIFE-PHENOMENA OF THE ORAL POLE OF TUBU-
LARIA MESEMBRYANTHEMUM

Doubt might arise as to whether the two heads of a bioral
Tubularian manifest the same life-phenomena ; as to whether
the two morphologically equal poles are also identical physio-
logically. I shall show that this is, indeed, the case, and in
doing so shall dwell a little more upon the differences in the
irritability of stem and root.

1. The stem and root of Tubularia mesembryanthemum
have an entirely different contact-irritability. If the root is
brought in contact with a solid body, it attaches itself to it,
and in its further growth remains closely attached to the sur-
face of the solid. If an attempt is made to lift the stem
from the solid body, it tears off close to the root, the latter
remaining attached to the base upon which it grew. The
polyp has exactly the opposite irritability. When the polyp
c:>rnes in contact with a solid body for example, when the
stem lies horizontally upon the bottom of the aquarium it
soon grows away from it. The yrowiny region of the stem
(which is situated close behind the polyp) becomes convex
against the solid substratum.

o

This (stereotropic) bending occurs OH/// in flic <jroiriiuj
jut ii of the stem, and persists when growth has ceased, just

126 STUDIES IN GENERAL PHYSIOLOGY

as do geotropic or heliotropic curvatures in many growing
plants. To bring about this stereotropic curvature it is
necessary that the / ><>!///> itxelf should come in contact with
the solid body. If any part of the stem alone comes in con-
tact with the solid, 110 bending occurs, even though the

o o

growing part of the stem, close to the polyp, touches the
solid. The contact-irritability of the polyp is opposite in
kind to that of the stem ; the stem is positively, the polyp is
negatively, stereotropic.

The negative stereotropism of the polyp may be clearly
demonstrated iu the following simple manner: Beheaded
Tubularians \vrre fixed in a beaker half-filled with sand in
such a way that one end was fixed in the sand, while the
other end just touched the side of the vessel. As soon as
tin- new polyps were formed and the Hydroids began to
grow in length, the tips of all the stems bent away from the
glass sides of the vessel. The direction of the rays of light
had no effect upon this process.

In all these e.i-()erhnents the polyps formed at tJie aboral
end helm ml < -.racily like- those formed <it lite oral end.

2. I have not succeeded in bringing about either helio-
tropic or geotropic curvatures in Tubularia mesembryanthe-
iimrn. When I fastened the stem in the middle, and when
both ends were surrounded by sea-water on all sides, the
stem of the bioral Tubularian continued to grow in the
direction of the old piece; it mattered not whether it lay in
a vertical or in a horizontal position, or in which direction
the light struck it. This is a remarkable fact, for, in looking
at a colony of Tubularians, one might easily be led to
think that they possess heliotropic or geotropic irritability,
as the stems of such a colony upon the surface of a solid are
all arranged in the same way. Yet the similarity in the
orientation might be determined in the main by their
contact-irritability. The oral ends of the young stems

HETEROMORPHOSIS 1-7

grow almost perpendicularly away from their substratum.
If, in addition, the separate stems stand very close together,
as is usually the case, the contact of the polyps with each
other influences their orientation. This has the same effect
as would be brought about by causing each separate polyp
to grow in a narrow hollow cylinder. The individual stems
must thus not only grow away from the surfaces to which
they are attached, but they must grow away from it in
approximately straight lines.

3. Daly ell has observed in Tubularia indivisa a form
very similar to Tubularia mesembryanthemum that the
polyps drop off after they have existed a certain length of
time, and that after a longer or shorter period new polyps
are formed in their places. As soon as a new polyp has
been formed, the stem begins to grow in length immediately
under it. The growth continues as long as the polyp exists ;
as soon as it drops off, growth ceases. 1

I observed the same condition of growth in Tubularia
mesembryanthemum. The longitudinal growth of the stem
was continued to a region just beneath the polyp, and it
continued as long as the polyp existed; when the latter
dropped off, growth ceased; when a new polyp was formed,
the stem again grew in length. In the bioral polyps an
increase in length occurred simultaneously at both ends of
the stem, so that these stems reached a much greater length
in a shorter time than any of the normal specimens that
were ever brought to me by the collectors of the Zoological
Station in Naples. That the stem grows in length close
behind the polyps at both ends of the bioral animal is
clearly shown by the fact that the newly formed part is thin
and transparent, and thus can be readily distinguished from
the older opaque portions of the stem. Then-fore in /7s
<in>irtli a/so I lie dboral pole of Tubularia or/m/vs like the oral.

i ISiiri ini'l l:<-iiiiirlnlili- Aiiiniiilx i if No )//</// (London, 1M7'.

128 STUDIES IN GENERAL PHYSIOLOGY

4. I did not succeed in observing the polyps in the
process of taking up food. Yet I have noticed in both the
oral and the aboral polyps the same sudden closure of the
tentacles which occurs in Actinians when they seize their
food and swallow it. I shall show later in Actinians that
heads which have been newly formed in abnormal places
behave like normal heads in the matter of taking up food.

IV. THE DISCREPANCY BETWEEN ALLMAN'S THEORY OF PO-
LARITY AND THE BEHAVIOR OF TUBULARIA MESEM-
BRYANTHEMUM

1. I mentioned in the introduction that Allman based
his theory of polarity on observations made upon Tubularia.
The discrepancy between Allman's ideas and my observa-
tions compels me to enter into a more detailed discussion of
his theory.

The passage in Allman's treatise which is of interest to
us is the following:

There is thus manifested in the formative force of the Tubu-
laria stem a well-marked polarity, which is rendered very apparent
if a segment be cut out from the center of the stem. In this case,
no matter in what position the segment may be, that end of it
which was directed downward or proximally, while it formed a
part of the unmutilated hydroid, will never develop a polypite, but
will extend itself as a simple prolongation of the cosnosarc; while
the upper or distal end, instead of becoming simply elongated, will
shape itself into a true polypite; and all this is true, though of
course not the least difference in structure or form can be detected
between the two extremities at the time of section. 1

Allman adds in a note that the observations of Daly ell,
who made numerous regeneration experiments upon Tubu-
laria indivisa, are in perfect accord with his own. By
reading Dalyell's paper one, indeed, finds the same idea
expressed as by Allman, although the term "polarity' 1 is
not used. It might be thought that Tubularia iudivisa, upon

lLoc. cit., pp. 392 ff.

HETEROMORPHOSIS 1_ ( .)

which Allman and Dalyell experimented, behaves typically
differently from Tubularia uieseuibryantheimim, upon which
T made my experiments. A certain difference seems, in-
deed, to exist. Dalyell states that the stem of the Tubularia
indivisa bends upward, when laid horizontally; this I have
not observed in Tubularia mesembryanthemuni.

Vet I do not believe that the conditions determining the
form of Tubularia indivisa differ in principle from those in
Tubularia mesembryanthemum. For Dalyell notes that he
once observed the growth of polyps from huili ends of a
piece cut from the middle of a Tubularia indivisa. ''It
may be conjectured that the summit of both had originally
constituted a single embryo, which by partition developed
into two, becoming progressively symmetrical in maturity."

To explain the formation of a head at both ends of the
stem in the single case just described, Dalyell therefore
assumes that the new head divided in the course of its
development. Such a division would, in consequence, (/lira//*
have to occur in the case of Tubularia mesembryanthemum,
which without <'.n-<'i>/ion forms a head at both ends, if both
ends are surrounded by water and have a sufficiently great
diameter, and a dividing embryo would therefore have to
exist in every piece of the stem of Tubularia mesem-
bryanthemum. Even if one were willing to consider this
hypothesis, it yet could not be made to harmonize with
Alluiaivs theory of polarity; for, according to this theory,
buih embryos would necessarily have to develop always
at the same end, namely, at the oral one ; yet I have never
found two heads to develop here side by side.

2. I might mention that it is possible apparently to ob-
tain such results in Tubularia mesembryanthemum as All-
man describes, if the stems used in the experiments are cut
off close to the root, and if care is taken, in choosing the

1 Loc. cit.. p. I'M.

130 STUDIES IN GENERAL PHYSIOLOGY

specimens, to select only those which are very thin at the
base. Such a selection might easily be made accidentally in
an experiment. In this case one might notice that polyps
arise only at the oral end, especially if the experiments are
not continued for a very lung time. Just as Allman regards
such a behavior a the expression of polarity in the animal
body r some botanists speak in analogous cases of "mor-
phological forces." I believe that the "morphological
force" which decides that a polyp forms first at the oral
end of a Tubularian segment is essentially nothing more
than that the diameter of the tube is very small at the
aboral end of the stern. Yet I prefer not to enter into
a discussion of such hypothetical things in this paper.

V. HETEROMORPHOSIS IN AGLAOPHENIA PLUMA

While in Tubularia we dealt with but a single stem which
under ordinary conditions ends in a root at one end and in a
polyp at the other, we have to deal in what follows with
colonies of animals. The place of the head is here taken
by a more or less ramified stem possessing many polyps.
At the other end is formed a root (as in Tubularia). We
shall confine ourselves to experiments upon the stems.
We shall call the end directed toward the root the aboral
or basal end of the animal; the other, free end, the oral
or apical end. I wished to determine whether it was pos-
sible to make a new tip grow in place of the root at the
basal end of the stem, or vice versa, and how we might
accomplish this.

1. Aglaophenia pluma (see Figs. 17-19) consists of a
main stern from which lateral branches are given off on both
sides. These side branches carry polyps upon their upper
surfaces ; they are slightly convex toward the tip of the main
stern and arise from it at an acute angle, which opens toward
the tip of the main stem. The side branches are the shorter

HETEROMORPHOSIS

131

the nearer the tip they are. These points enable one to dis-
tinguish between the basal end (originally directed toward
the root) and the apical end (originally directed toward the
tip) of a stem from which the tip and root have been cut.
2. I cut off some stems of Aglaophenia pluma close to the

FIG. 17

FIG. 19

root, and fixed them vertically, but with their tips down-
ward, into the sand. The tips were planted just deeply
enough to keep the animals in a vertical position. The
remaining part of the stem was surrounded by water. In a
of these, (iii/n/dls new ti]>x, irliiclt roidinncd lo grow
, were formed at 1//c Ixixtd end* (Figs. 18, 19). At
first the old main stem grew in length by growing vertically
upward. From this there thru arose the lateral branches.
The new polyps which were formed grew only upon the

132 STUDIES IN GENERAL PHYSIOLOGY

tipper surfaces of the lateral branches, and were therefore
directed, not toward the old, but toward the new tip of the
animal. Furthermore, the acute angle at which the new
lateral branches arose from the main stem opened toward the
zenith ; the convexity of the new branches was also directed
toward the zenith. In this way animals were therefore
formed irhiclt ended in a tip at both cuds animals that were
biapical ; just as though one were to grow a new top upon a
tree in place of the roots, without, however, allowing the old
apex to go to pieces. In the specimens illustrated in this
paper the tips are still relatively small. My stay at Naples
was too short to allow me to wait for them to reach maturity.

3. When stems of Aglaophenia which had been cut off
close to the roots, the tips of which, however, were left intact,
were suspended vertically and in an upright position in
water, a new root was invariably formed at the basal end,
and never a new tip.

It therefore seems that the position of the stem of Aglao-
plienia determines to a certain e,rtent whether a heteromor-
_/>Ao.s7s, or only <i regeneration, of the lost part occurs at the
basal cut end.

4. This fact is further supported by the following obser-
vation : When stems of Aglaophenia from which the tips and
roots had been cut were suspended vertically in the aquarium
so that both cut ends were surrounded by water, a root was
always formed upon the end directed downward, it mattered
not whether it was the basal or apical end.

In many cases branches were formed at the end directed
upward, yet in other cases a root was formed here also. A
root was formed most frequently upon the ends directed
upward when the basal end of the stem was pointed in that
direction; branches were formed most frequently when the
apical end of the stem was directed upward.

It is therefore possible to create bi basal Aglaopheniae,

HETEROMORPHOSIS

and, according to the rx|M'riimMits thus far made, the method
by which this is accomplished consists in cutting off the tips
and roots of the stems, and suspending the stems vertically
in the aquarium with their bases pointing upward.

5. When such segments from which the tips and the roots
have been cut off are laid horizontally, and in such a way
that they are surrounded by water on all sides, roots grow
only from the aboral ends of the fragments, while, as a rule,
new tips grow from the oral ends. Exceptionally, however,
roots grow from the oral ends also.

6. Bearing in mind the fact that roots may arise from
either cut end and under all conditions, we may say that
biapical animals may be produced by leaving the tips intact
and cutting off the stems close to the root. If such stems

O

are suspended vertically, with their tips downward, new tips
may arise at the aboral end. If bibasal animals are desired,
stems deprived of their tips are cut off close to the root, and
are suspended vertically in the aquarium, with their tips
downward. In all my experiments thus far performed only
roofs have been formal at the cut ends directed downward,
irlu'le tips or roots Jiare I >cen formed at the cut ends directed
tijnrard. Besides the influence which the position of the
stem has upon the formation of organs, another at present
unknown, and therefore uncontrollable, factor exists which
renders possible the growth of a root at the cut end which is
directed upward. Yet I believe it possible that purely
e.Hei'nal conditions (which were satisfied in the aquarium,
and which possibly some day may be brought under con-
trol) determine this strong tendency toward the formation of
roots.

It still remains to be investigated whether gravity or light
or both circumstances have an influence upon the formation
of the organs in this case. In all my experiments performed
thus far in the dark room, no regeneration whatsoever of the

134 STUDIES IN GENERAL PHYSIOLOGY

lost organs occurred a fact probably not entirely due to
lack of light.

7. The roots were characterized by a distinct kind of
contact-irritability and by a tendency to bend downward,
which I shall now discuss.

When a root was formed at the cut end of a vertical stem,
it at first grew horizontally for a short distance when it
did not come in contact with solid bodies and then down-
ward (Fig. 19, lOj). In stems lying horizontally the root
grew directly downward. In animals thus operated upon,
adventitious roots were also often formed at the middle of the
stem. I have never found these adventitious roots upon the
uninjured animals taken from the ocean. They grew directly
downward toward the earth (Fig. 19, w 2 ). The phenomenon
seemed strangest of all when such adventitious roots arose
from a stem fixed in the sand in an inverted position (with
the tip down) ; in this case the root grew toward the apical
end of the animal. At times these downward-growing roots
showed torsions such as are found in winding plants.

8. The newly formed main stems behave in a way oppo-
site to that of the roots ; they grow vertically upward. This
contrast between the root and main stem is shown most beau-
tifully when new stems with polyps arise from the newly
formed root itself. In Fig. 19 is shown a branch which,
after having been deprived of its tip, was fixed vertically in
the sand with its tip directed upward. In place of the tip
a new root w^ grew from the main stem, at first horizontally
and then downward. A young branch s arises from the
root u\ and grows vertically upward.

In another stem all the lateral branches had gone to
pieces; it had been suspended vertically. I believed that
the animal had died, when from the middle of the stem
branches began to arise, which proved to be both roots and
polyps ; the roots sprang from the lower portions of the stem,

HETEROMORPHOSIS 135

the new stem from the upper portions. The new stems grew
upward, the roots downward. I have seen such new stems
arise, not only from the main stem and the main roots, but
also from the adventitious roots. Here also the new stems
always grew upward. Finally, I have seen new stems, which
also grew upward, arise from stems lying horizontally.
When, however, I cut off the tip from stems lying hori-
zontally, and regeneration occurred without heteromorphosis
or deformity of any kind, the new tip showed, so far as
my present experience goes, no tendency to bend upward.

9. All newly formed stems arose from the upper surface of
the stem or root (see Fig. 19, s), it mattered not whether they
grew upon the main stem or upon the accessory roots. The
accessory roots sprang from the lower surface of the stems
when these lay horizontally. Whether all these phenomena
are determined solely by gravity I shall attempt to decide
by further experiment.

10. That form of contact-irritability which I have called
stereotropism plays an important role in the growth of the
root of Aglaophenia. When the roots come in contact with
a solid body, they attach themselves to it (by means of a
secretion?) and grow along its surface. This attachment is
a phenomenon of irritability which is called forth by contact
with the solid body itself; for when the root is brought in
contact with a solid body, it does not immediately stick to it,
but only after contact has lasted for some time (often as long
as twenty-four hours). Only the growing part (tip) of the
root is able to fasten itself to the surface of a slide. The
root adheres so firmly to the solid body that it is impossible
to separate the two by traction ; the root tears before it can
be pulled from the solid body. I have not as yet observed
the branches of Aglaophenia haul <t/r<i// from a solid body.
Yet I have proved with certainty that growing branches car-
rying polyps never attach themselves to a solid body with

136 STUDIES IN GENERAL PHYSIOLOGY

which they are brought in contact, no matter how long one
may wait; while, on the other hand, this reaction is always
obtained with certainty in growing roots. I have also
assured myself of the fact that when (idreiititious roots are
brought in contact with the wall of a beaker, they imme-
diately attach themselves to it, and grow on its surface.

11. As soon as a root comes in contact with a solid body,
it grows rajiidl// in length, and in a few days exceeds the
length of other newly formed and equally long roots, which
are prevented from contact with solid bodies. 1

The attachment of the root to a solid body influences form
in a second direction. While we observed above that new
stems grow from the upper surface of a root when it is sur-
rounded on all sides by water, we find, when the root is
attached to a solid substratum, that the stems arise from that
surface of the root which lies opposite the solid body. In
their further development these stems also grow only straight
upward, though not entirely vertically.

1 I have observed in the roots of Aglaophenia a phe-
nomenon relative to growth which has thus far been known
only in plants.

The longitudinal groicth of the root is confined to a nar-
row region situated near tlte tip (while no longitudinal
growth occurs in the remaining portions of the root). This
could be shown in the following way: I permitted the roots
of Aglaophenia to attach themselves to a slide and grow
upon it. A slight bulging out soon occurred just behind the
tips the beginning of a new stem, which on the next day
reached a length of imm. and soon thereafter bore polyps.
I marked the position of the beginning stern on the glass,
by etching a line into the glass. The position of this new

1 DALTELL observed in Sertularia halecina that new growths occur which
adhere to other solid bodies and thereby become abnormally long. "These com-
ing iu contact with a solid surface have a tendency to adhere and to extend in
irregular prolongations surpassing the natural increment," Rare and Remarkable
Animals of Scotland (London, 1847), p. 163.

HETEROMORPHOSIS

187

stem upon the glass remained permanently the same,
though the tip itself moved forward at the rate of about
lium. each day. The longitudinal growth must, therefore,
have occurred in the narrowzone lying in front of flic iicir slcni.
13. These experiments were made in April when the

Aglaophenise were sex-
ually mature. One day I
observed a number of small
(about ^mm. long), whitish,
cone-shaped larvae that moved
over the bottom of the
aquarium toward the window,
and remained there. The
next morning, however, they
had all disappeared, so that
I can only suspect that these
organisms, which in the
moment of observation were
positively heliotropic, may
have been larvae of Aglao-

O

phenia.

VI. HETEROMORPHOSIS IN
PLUMULAKIA PINNATA

1 . I have made a series of
FIG. 206 experiments, similar to those
made upon Aglaopheuia, upon Plumularia pinnata, which in
form closely resembles Aglaopheuia pluma. I wish to
describe one of these experiments here.

A series of stalks were cut off close to the root arid fixed
vertically in the sand, so that the apical ends were covered
by it. In individual instances, but only very rarely, a new
tip was immediately formed at the aboral end, so that I ob-
tained biapical animals similar to those of Aglaophenia

FIG. 20a

138 STUDIES IN GENERAL PHYSIOLOGY

pluma ; the new apex, however, was not so regularly formed.
Occasionally a lateral branch was missing upon the side of
the main stem. In the majority of the experiments, how-
ever, such figures as are shown in Figs. 20a and 206 were
formed. A new piece be at first grew vertically upward
from the main stem ah until the new stem ce was formed,
which usually grew vertically upward; the polyps of this new
stem ce were all located upon the upper surfaces of the lateral
branches, so that the new stem ce was oriented symmetri-
cally to the old stem ah with respect to a horizontal axis.
After this the main stem began to grow horizontally and
finally downward.

2. The newly formed parts he arising from the prolonga-
tion of the main stem, and growing at first horizontally and
then downward, all possessed the contact-irritability of
roots, namely, positive stereotropisni. When brought in
contact with solid bodies, they attached themselves to
their surfaces and behaved like the roots of Aglaophenia
pluina. Only the growing parts of the roots were able to
attach themselves in this way. Here also the influence of
contact stimuli in determining the point of origin of
branches again showed itself. While the branches arose,
almost without exception, from the upper surface of the root,
when it was surrounded by water, they were formed upon the
side opposite the solid substratum upon which the roots grew,
in roots which were attached to the surface of a solid body.

The experiments were made in an aquarium which was
far removed from a window, and into which only very weak
light fell almost horizontally. In spite of this, the branches
grew vertically. This seems to indicate that light has no
influence in this case in determining the place where new
organs are formed.

The protoplasm retracted from that portion of the branch
which was buried in the sand.

HETEROMORPHOSIS

VII. HETEROMORPHOSIS IN EUDENDRIUM (RACEMOSUM ?)

Eudendrium (racemosum?) (Figs. 21 and 21/>) consists
of a main stem which terminates in a polyp at its upper end
and in a root at its lower end. The root adheres to solid
bodies. Stout lateral branches arise from the stem and

grow upward. They
also carry polyps at
their tips. New branches
may again arise from
these, all of which are
directed toward the tip
of the main stem. I
cut off the tips and roots
from stems of Euden-
drium and suspended
them in part with the
tip, in part with the
base directed downward
in the aquarium. Both
ends were surrounded
by water. The stem
began to grow from the
two extremities, and
polyps u'cre formed at

FIG. 21o FIG. 216 fo^ cn d S (FigS. 21(1

and 216). AH Eud&ndria became biapical (just as </<><'*
Tiibiildria toesembryanthemum under similar conditions);
with this difference, however, that in addition to the new
tip, roots were at times formed, at one of the cut ends, which
was never the case in Tubularia.

To maintain the pieces of Eudendrium stems in a vertical
position in the aquarium, I pushed them through lead plates
in which fine holes had been punched. The plates lasted

140 STUDIES IN GENERAL PHYSIOLOGY

upon beakers. While the upper end of the animal was,
therefore, in the aquarium, in which the sea water was con-
tinually renewed, the lower ends of the stems dipped into
the beakers in which the circulation of the water was much
less perfect than in the rest of the aquarium. Striking
differences existed between the new growth which occurred
at the lower end and that at the opposite end. The lower
end in the poorly aerated water formed a new polyp upon
the main stem, but its growth was slow, and the formation
of new lateral branches occurred either not at all or only

/

slightly as compared with the corresponding processes at the
other end. (Possibly light and gravity may also have played
a role in bringing about this result.) In what follows we
shall consider only the new growths which occurred at the
upper end of the vertically standing stem.

2. When the basal end of the stem was directed upward,
and new side branches were formed, they were directed, not
toward the old tip, but toward the new tip. In Figs. 21
and 216 ab is the old stem, be the regenerated tip, and ad
the heterouiorphic tip at the aboral end. The newly formed
branches s are all directed toward the heteromorphic tip d.

In a larger number of cases new stems were formed also
upon the old lateral branches after the stem had been turned
upside down. Some of these did not grow downward toward
the old tip, but in the opposite direction, upward, toward
the new tip. Fig. 216 illustrates such an instance. After
the whole stem had been suspended in an inverted position
in the aquarium, a new branch s, was formed upon the
lateral branch e, and grew upward, toward the new tip. It
had, moreover, been formed upon the upper surface of the
branch e.

In the arrangement of new organs in Eudendrium we do
not, therefore, deal with a "polarity" which is determined
solely by internal structural relations, but with the effects of

HETEROMORPHORIR 141

stimuli in irhicli riot only the internal *lrncfnral con<li/i<>ns,
but tln'xc (uid lite c.i'terndl xfiuiitli fo</eflier, determine Hie
rex it It. The external stimuli with which we deal here seem
to be light and possibly gravity.

3. The effect of light upon the formation of new polyps
in Eudendrium is shown in an unmistakable way. When one
compares the number of new polyps and branches formed
upon the window side of the stem with those formed upon
the room side, one finds that the number \ipoii the window
side is very much the larger.

The branches are, moreover, positively heliotropic. I
cut off a Eudendrium at its base, close above the root; a
new polyp was formed upon the tip of the stump that
remained. The stem then began to grow rapidly. The
growing, apical portion of the stem bent toward the window
side of the aquarium. That part of the stem which was not
growing actively showed no heliotropic curvature.

4. I have made a single observation which seems to indi-
cate that currents in the water, if they are continued for
some time and always in the same direction, can pro-
duce curvatures in a growing Eudendrium stem. The anal
opening of a large Ascidian was situated near a growing
Eudendrium stem, so that the stream of water ejected by
the Ascidian struck the Eudendrium. The growing part of
the Eudendrium which was struck by the current of water
bent so as to have its concave side directed toward the
source of the current. The other stems of the same
culture which had been subjected to otherwise similar treat-
ment had all bent toward the source of light. This observa-
tion also shows that the rheotropism of these Eudendrium
stems if we are, indeed, dealing with this phenomenon-
is able to overcome and to veil their heliotropism.

5. In a few cases in which contact had been especially
close, roots were formed in the middle of the stein, where it

1412

STUDIES IN GENERAL PHYSIOLOGY

had been in contact with the lead plate. These roots attached
themselves to the lead plates and spread over their surfaces.
I have not yet succeeded in producing bibasal Eudendria.

VIII. HETEROMORPHOSIS IN SERTULARIA (POLYZONIAS ?)

Heteromorphosis can be produced in Sertularia; but just
as the heteromorphoses in no two of the animals thus far

FIG. '

FIG. 226

FIG. 22c

considered are exactly similar in every detail, heteromor-
phosis in Sertularia has its specific characteristics also. I
cut off stems of Sertularia close to the root and fixed them
in an inverted position in the sand. Both roots and stems
(Figs. 22 and 226) grew from the cut basal end. But
while the stems grew upward (and in the direction of the
rays of light), the roots grew downward (and away from the
source of light). 1 As shown in my earlier observations, the
roots of Sertularia are negatively, the branches positively,
heliotropic. Biapical stems can be easily produced in the
manner described. Not uncommonly the condition repre-
sented in Fig. 22c is found, in which a negatively helio-

1 See paper ii, p. 89.

HETEROMORPHOSIS 143

tropic root arises from the main stem, and from this (iifioii
1/ie $idc directed toirard tin', light) spring new positively
heliotropic stems. This arrangement corresponds with that
so often found in Plumularia pinnata.

The roots are positively stereotropic as in the other
Hydrozoa. When brought in contact with a solid body tlu-v
attach themselves to its surface. As soon as the roots have
attached themselves, the position of the new stems forming
upon the roots is determined by the contact stimulus; the
new stems arise from those points on the surface of the root
which are diametrically opposite the substratum to which the
root is attached.

The fact that roots and stems may arise simultaneously
and side by side from the basal end of an organ has also
been observed in certain organs of plants ; c . g., in fragments
of leaves which form both roots and stems at their bases.
The protoplasm retracted from that piece of the Sertularia
stem which was covered by sand.

2. When the tips were cut off of stems which were fixed
in the sand in a vertical and upright position (with the tip
upward), simple regeneration of the tip followed in the great
majority of cases. Only once or twice did I see a root arise
from the tip of a vertical and upright stem.

3. In inverted stems occasionally new stems arose from
the middle of the old stem upon the side directed toward the
source of light. These grew in a direction determined by
their positive heliotropisui ; when the light came from above,
they grew upward toward the basal cut end.

Roots which were formed in the middle of inverted stems
(Fig. 22o.) grew downward and toward the room side of the
aquarium, when the light fell upon them from above.

4. Driesch has observed a phenomenon of growth in
specimens of Sertularella polyzonias which were cultivated
under ''unfavorable" conditions that I have never found in

144

STUDIES IN GENERAL PHYSIOLOGY

my specimens of Sertulariae.
"In place of normal individuals,
stolons were formed." I have
observed these abnormalities,
however, in Gonothymea Lovenii,
which I givw in poorly aerated
aquaria.

HETEROMORPHOSIS IN GONO-
THYK.EA LOVENII

Stems of Gonothyraea Lovenii,
which were cut off close to the
root and fixed vertically in the
sand with the tip downward,
formed new tips at their upper
basal ends, so that biapical stems
resulted (Fig. 23).

The influence of contact
stimuli upon the longitudinal
growth of fhe roots was very
marked. Roots that were
brought in contact with
the surface of solid bodies
grew many times faster
than roots growing in the
middle of the aquarium
without any such contact.

FIG. 23

1H. DRIESCFI, "Heliotropismus bei Hydroidpolypen," Zoologische Jahrbiicher,
herausgegeben von SPENGEL, Vol. V, p. 150.

HETEROMORPHOSIS

145

When the roots were brought in touch with the surface
of the water, the latter acted upon the root as a solid body.
The root began to grow rapidly in length, attaching itself to
the surface of the water (as if it were the surface of a solid
body). Whenever a root adhered to a solid body new stems
arose from that surface of the root which lay diametrically
opposite to the solid body. Usually these branches then grew

FIG. 24

FIG. 25

perpendicularly away from the surface of the solid body.
When new stems arose from roots growing along the surface
of the water, the stems grew vertically downward.

X. OX THE FORMATION OF TENTACLES IN CERIANTHUS

MEMBRANACEUS

1. I shall now discuss some experiments upon animals
which seem to behave in accordance with Allman's theory of
polarity, inasmuch as in these I did not succeed in producing
a head in the place of an aboral pole. The experiments
led however to the production of several heads lying one
above the other in one and the same animal (see Figs. 24,
25). The irritability of the new heads could easily be com-
pared with that of the old. In these experiments, more-

146

STUDIES IN GENERAL PHYSIOLOGY

over, I had the opportunity of studying new relations be-
tween irritability and body form, and in addition one of the
fundamental conditions which underlie growth. These lat-
ter observations point to a greater similarity between the
general life-phenomena of animals and plants than has thus
far been known.

..-b

FIG. 26

The experiments were made upon Cerianthus niembrana-
ceus. The animal consists of a long, soft, and smooth
cylindrical body, carrying a heavy crown of tentacles at its
oral end (Fig. 26, ), while at its aboral end (Fig. 26, I,) it is
smooth and rounded.

The tentacles at the oral pole are arranged about the oral
plate in two concentric circles; the outer circle consists of
long, the inner of thin and short tentacles. In the middle of
the circle is situated the oral opening, which serves also the
functions of an anus. The body of the animal is hollow

HETEROMORPHORIS

147

and the reader who is unacquainted with zoology can best
picture the animal to himself by imagining a sack made up
of an elastic, contractile wall, the opening of which is sur-
rounded by tentacles.

The animals which I obtained in Naples were 5-10cm.
long. They are very common in the Mediterranean and are
especially adapted to physiological experi-
ments because they are very tough and
comparatively long-lived. The animal lies
buried in a mucous envelope in the sand. .It

V\

FIG. 27

FIG. 28

FIG. 29

only thrusts its head outside of the envelope to catch small
marine animals for its prey. The envelope is formed by a
secretion from the skin.

I made diagonal incisions (fich, Fig. 27) into the middle
of a large number of such Cerianthi. After a few days
new tentacles begin to spring from the cut surface /><-,
which grow rapidly and correspond in form, color, and mark-
ing with the tentacles at the oral pole. 1 hare i/crcr .sw//
crcn mi iiulicdh'oii of the fonndlion of iic/r fcii/arli's (tf
the oilier cut x/ir/'iK'r, <ic. Figs. 28 and 2' I ivpivsml such an
animal eight days after an incision had been made. Tentacles

148 STUDIES IN GENERAL PHYSIOLOGY

spring from the lower cut surface; the upper cut surface/
is free from tentacles. The cut surface ac (Fig. 27) suffers
changes which lead to a rounding off of this part, and make
it resemble a foot. I have made more than a hundred such
experiments, and yet have always obtained the same result.
In order to have new tentacles form it is necessary to pre-
vent the lips of the wound from healing together during the
first few days after the operation. I attained this end most
easily by laying the operated animals upon a wire screen;
the animals would push their aboral ends through the
meshes of the screen up to the incision. The wire then
pushed itself between the lips of the wound, and so prevented
the edges from healing together. First the outer row of
tentacles and an oral plate were formed; then an inner r<>\v
of tentacles originated; so that finally such an animal pos-
sessed two morphologically identical heads the one situated
above the other. Such animals are represented in Figs. 24
and 25 ; a is the old, b the new head. The new head in
Fig. 24 is about three months old; that in Fig. 25 is much
younger. By similar means I also succeeded in producing
animals with three heads, situated one above the other.
There was nothing to prevent the production of a still larger
number of heads lying one above the other, if there had
been any object in doing this. I noticed that the formation
of a new head and the growth of the new tentacles generally
occurred more quickly and were the more considerable the
nearer the incision lay to the oral pole. In animals with
three heads, that lying nearest the foot had the smallest
tentacles.

When the incision was made very near the aboral pole,
no new head whatsoever was formed. Fig. 25' shows an
animal into which I made two incisions at the same time
6 near the middle and c near the aboral end of the animal.
It will be seen that new tentacles have grown from the

HETEROMORPHORTS 149

incision !>, while none have grown from the incision c near
the foot cud, even 1 hough the lips of the wound wore pre-
vented from healing together. In the drawing which was
made from life, the cut is relatively far removed from the
foot end of the animal. This is because the aboral end <<!
lying behind the second cut is greatly stretched, while that
portion of the animal lying anterior to the incision is con-
tracted. It will also be seen that after such an incision both
parts become independent of each other to a certain extent ;
much as after transverse section of the spinal cord in one of
the higher animals the incision renders the two parts of the
animal comparatively independent.

3. I found no statement in the literature as to whether or
not such observations had already been made upon other
animals. It is, however, known that a new Hydra may spring
from the body of an old one, which increases in size, and
after a certain time separates from the mother to lead an
individual existence. As long as it remains attached to the
mother, the whole is to be regarded as an animal with two
heads situated one above the other; for the body cavities of
the young and the old animal communicate with each
other. Yet such a Hydra is essentially different from our
Cerianthus. While in Hydra not only a head, but a whole
body is formed, only the oral plate is formed in Cerianthus.
While the new animal becomes detached after some time in
Hydra, the new head in Cerianthus remains permanently
attached to the mother. Furthermore, while in Hydra the
newly formed individual has the same number of tentacles,
and the same cylindrical form as the mother, the number of
tentacles that grow in Cerianthus is dependent upon the size
of the incision. The smaller the incision, the smaller is the
number of tentacles that are formed. Only a segment of a
head, corresponding to the size of the incision, is therefore
formed when an incision is made into a Cerianthus. A

150

STUDIES IN GENERAL PHYSIOLOGY

study of the illustrations shows that the number of new
tentacles corresponds to the size of the incision, and increases
or decreases as the length of the incision increases or
decreases.

4. I cut rectangular pieces from the wall of Ceriantlms
by cutting off the head and the foot trans-
versely, and dividing the resulting hollow
cylinder by a longitudinal cut. These
rc<-f<iii</nl(ir ymvrs formal f<'ii/<ict<'x upon
only our of tltc four cut <W//rs. This
was upon that edge which was originally
directed toward the oral pole of the
animal. If <iltc<l, Fig. 30, represents
such a piece, the new tentacles sprang
only from the edge alt. The three
remaining sides remained
absolutely free from all
evidences of new tentacles.
Thus far this experi-
ment corresponds in behavior with that of an
analogous experiment upon Hydra. 1 But
while the piece removed from Hydra forms a
new cylinder, and a closed body-cavity before
the new tentacles sprout. 2 the tentacles upon
the pieces of Cerianthus are formed without a
new body-cavity originating, and even while
the entoderm is still exposed. The cut edges
that are free from tentacles may never heal
together, and a new body-cavity may never
be formed. In Fig. 31 is given a picture of a fragment of
Cerianthus bearing large tentacles while the body- cavity is
still open and the entoderm still exposed. The cut edges
show inversions and puckerings, to which I shall return

1 See ISCHIKATVA, Zeitschrift fur wissenchaftliche Zonloc/ie, Vol. XLIX, p. 441.
- NUSSBAUM, Archivfur mikroskopische Anatomic, Vol. XXIX.

FIG. 30

FIG. 31

FIG. 32

HETEROMORPHOSIS 151

later. Fisf. 32 shows the animal two months later. The cut

O

edavs have coalesced, and the animal resembles somewhat a

O

normal Cerianthiis. The inner row of small tentacles has
also been formed in addition to the large peripheral ones;
but and herein these phenomena again differ from those
observed in Hydra the number of tentacles is only a frac-
tion of the number of tentacles of the normal head, corre-
sponding to the size of the oral cut. Nor do the ends of
the two rows of tentacles meet to form a closed circle.

After what has been said, it need not be emphasized that
the formation of tentacles is independent of food-supply, as
the taking up of food is impossible without a body-cavity.
When the pieces are too small, no tentacles whatsoever may
be formed.

From all that has been said, the following observations
are easily understood. When a Cerianthus is cut completely
in two transversely, the aboral piece forms a new head,
bearing a normal number of tentacles, while at the lower end
of the oral piece, which has to regenerate afoot, new substance
is deposited upon the cut surface which restores the con-
tinuity of the ectoderm at this end and assumes the rounded
form of the foot.

If head and foot are both cut from a Cerianthus, the
middle piece forms new tentacles at the oral and a new foot
at the aboral cut surface (the latter is formed more quickly
than the former). But this is possible only as long as the
pieces exceed a minimal size.

5. I have tried to control the place where new tentacles
are formed by contact stimuli or by orienting the animal in
different \v;ivs against gravity and light. All of these experi-
ments have thus far been unsuccessful, if for no other rea-
son, because it was impossible to maintain the animal in any
abnormal position for any length of time. The following
chapter will give the reasons for this behavior.

152 STUDIES IN GENERAL PHYSIOLOGY

For the time being, therefore, the place where the ten-
tacles in Cerianthus are formed is determined by the following
law, which is only a somewhat modified expression of All-
maivs theory of polarity:

The filace irhere the tentacles arc formed on a frai/inenf
of Cerianthus is dependent }i/><t the orientation irhicti lite
frafjinent Inul in the uninjured animal. The tentacles al trans
(jroir ii/xni 1he cut surface irhieh in is directed toinird the
and i>ole of tl/e uninjured animal.

If for any reason, therefore, we might wish to know how
a fragment of Cerianthus had been oriented in the uninjured
animal, we should only have to wait until new tentacles were
formed ; the side upon which the tentacles sprouted would be
that which was directed toward the head.

XI. RELATIONS BETWEEN FORM AND IRRITABILITY IN

CERIANTHUS

As is well known, it is possible to determine from the
physical behavior of a fragment of a crystal how it was
oriented in the crystal. I have tried to determine whether
or not relations between body form and irritability can be
shown to exist in living animals comparable to those existing
between the geometrical form and the physical behavior of
crystals. Such a relation, indeed, exists in Cerianthus, and
this can be recognized, not only in the uninjured animal,
but also in the animal deprived of its head or foot. This is
true, under certain conditions, even in fragments of an ani-
mal. In this way it is sometimes possible to recognize from
the behavior of a fragment toward external conditions which
of its ends was originally directed toward the oral pole.

When the external conditions permit of it, Cerianthus
membranaceus assumes a position in which its long axis is
absolutely or nearly vertical, and in which its oral pole is
directed upward and its aboral pole downward. If the head

HETEROMORPHOSIK 153

or the foot of Cerianthus is amputated, or if a piece is cut
out of a Cerianthus, the fragment, if not too small, and if
external conditions permit of it, again assumes a vertical
position, with its oral end directed upward and its aboral
end directed downward. I shall now describe these phe-
nomena in greater detail.

1. If a Cerianthus is laid upon the bottom of a vessel
covered with sand, after a few minutes the foot of the ani-
mal begins to bend downward near its tip and to bore into
the sand. In half an hour or less (at the proper temperature
and with irritable animals) the entire animal has buried itself
vertically in the sand up to its head. It remains perma-
nently in this position, if other circumstances do not induce
it to move.

2. A wire net, the meshes of which are so narrow that
the body of a Cerianthus can only with difficulty be drawn
through them, is supported horizontally upon a glass vessel
and set into the aquarium. A Cerianthus is laid upon the
wire net. After a few minutes the foot of the animal begins
to turn downward, and to bore through one of the meshes of
the wire net. That portion of the foot which has passed
through the wire net assumes an absolutely or nearly ver-
tii-til position, and minn'ii* so permanently. No change
occurs at the oral pole, except that the tentacles close together
so that they look like a brush, the handle of which is formed
by the remaining portion of the animal. The animal crowds
its body more and more through the mesh in the net, until
it finally attains the vertical position shown in Fig. 26.

3. This orientation can also be reached within half an hour.
But while the Actiniaii generally remains in the sand after
having buried itself vertically in it, an animal upon the wire
screen rarely retains the orientation described longer than two
days; it either works itself entirely through the wire screen,
or else retracts its foot to bore it through another mesh

154

STUDIES IN GENERAL PHYSIOLOGY

in the screen, or to move off the screen entirely. If,
as soon as the animal has assumed the position shown in
Fig. 20, the wire screen is turned over so that the foot of the
animal is directed upward, the foot is not withdrawn, but
begins io bend rcrficaUy downward from the fi}>. The bend-
ing then passes from one ele-
ment of the body to the next,
from the foot to the head. As
soon as the tip of the foot again
touches the screen, it pushes
itself through it as far as pos-
sible. If the wire net is again
turned over, the whole process
is repeated anew. In this way
the animal can be compelled,
by the help of gravity alone, to weave itself through the
meshes of the screen several times "of its own accord."

Fig. 33 shows a Cerianthns which has thrice passed
through the meshes of the screen in this way. The drawing
is taken from life.

4. Such a bending downward, which has been accurately
studied in negatively geotropic roots, has never been demon-
strated, so far as I know, in animals. I will therefore cite
another experiment which better illustrates the course of
this reaction. If a Cerianthus be put into a test-tube filled
with sea- water, and the test-tube be placed so that the head
of the animal is down and the foot up, while the long axis of
the animal is vertical, the tip of the foot begins after some
minutes to bend vertically downward. In Fig. 34 is shown
the course of such an experiment. Several minutes before
12 o'clock the animal was placed in a test-tube in the posi-
tion described. At 12 the foot of the animal had begun to
bend downward (Fig. 34, ft); in the next thirteen minutes the
bending gradually advanced toward the head (Fig. 34, 6).

HETEROMORPHOSIS 155

Five minutes later -the foot reached the bottom of the test-
tul>e (Fig. 34, c). The bending spread gradually to elements
lying nearer the head; as the foot could no longer advance
vertically, it was pushed horizontally over the bottom of the
test-tube; and at the same time the head, which until now
had played no active part, was slightly raised (2:35 P. M.,
Fig. 34, d). The bending then passed from one element of
the body to another, until the head was brought into an
erect position (Fig. 34, e}. Finally the entire animal righted
itself so that at 1 o'clock it had the position shown in Fig.
34,/). The whole righting process had therefore occupied
an hour. The animal remained in this position for two days,
when it crawled out of the test-tube. I have repeated the
experiment many times, but always with the same result.

5. If a Cerianthus is divided transversely in the middle,
and both pieces are laid upon the wire screen, they work
their way (often immediately after the division) through the
screen with their aboral ends directed downward. If the
head and foot of an animal are amputated, the middle piece
may still show this reaction. When this occurs, the amoral
cud always bends downward, and works its way through
the wire screen. Never have I seen the reverse occur-
that such an animal assumes a position in which the oral
pole is directed downward and the aboral pole upward. I
wished to determine whether light or gravity had any effect
upon the position of the new organs formed in these headless
and footless animals when fixed vertically in sand, with
their aboral ends directed upward; in no case, no matter how
often I fixed the animals in an inverted position in the sand,
did I succeed in retaining them in this position longer than
two days. Nor did they remain with their aboral cut ends
directed downward in a narrow test-tube the long axis of
which stood vertically. In all cases they turned their oral
poles upward.

156

STUDIES IN GENERAL PHYSIOLOGY

V-;

HETEROMORPHOSIS 157

6. We have seen that in the uninjured animal it is the
foot which first bends downward on the wire screen arid
assumes a vertical position; while the head is the last to
assume this orientation. If an animal is cut across trans-
versely, and the two pieces are laid side by side upon the
wire screen immediately after the operation, the aboral frag-
ment carrying the uninjured foot begins to bend down
vertically sooner than the oral fragment which carries the
head.

This difference in the irritability of the two portions of
the animal can be shown very prettily by making a trans-
verse incision at about the middle of the animal, so that
both pieces still hang together. If such an animal is laid
upon the wire screen, immediately after the operation, the
foot works itself through the rnesh in the net to the incision
and assumes a vertical position, while the oral piece extend-
ing from the incision to the heads usually remains lying hori-
zontally upon the wire screen.

7. If the heads of Cerianthus, which have been cut off
close to the oral plate, and which no longer work their aboral
poles through the wire mesh when laid upon it, are laid upon
the sand for a time, they also at length assume a position in
which their long axis is in a vertical position. One receives
the impression at first that one is dealing with normal
animals buried deep in the sand. The method by which
they retain their vertical position is remarkable. Certain of
the cells of the ectoderm secrete a mucoid substance to which
kernels of sand become attached. But this secretion is
formed only on the base of the pieces which have been cut
off just below the oral plate. The kernels of sand which
adhere to the base have a greater specific gravity than the
animal itself, and this keeps the animal in an upright posi-
tion.

8. All these experiments succeed equally well in the light

158 STUDIES IN GENERAL PHYSIOLOGY

and the dark. The fact that the animal assumes a vertical
position in every case seems toindicate that gravity is the deter-
mining factor. I have tried to see whether the animal would
assume upon the centrifugal machine the position of one of the
radii, that is, with its foot directed toward the periphery
and its head toward the center of the rotating disc. But
the animal had always to be kept in a vessel of water in
these experiments, and the currents set up in the water by
the rotation interfered with the movements of the exceedingly
soft animal. Even when the animal was fastened to the
wall of the vessel by a needle, its free ends were always set
in motion by the water. Nothing remained, therefore, but
to introduce the animal into a long test-tube which was fas-
tened radially upon the revolving table, and to observe
whether the animal directed its foot or its head toward the
center of the revolving table. The experiments which have
been performed thus far have not given a uniform result.

9. The animal retains a vertical position permanently
only when at the same time contact stimuli act constantly
upon its entire surface. The animal retains a vertical position
permanently in the sand, but only for a few days at the best
upon a wire screen. Iwas also able to keep the animal perma-
nently in a horizontal position in a closely fitting fr^t-tabe.
The head which projected beyond the lips of the test-tube
was directed vertically upward.

How strongly these animals are compelled to bring as much
as possible of their bodies in contact with other solid bodies is
evidenced by the fact that they crowded themselves forcibly
under lead blocks and lead plates which I had laid upon the
bottom of the aquarium. This is the same form of contact-
irritability that is found in Forficula, Iarva3 of Musca, winged
ants, etc. a phenomenon which I have described in greater
detail in previous publications. 1

1 "The Heliotropism of Animals," p. 1, and also " Further Investigations on the
Heliotropism of Animals," p. 89.

HETEROMORPHOSIR 1 ">'.)

10. The morphogenetic polarity discussed in th> \ (reced-
ing chapter therefore corresponds with a polarity in regard
to the orientation of a Ceriauthus toward gravitation. Since,
however, we are as little acquainted with the structural con-
ditions which determine the orientation of a Cerianthus as
with the structural conditions which determine that the for-
mation of tentacles only occurs at the oral end of a fragment,
the question as to whether the same conditions underlie both
phenomena cannot as yet be discussed.

XII. FURTHER REMARKS ON THE FORM AND LIFE PHENOMENA
OF THE NEWLY FORMED HEADS IN CERIANTHUS

1. If a transverse incision such as described in sec. x
be made fairly close to the head, the edges of the wound do
not draw together so easilv. In this case new tentacles, a

o /

new oral plate, and a new mouth are formed at the oral cut
edge. The part above the incision may persist for months,
but finally it drops off like a wilted leaf.

If, on the other hand, the incision is made in the middle
of the animal, the tendency for the edges of the wound to
heal together is very great. New tentacles (external and
internal) and a new oral plate are formed; but never has a
mouth formed in any of the cases observed thus far. The
newly formed head was therefore of no use whatsoever to the
animal. If we look more closely at such a head (Fig. 24. 1> ),
the ectoderm is seen to pass over into an oral plate at /;,
which is covered with two rows of tentacles. An opening no
longer exists in the ectoderm.

We saw, moreover, that quadrangular pieces cut from the
wall of a Cerianthus formed tentacles upon one side only.
A second circumstance to be considered is the fact that the
elastic tension of the inner layer of the wall is greater than
that of the external. In consequence of this, the three
remaining cut edges, upon which no tentacles are formed,

160 STUDIES IN GENERAL PHYSIOLOGY

roll inward, so that only the ectoderm is visible externally
(Fig. 35). Because of these mechanical conditions the part
assumes after some time especially when the new tentacles
begin to grow an appearance which reminds one in some
ways of a normal Ceriaiithus. Of course, many pieces remain
permanently monstrosities. So far as the experi-
ments performed hitherto are concerned, a moid Ii
lius never been formed in these pieces. This is a
remarkable fact, and seems to indicate that the
animals have a source of food-supply which
differs from that of the uninjured animal, for they remain
alive, and do not diminish markedly in bulk even in the
course of months.

2. These moathless heads when brought in contact with
food reacted exactly as normal heads. The reader is prob-
ably acquainted from personal observation with the behavior
of an Actinian when a piece of meat is laid upon the tip of
one of its tentacles. The tentacle becomes concave toward
the piece of meat, winds itself about the meat as a vine
about a support and finally bends so that the piece of meat
reaches the middle of the oral plate, where the mouth is
situated in normal animals. In Cerianthus the inner ten-
tacles then fold over the meat ; some or all of the external
tentacles then follow in a similar way, and it looks as though
the tentacles were pressing the meat into the mouth. The
meat reaches the stomach, and the tentacles then unfold.
But this reaction is certain to occur only when the sub-
stance laid upon the tentacles has certain chemical and
mechanical characteristics. If a grain of sand is laid upon
the tentacles instead of the meat, the tentacles do not bend
in as described.

If a piece of meat is carefully laid upon the tip of the
external tentacles of a newly formed head, which has no
oral opening, they also seize it in the manner just described;

HETEROMORPHOSIS 101

they carry it to the middle of the newly formed oral plate,
and the inner tentacles cover the meat and press it against
Ilu> oral plate; the outer tentacles then also cover the meat,
and the animal struggles several minutes in vain to press the
meat into a mouth which does not exist. The external ten-
tacles are then withdrawn from the center of the oral plate
and expanded, and the same is done with the internal ten-
tacles. The piece of meat again reaches the edges of the
tentacles (probably through ciliary motion) and drops off.

The experiment can be repeated with the same result any
number of times upon the newly formed heads which have
no oral opening; they always react when a piece of meat is
laid upon the tentacles. No trace of memory is present.

In order to have the new head react with certainty, it is
necessary that the substance laid upon the tentacles have the
same characteristics as that necessary to call forth the
reaction in the old head. Pieces of meat are always carried
to the center of the oral plate by the tentacles of the new
head, but this does not occur when kernels of sand are used.
If the head of a Cerianthus is amputated, the animal does
not again take up food until the new mouth has been formed
and the tentacles have attained a certain size. We shall see
that other Actinia behave differently in this respect.

H. The fruitless attempts of the mouthless heads to take
up food is somewhat comical in the light of an optimistic
teleology. The physiologist, however, takes it for granted
that the tentacles of the moiif/ilcss head intist react in a
similar way to chemical and mechanical stimuli as the ten-
lades provided with a mouth, because they have the same
external form, and possibly also the same structure. That
I lie meat is finally brought to the mouth through these
reactions (bendings), and that when the meat has reached the
mouth the bendings again are reversed, does, of course, not
influence the immediate effect of the contact between ten-

162 STUDIES IN GENEEAL PHYSIOLOGY

tacles and meat; just as a moth must react to a flame with
progressive heliotropic movements, even though it after-
ward derives no actual benefit, but actual harm, from this
sort of reaction.

4. Cerianthus remains permanently in one place if its body
is in contact with solid bodies and if it is properly fed. If
the feeding is interrupted, it occasionally leaves its tubes in
the sand to burrow anew after some time in some other part
of the sand. If, however, the head of the Cerianthus is
amputated, this otherwise sessile animal becomes a complete
noniad. It burrows, remains for a few hours in its tube,
crawls out again, buries itself anew in some other place in
the sand, only to leave its new home after a short time, etc.
When the tentacles have again grown, the animal becomes
more sessile again.

XIII. THE IMPORTANCE OF TURGOR FOR THE GROWTH OF
THE TENTACLES IN CERIANTHUS

1. Although the analysis of the mechanical conditions
which influence the growth of plants has made great strides, a
physiology of animal growth does not exist even by name in
the modern text-books of animal physiology. It may there-
fore be permissible to describe here a very simple experi-
ment which shows that one of the fundamental conditions
necessary for the growth of vegetable tissues turgor
must be fulfilled in animal tissues also in order that growth
may occur.

It may be known to the reader that the growth of plants
decreases or entirely stops when they wilt, but that it in-
creases when a plentiful supply of water is at hand. It is
believed that the cell contents of the growing part of a
plant take up water energetically from their surroundings
(due to the salts of the organic acids contained in them).
In consequence of this absorption of water, the cell-walls

HETEROMORPHOSTS 103

are stretched. This stretching of the cell membrane permits
the deposition of new material in the cell growth. When
the hydrostatic pressure in the cells of an organ attains a
height in which the cell membrane is tensely stretched, the
organ is said to be turgescent.

In the course of the experiments detailed in the preced-
ing chapter I accidentally discovered a means by which the
turgor of a part of the tentacles of a Cerianthus can be
diminished, while in the others it remains unaltered. I used
this method to determine whether a diminution in the turgor
would decrease or stop the growth in animal organs. If a
transverse incision is made into the body of a Cerianthus
such as is necessary to cause the growth of a second head,
the incision has a striking effect upon the behavior of the
tentacles. If one watches such an animal when its tentacles
are stretched out, it is seen that those tentacles which are
situated above the incision are distinctly, often as much as
one-half, thinner and shorter than the remaining tentacles.
This difference is shown distinctly in Fig. 21) which repre-
sents the same animal as Fig. 2S, only viewed from another
side. This difference in the turgor of the tentacles is per-
manent when the incision is made near the oral plate, and
when the edges of the wound are not allowed to heal to-
gether. As soon as the wound heals, the turgor of the
tentacles is re-established. If the irritability of such wilted
tentacles is compared to the irritability of the turgescent
tentacles of the same animal, it is found that the irritability
is not markedly changed >(nrin<j //ic Jirxl far <l<i>/x aj'/rr
the incision.

If a piece of meat be carefully laid upon the tip of such
a wilted tentacle, it is carried to the mouth in the same way
as by an erect tentacle. Only it seemed to me that the
movement of the wilted tentacle was slower and more awk-
ward than that of the turgescent tentacle.

164 STUDIES IN GENERAL PHYSIOLOGY

The explanation is sometimes given that the tentacles of
Actinia are stretched by a contraction of the muscles of the
body- wall which forces water out of the body-cavity into the
hollow tentacles. The turgor of the tentacles of Cerianthus
cannot well be brought about in this way ; for, if this were
the case, the turgor of all the tentacles would have to be
decreased when the body-cavity is opened; but only the
turgor of the tentacles above the incision is diminished,
while it remains the same in the others.

2. I amputated the heads of a large number of Ceri-
anthi. After some time which was, within certain limits,
shorter as the temperature of the water was higher new
tentacles were formed at the cut edge. I waited until the
newly sprouted tentacles had reached a length of 5 10mm.
when stretched out. I then made a partial transverse inci-
sion into the body and prevented the wound from healing
together. The tentacles above the incision lost some of their
turyor, and ceased to </)<>/< from that time on. The re-
maining tentacles, lio/rerer, continued to i/row and after
sereral irceks readied a lenyfh of 30 nun. or more when
stretched out.

As I had to bring my experiments to a close, I could not
determine whether the wilted tentacles could again be made
to grow by restoring their turgor. I hope to be able to
make this and further experiments on growth at another
time.

The fundamental condition for growth in plants is there-
fore also found in animals.

XIV. ON THE EXTERNAL CONDITIONS WHICH DETERMINE THE
FORMATION OP TUBES IN CERIANTHUS MEMBRA NACEUS

If a Cerianthus is laid upon the sand, and a sufficient
time is allowed the animal to burrow, it is noticed after
several days that the hole in which it lies is covered with a

HETEROMORPHOSIS 105

smooth coating. If the Cerianthus is pulled out of the sand,
it is found to be incased in a soft tube which is smooth
internally and covered on the outside with tine grains of
sand. The animal can be drawn out of the tube without
injury. It seems very human that the animal should arrange
itself comfortably in the sand and protect itself against
unwelcome visitors by building a tube about itself. This
formation of tubes by Cerianthus appears to belong to those
cases to which the old phrases of "instinct" and "artistic
impulse of animals" might be applied. It may perhaps be
of interest to some of the readers to become acquainted with
the following simple experiments which show upon what
foundation anthropomorphic conceptions of life-phenomena
are occasionally based.

1. If a Cerianthus is carefully drawn out of its tube and
laid upon a very thin layer of sand in which it cannot burrow,
a secretion is soon formed at the surface of those portions of
the body which rub against the sand. The surrounding
particles of sand stick to this secretion. The continued
movement of the animal, and the propagation of stimuli from
those parts of the animal which are rubbed in the sand to
neighboring parts of its surface, cause the secretion to be
poured out over the entire surface of the body of the animal.
The tube is then completed. It is at first very thin, but
becomes thicker in the course of time, as more secretion is
poured out in consequence of the continued friction.
According to these observations, therefore, the entire process
of "tube-building" is nothing but a process of secretion, the
stimulus for which is found in the friction of the surface of
the body against solids. This is confirmed by the following
experiments.

_!. The thickness of the tube is dependent upon the
degree of friction. If one Cerianthus is laid upon sand,
while another is introduced into a carefully cleaned test-

1()G STUDIES IN GENERAL PHYSIOLOGY

tube, the former constructs a firm tube of mucus in the
course of a few hours, while the latter which is kept in a
test-tube, and the skin of which encounters but little friction
from the smooth glass, forms a scarcely perceptible veil in
the course of twenty-four hours. The greater amount of
friction brings about a greater secretion and a more exten-
sive tube-building.

3. I fastened a Cerianthus to the underside of a cork
floating at the surface of the aquarium. The Cerianthus
was fastened to the cork by passing a pin through its body.
The head and foot of the animal hung down loosely upon
either side of the pin. I waited four weeks, but no mem-
brane was formed upon the parts which did not come in con-
tact with solid bodies. But a secretion occurred at those
places where the Cerianthus rubbed against the cork or the
head of the pin. The mass of mucus secreted at these points
attained the thickness of a finger in four weeks.

The wound occasioned by the pin was not the cause of
this secretion, but only the friction, for I observed the same
phenomena in uninjured Cerianthi which remained for some
time in the meshes of a wire screen. Only in the latter case
it is very difficult to keep a Cerianthus very long in this
position without movement.

The formation of a tube by Cerianthus offers therefore
the same evidence of "artistic impulse" as the secretion of
saliva during mastication.

XV. EXPERIMENTS ON ORGANIZATION AND IRRITABILITY IN

SOME OTHER ACTINIA

1. I have made experiments similar to those upon Ceri-
anthus 011 the determination of the situation of the new head
in a. number of other Actinians Actinia equina of the Bay
of Naples and the East Sea, Actinia cari, Adamsia Rondel-
letti, Anemoiiia sulcata, Cereactis auraiitiaca, etc.

HETEBOMORPHOSIS 1<>7

I always t'ouncl that new tentacles were formed only
the oral edge of the pie.v cut from the animal, while a IM-W
foot was formed oidy at the aboral end. Kven though I
have not thus far been aide to cause a head to develop at the
aboral end in Actinia as in Tulmlaria, still 1 consider it
probable that this also will succeed because of a note I found
in Contarini's Tr<ifl<ito (Idle AtlinicJ according to which
Diquemare." who worked on regeneration in Actinia, once
noticed such a heteromorphosis. A piece cut transversely
from an Actinian " riprodurre in vece di un piede degli altri
tentacoli e un altra bocca, cosi mangiara da due parti nello
stesso tempo."

2. In these experiments the Actinia equina of the East
Sea* behaved in some respects differently from Cerianthus.
When I cut transverse pieces from Actinia equina, they
formed fcitfaclcx oxl'l, and these without exception at the
oral pole; I uerer saw a new foot formed upon transverse
pieces of this animal. The wound only healed at the aboral
pole; no regeneration whatever occurred here, and the body-
cavity of such an animal communicated with the outer world
at both poles. Most remarkable, however, was the fact that
both poles took up food, the aboral mouth being even more
voracious than the normal mouth at the oral end. Substances
were swallowed by the aboral mouth which the oral mouth
as a rule does not take up. If a paper wad soaked in sea
water is laid upon the normal mouth of an Actinia equina
of the East Sea, it is not swallowed; while a piece of crab
meat, which by our tongue cannot be distinguished from the
taste of the paper wad, is immediately taken up by the
animal. I tied a paper wad to one end of a stout thread

i X. COXTAKIXI, Ti-ntt,it<i delle Attinie 'Vene/.ia. 1MI).
-' I li.-ivc not been able to ol)t:iin I>ii|iieniare'~ work.

Very recently Professor Torrey. of the VniverMty of California, has observed
heteromorphosis in an Art i man. [ I'.l'l.'! I

Tin- Actinia equina of the Easl Sea i- not identical in its physiological behavior
with the Actinia equina of the liaj of Naples.

IBS STUDIES IN GENERAL PHYSIOLOGY

and a piece of meat to the other, and then threw the whole
upon the outstretched tentacles of a hungry animal. The
tentacles which came in contact with the meat reacted at once
by bending by which the meat was carried to the mouth ; the
tentacles in contact with the paper did not react. I removed
the thread and reversed the position of the meat and paper
upon the oral plate, so that the tentacles which had before
been touched by the paper were now in contact with the
meat. The tentacles touched by the meat carried it to the
mouth, while the tentacles touched by the paper let it fall.
The meat was thru crowded into the mouth, and the thread
was pulled in after it; but the paper and a piece of the
thread remained outside of the oral opening. No change
occurred within the next twenty-four hours. After this
period the thread was ejected, but without the meat. The
latter had probably been digested. I have often repeated
this experiment with the same result; only occasionally the
thread was ejected earlier, and then a piece or all of the
undigested meat was still attached to the thread.

I divided an A. equina into two pieces by a transverse
incision. The oral piece which I shall call the head piece
-had the old normal mouth at its oral end; the body-cavity
was open also at the aboral end of the head piece, and food
was taken up here likewise, even though no tentacles were
present. The old oral mouth of the head piece showed the
same choice in the taking up of food after the division of the
animal as before. But the aboral mouth of the head piece
at times took up paper wads and swallowed them. Yet I
saw it refuse paper wads while at the same time it eagerly
took up pieces of crab meat.

While the old mouth at times refused meat, the aboral
mouth was nearly always ready to take up food.

3. The following means, however, served to decrease the
irritability of the mouth toward chemical and mechanical

HETEROMORPHOSIS K; ( .)

stimuli. \\'hen I stood the head piece upon its oral mouth
for but out 1 hour, so that the mouth was in contact with the
bottom of the vessel, the animal would take up no food for
a loug time (often as long as twenty-four hours) when again
turned over. By similar means I could bring about the
same effect at the new aboral mouth. But it was necessary
to keep the animal with its aboral mouth downward a much
l<i(/er time half a day perhaps and then the effect lasted
only an hour. In this experiment it cannot be that every abnor-
mal stimulus simply inhibits the irritability of the mouth.
For a series of artificial mouths made by transverse incisions
into an animal <tie meat despite the wounds, immediately
after the dirixion of the animal. The (transitory) loss of
the specific irritability of the mouth is probably due to the
contact stimuli which act upon the mouth. 1

4. I laid pieces of Actinia which took up nourishment at
both ends upon the side and tried to see whether both mouths
would take up food at the same time. I first held a large
piece of meat against the aboral mouth, which was, as
usual, tightly closed in consequence of the contraction of the
circular muscle fibers. The meat caused the mouth to open
and to seize and slowly crowd it into the body-cavity.
Before the piece of meat had been entirely swallowed I
offered another piece to the oral mouth. This was also taken
up. At the same moment the act of deglutition was inter-
rupted at the other mouth by a firm contraction of the ring
muscles. After a few moments, when the meat had been
crowded into the oral mouth, the musculature at the aboral
end relaxed and the piece of meat dropped out of the mouth.
When I fed the two mouths successively, that which had
been fed first gave up its food when the other began to take
up its food. Peristaltic waves seemed to pass frequently in
both directions in these animals in which the body-cavity

1 Or the lack of oxygen. [1903]

170 STUDIES IN GENERAL PHYSIOLOGY

is opened at both ends. The meat after being swallowed
was often ejected undigested. Actual nutrition of these
animals was, of course, impossible.

5. We have thus far considered only the head piece of a
divided Actiiiian. The foot piece which possesses an
uninjured foot at its aboral end, and a cut edge at its oral
end, soon regenerates tentacles at this end and a normal
mouth is formed. But even before the tentacles and mouth
have begun to regenerate, the oral opening assumes the
functions of a mouth. Pieces of meat are taken up and
swallowed, while I have never seen it take up wads of paper
or grains of sand.

(5. While the entire body of Cerianthus, with the excep-
tion of the oral plate, is endowed with contact-irritability,
this contact-irritability is confined to the basal surface of the
body of Actinia equina. By means of this surface the ani-
mal attaches itself to solid bodies. The surface of the foot
has here the same function as the roots of Tubularia, only
with this difference, that through (spontaneous) internal
changes the Actiiiian can again leave the surface of the body
to which it is attached, while this is not possible in Tubu-
laria. It is interesting to note that the nature of the sur-
face of the solid body is not a matter of indifference in call-
ing forth these reactions. When no other object was near, the
animal attached itself to the glass wall of the aquarium and
slid about upon it ; when, however, I placed the shell of a
Mytilus in the aquarium, and the animal came in contact
with it in the course of its movements, it immediately
attached itself to it and remained there, it mattered not
whether the shell was empty or occupied.

The surface of an ulva leaf in the aquarium acted in the
same way as the surface of the mussel. While the animal
would always leave the glass to which it was fastened, to
attach itself to an ulva leaf when brought in contact with it,

HETEROMORPHOSIS 171

the ivvrrsr phenomenon namely, that the animal would
leave the ulva leaf or the mussel shell to attach itself to the
glass occurred but rarely. This contact-irritability of I he
foot docs not clitnnjc ir/icii Ihc head or flic l<ir</cr jxtrlioit of
the oral part of the (iiu'mal is amputated. I kept the oral
end of such a piece of an animal in contact with the bottom
of the aquarium, while its foot extended upward. A slide
was placed in contact with the foot, to which the animal
might easily have attached itself, but this it did not do. As
soon, however, as an ulva leaf floating in the aquarium came
in contact with the foot, the animal attached itself to it
immediately.

7. The aboral pieces of transversely divided Actinia
which still had a foot remained alive longer than the oral
pieces. The latter usually succumbed to a fungus disease
after a few weeks. The disease began ordinarily in the ten-
tacles which had lost their turgidity after the operation.
Contarini seems to have made a similar observation in opera-
tions on Actinia (Aiptasia) diaphena.

XVI. SUMMARY OF RESULTS

In conclusion I wish to summarize briefly the chief results
of these investigations.

I. We saw first of all that there are certain animals in
which it is possible to control the place where an organ is
formed by external conditions, in such a way as to substitute
in place of a lost organ one which differs from it in form and
function (heteromorphosis). It is thus possible to produce
at will forms with normal vitality, which differ from the
hereditary forms produced by nature in a definite way. In
detail the heteromorphoses thus far accomplished are the
following:

1. In Tubularia mesembryanthemum:

a) If a piece of stem, which must not be below a certain

172 STUDIES IN GENERAL PHYSIOLOGY

minimal size, is cut from Tubularia mesembryanthemum,
and both its ends are surrounded by water, a head is formed
at both ends (Fig. 16). The head is usually formed more
rapidly at the oral than at the aboral end. In this way it is
possible to produce any number of bioral animals without
interfering with their vitality.

6) If a piece of root is carefully separated from the base
to which it was attached, so that it is surrounded on all
sides by water, a head is formed at its aboral end. The root
continues to grow but forms no polyps if it is allowed to
attach itself anew as a root.

2. If a portion of the stem is cut from Aglaophenia and
suspended vertically in the aquarium, it always forms a root
at its lower end, according to observations made thus far ; it.
matters not whether the apical or the basal end is directed
dc >\vmvard. Either a tip or a root is formed at the end directed
upward (toward the zenith), but a tip is formed the more
readily when the apical end is directed upward.

It is therefore possible to create biapical and bibasal
forms (Figs. 17 and IS) in Aglaophenia; yet the certainty
with which bibasal animals can be created is greater than
that with which biapical animals can be produced.

3. If the stems of Plumularia pinnata are cut off close to
the root and fixed in a vertical position, but with the tip
downward, a new tip instead of a new root may arise from
the basal end, which continues to grow upward; more fre-
quently a root first springs from this end, from which arises
a stem that grows upward (Fig. 20, , 6).

4a) If a piece is cut from the stem of Eudendrium, and
both ends are surrounded by water, new tips are formed at
both extremities (Fig. 21, a, b). Yet a variation occurs at
times which I have not observed in the beforementioned ani-
mals; namely, a new tip and a new root may grow from the
same cut end.

HETEROMORPHOSIS 173

li) It was possible to cause the growth of roots in the
middle of a stein by bringing this point in contact with
solid bodies.

5 a) It was possible to grow tips from the basal cut end of
a stern of Sertularia polyzonia by fixing the stems in the
aquarium with their basal ends upward (toward the source
of light). New roots were usually formed at this end in
addition to the new tips (Fig. 22, a, b).

6) So far as my present experiments go, new branches
were formed only on that side of the old stem or root which
was turned toward the light.

6. It was possible to produce biapical animals in Gono-
thyrae Lovenii.

II. In a long series of animals, particularly Actinians, I
have not yet succeeded in causing a heteromorphosis of any
kind. In these animals the position of the regenerated
organ is determined (as far as our present knowledge goes)
by the orientation of the fragment occupied in the uninjured
organism, a new head is formed at the oral edge of a piece
of such an animal, while a new foot is regenerated at the
aboral end. This law governs regeneration, not only in
Actinia, but also in Hydra, in certain starfish which I have
studied, in a series of worms, snails, crustaceans, and animals
still higher in the scale,

III. The same behavior in organization is therefore found
in the animal kingdom as in the vegetable. As is well
known, it is possible to control the position of a new organ
(just as in the animals given under heading I) in certain
plants, while in others (as in Actinians) regeneration is
dependent upon conditions which are at present unknown,
and are apparently internal

IV. The following are mentioned as some of tin- diUVivMi
forms of irritability which influence the orientation of animal
organs :

174 STUDIES IN GENERAL PHYSIOLOGY

1. Contact-irritability (stereotropism).

) In a series of Hydroids the root attaches itself to the
surface of a solid body as soon as it conies in contact with it,
and continues to grow over its surface, adhering to it as
closely as possible.

The stems of these Hydroids do not possess this irrita-
bility. In Tubularia, in which the polyps are large enough
to permit one to experiment upon them, it can be shown that
they possess the opposite kind of contact-irritability. When
brought in contact with the surface of solid bodies they bend
away from it. By taking advantage of this irritability of
the polyps, it is possible to bring about permanent (stereo-
tropic) curvature in the Tubularian stem.

h) Only the tip of a root which is growing attaches itself
to the surface of a solid body.

c) In order that the root may attach itself it is necessary
that the contact stimulus should act for some time.

<l) It has already been mentioned that contact-irritability
can cause the growth of roots in the middle of a stem for
example, in Eudendrium.

c) When in a number of Hydroids stems arise from the
roots which have become attached to a solid body, these new
stems originate on that side of the roots which lies diametri-
cally opposite the solid body.

2. Heliotropism.

a) In Sertularia (polyzonias ?) the branches are positively,
the roots negatively, heliotropic. Only the growing parts
show any heliotropic curvatures.

3. Geotropism (?).

a) When possible, Cerianthus membranaceus always
assumes a position in which its long axis is vertical, its oral
pole upward, and its aboral pole downward. If the animal
is placed in a different position, the foot tries to gain its nor-
mal orientation by bending vertically downward.

HETEKOMORPHOSIS

/>) The main root and the adventitious roots of Aglao-
phfiiia, when they do not come in contact with the surface of
solid bodies, bend downward and continue to grow in this
direction; while the stems bend upward, grow toward the
zenith, and arise from the upper surface of a horizontally
growing root.

V. The following phenomena are of importance in the
general physiology of animal growth :

1. For the growth of the tentacles of Cerianthus, as for
the growth of plant tissues, it is absolutely necessary that
the hydrostatic pressure in the cells of the organ exceeds a
certain amount (that the organ is turgescent).

2. The growth of the roots of Aglaophenia, Sertularia,
and other Hydrozoa occurs only in a small region near the
tip of the roots as is the case in the analogous plant tissues.

3. When the roots of Aglaophenia, Gonothyrsea, Plumu-
laria. and Sertularia become attached to solid bodies, they
grow in length much more rapidly, and their absolute growth
is much greater than when they are surrounded on all sides
by water. This has already been demonstrated by Dalyell
in other Hydrozoa.

VI. Of the special results the following only may be
mentioned: If a transverse incision is made into the body-
\vall of Cerianthus near the oral plate, only those tentacles
situated above the cut lose their turgidity, while the remain-
ing tentacles retain theirs. The turgidity can therefore not
depend upon a contraction of the body-wall which forces
water into the tentacles.

V

GEOTROPISM IN ANIMALS 1

I. GEOTROPIC CURVATURES IN ANIMALS

As A continuation of observations which I have already
published' I wish to give in the following pages some
further facts which show that certain animals are compelled
to orient their bodies in a definite way toward the center of
the earth, as are certain plants.

In order to show more clearly the similarity between the
behavior of animals and that of plants in this respect, I
quote the following passage from Sachs on geotropism in
the plant kingdom:

Whenever portions of a plant are moved by any cause whatso-
ever from their habitual position into ad ifTerent position with refer-
ence to the horizontal, they bend until they again assume the same
relation with the horizon which the}' had originally. This bending,
which is brought about through the mere change in position, is the
effect of a geotropic stimulus, the consequence of some property
of the organs which does not give them any rest until they are
again at their proper angle with the direction of the force of gravi-
tation. 3

These geotropic bendings of plants, as Sachs adds, "are
brought about exclusively through growth, and only those
organs which are still capable of growth can therefore regain
their normal position with reference to the horizontal."

I have pointed out in an earlier paper that the roots of
Aglaophenia pluma, a Hydroid, have the tendency to grow
downward. Curvatures at the same time take place in this
animal which are determined by internal causes, and which

iPfluyers Archiv, Vol. XLIX (1891), p. 175.

i.Sitzungsber. der Wurzburger physik.-med. Gesellschaft, 1888, and Part I, pp. 1
and 89.

3 J. SACHS, Vorlesungen iiber Pflanzen- Physiologic, 2d ed. (Leipzig, 1887), p. 717.

176

GEOTROPISM IN ANIMALS 177

2Oiuplicate the phenomena of geotropism. Recently, how-
ever, I have discovered geotropic bendiiigs in the growing
portions of a different Hydroid (Antennularia aiitennina)
which are not masked by any secondary phenomena.

Antennularia aiitennina consists of a main stem about
liuin. in diameter, and often 20cm. long, which usually
arises perfectly perpendicularly from a felt-like mass of very
fine rootlets. From the main stem spring in regular order
very delicate, short, unbranching lateral twigs, upon the
upper surface of which are found polyps and nematophores.
If such a stem is placed in any position deviating from the
vertical, the tip of the stem, if it grows at all, bends sharply
back toward the vertical and continues to grow vertically
upward. Only the newly growing portion of the tip is able
thus to change its orientation. If the orientation of the
animal is riot again altered, the stem grows absolutely ver-
tically upward; it is negatively geotropic. The roots, upon
the other hand, grow vertically downward ; they are positively
geotropic ; yet the direction of their downward growth is not
so perfectly straight as that of the upward-growing stem.
As often as the orientation of the stem with reference to the
vertical is changed, if any new growth whatsoever occurs,
the stem bends toward the vertical and grows upward in this
direction.

But not only the orientation of the organs, but also the
place where new organs originate, is dependent to a large
extent upon gravitation. But I will speak of these facts at
another place, and publish therewith the pictures necessary
to illustrate them.

These curvatures during growth are independent of the
light. They occur equally well whether the stems are grown
in the dark or the light mom; when cultivated in the light,
so far as I have been able to see, their orientation is not
affected in the least bv the direction of the ravs of HHit.

< /

STUDIES IN GENERAL PHYSIOLOGY

Other conditions than light and gravity which might have
influenced the perfectly vertical growth of the stem, or its
geotropic bendings, when its position with reference to the
vertical was altered, were shut out in these experiments. I
was not able to make any experiments upon the centrifugal
machine, as it takes some twenty-four hours to bring about
the geotropic bendings, and because I could avail myself of
no motive power which ran uninterruptedly for so long a
time. There can be no doubt, however, that we are dealing
in this case with a geotropic phenomenon.

There are, on the other hand, animals which are capable
of geotropic curvatures through muscular- contractions with-
out any accompanying phenomena of growth. We find such
conditions in an Actinian, Cerianthus membranaceus. This
animal has the habit of burying itself vertically in the sand.
If the animal is brought into any other orientation toward
the vertical, it bends its body downward, beginning with the
foot, until the entire animal has again a vertical position. 1

II. GEOTROPISM IN FREE-SWIMMING ANIMALS AND ITS
SIGNIFICANCE FOR THE BATHOMETRIC DISTRIBUTION OF
CERTAIN PELAGIC ANIMALS

1 . Since the geotropic effects in the vegetable kingdom have
been studied, in the main, only upon sessile organs, and are
known in these only in the form of curvatures during growth,
objection might perhaps be taken to the fact that I intend to
speak of geotropism in free-swimming animals. But the
term "geotropism' 1 signifies only a dependence of the orien-
tation upon gravitation, without saying anything concerning
the mechanism of this dependence. It would therefore be
mere pedantry if we should declare that such dependence
could be designated geotropism only in sessile and not in
motile animals. I will lose no time in arguing this question,

1 Part I, p. 155.

GEOTROPIRM IN ANIMALS 179

and rather quote the words with which J. Sachs describes
the geotropisui of motile plants:

I have already called attention to the remarkable way in which
Plasmodia crawl up the stems of plants, flower-pots, and other
comparatively high objects. They can be induced to do this most
readily when moist glass plates are fixed vertically into tan bark
containing 1 young 1 Plasmodia which are just ready to creep to the
surface. In the course of a few hours the mesh-like bodies crawl
to the highest points of the glass plate, which can now be removed
and observed directly under the microscope in order to watch their
movements more accurately. It can scarcely be doubted that t/ti*
ii)ij>nln<' to cnttrl itjncard is to be considered the effect of a
geotrojnc stimulus; that is to say, that an as yet unknown effect of
gravitation upon the molecular structure of the protoplasm influences
the movements of the molecules in such a way that the effect which
we have described is finally brought about. It is scarcely necessary
to add that the individual mechanical factors which play a role in
this process are entirely unknown. 1

Others dispute the idea that the Plasmodia are geotropic.
They claim that these phenomena are the expression of a
rheotropism and hydrotropism.

I showed in one of my earlier papers that certain insects
behave in a way analogous to the movements of Plasmodia.
In the case of insects these movements are certainly not
dependent upon rheotropism and hydrotropism. If certain
insects, such as Coccinellse for instance, are introduced into
a closed wooden box (and are in addition kept in a dark
room in order to shut out every effect of light), they have a
tendency to creep up the vertical walls of the box and to
collect in the highest regions. The behavior of these
animals toward gravitation corresponds with that which
Sachs has described for Plasmodia.

A similar phenomenon is noticed in marine animals. In
this connection it contributes toward the understanding of
bathometric distribution of these animals. The reader is

ij. S.M-IIS. 'i/i. cit., p. ti.'r.i.

STUDIES IN GENERAL PHYSIOLOGY

undoubtedly familiar with the fact that many marine animals
are found only at certain depths in the ocean. Thus far it
has not been determined what conditions compel these
animals to behave in this way, Groom and I together have
shown how the heliotropism of certain pelagic animals must
lead to periodic depth-migrations. 1 I can show by several
examples that gravitation is also of importance in the distri-
bution of pelagic animals at various depths, and that it is
this force which compels certain animals to live iii the sur-
face regions of the water.

Everyone who will watch the animals found about the
rocks or piles near the surface of the ocean during a quiet
sea will notice the relatively large proportions of Echiuo-
derms. Some of these Echinoderms such, for example, as
Cucumaria cucuuiis which is found in great numbers in the
Bay of Naples live near the surface of the water, or not more
than HO in. below it. It can be readily shown, however, that
Cucumaria cucumis is, like the Plasmodia or the Coccinellas,
compelled to crawl upward on vertical surfaces, and that
apparently gravitation determines this behavior. Cucumaria
cucumis has an elongated pentagonal body some 10 or more
cm. long, carrying at its oral pole radially arranged arbores-
cent tentacles. Upon each of the five edges are found
two parallel rows of tiny feet by which the animal is
enabled to crawl upward, even upon smooth glass plates. If
these animals are introduced into an aquarium, they creep
about the bottom until they reach a vertical wall, up which
they climb to remain at its highest point, and when possible
immediately under the surface of the water. The animal
remains permanently in this position and behaves like a
sessile animal.

If such a Cucumaria cucumis is permitted to attach itself
to a vertical glass wall which can be turned about a hori-

i GEOOM DXD LOEB, Biologisches Centralblatt, Vol. X (1890).

GEOTROPISM IN ANIMALS

1st

? ^TT-

n

n

-n

-fi
A

-IZHEZ

- . .

^6^

ft

a

/^

&^=-:-

- 1

A " rJy - ~ =

xontal axis in the . aquarium, the animal endeavors unceas-
ingly to creep upward as often as the plate is turned through
an angle of ( .M) . We are not dealing in this case with a
compensatory motion brought about by centrifugal force;
while the plate is being turned the animal remains quiet,
and not until some fifteen or
twenty minutes later does it
begin to move upward.

Nor do we deal in this case
with the effect of daylight
entering the aquarium from
above. If the animals are kept
in an aquarium into which light is allowed to enter by suit-
able means only from below and without, the animals con-
tinue to creep up the vertical surfaces, the direction in which
they move not being influenced in any way by the light.

One might think that the need of oxygen determines the
movements of Cucumaria toward the surface of the water,
but it can be shown that this also is not the case. If a large
beaker, from which the air has been removed by filling it
with water, is placed in an aquarium upside down that is
to say, with the bottom of the beaker directed upward the
Cucumarire nevertheless continue to creep up to the bottom of
the beaker. They do this even when the experiment is made
as shown in Fig. 36. A bridge BB is suspended in the aqua-
rium ^4.4 so that its horizontal part B l B l is below the sur-
face of the water n in the aquarium. In the bridge is a
small circular hole o, over which the inverted beaker ahcd,
in which the air has been displaced by water, is placed.
Fresh water is supplied through o under slight pressure
by means of a suitably curved glass tube </. The Cucu-
niariie nevertheless leave the neighborhood o, collectingf

O

either on the bottom of the beaker cd or upon its vertical
sides near cd.

182 STUDIES IN GENERAL PHYSIOLOGY

Yet the hydrostatic pressure does not compel the animals
to move upward to the surface of the water, for when the
animals are put into a flat dish containing only 1-2 cm. of
water, so that the animals are just under the surface of the
water, they also move to the vertical wall of the vessel and
attach themselves to it, even though they are no nearer the
surface of the water in this position than anywhere else in
the aquarium. The nature of the conditions of the experi-
ment shuts out the possibility of rheotropisin and hydrot-
ropisin.

Experiments with the centrifugal machine yielded no
results, as the animals do not move at all during the rotation
of the machine. The one condition which compels the
animals to seek vertical surfaces and crawl upward is gravi-
tation. I imagine that the way in which gravity compels
the animals to move upward is similar to that which compels
insects, such as the butterflies which have just left their pupa
case, to crawl upward. Immediately after hatching, the
wings of the butterfly are not yet unfolded, and the animal
runs about in a restless way until it reaches a vertical sur-
face upon which it creeps and remains, with its head directed
upward, for a relatively long time, until the unfolding of the
wings or other conditions cause the animal to become rest-
less again. In cockroaches this dependence of rest and
unrest upon gravitation is still more apparent. They remain
perfectly quiet only when the weight of their bodies exerts
a pull upon their legs, but not when the weight of their bodies
presses upon their legs. Cucurnaria cucumis seems to behave
in a similar way. The direction of the pull, however, influ-
ences in addition the direction of the progressive movements
upward. Yet it is possible that these reactions in Cucuniaria?
and insects are dependent upon specific organs, as in the case
of vertebrates.

This relation of Cucumariae to gravitation compels the

GEOTROPISM IN ANIMALS

animals to live in tin- upper regions of the sea. It' a larva
were driven into deep water, it would be compelled to crawl
upward without ceasing, through its negative geotropism,
until it had reached the surface again, or until death brought
an end to these efforts.

2. Other marine animals, which we also know to live only
in the upper regions of the sea behave as does Cucumaria
cucumis. Actinia mesembryanthemum of the Bay of Naples
belongs to this class. If I remember correctly, however, I
have never observed this behavior in the Actiiiian of the East
Sea, which has the same name, but differs somewhat in its
characteristics, from that found in the Bay of Naples. But
certain starfish, such as Asterina gibbosa, are also negatively
geotropic. All the experiments which I have made on Cucu-
maria are successful also in Asterina gibbosa, with this
difference, however, that when the exceedingly voracious
Asterina has reached the highest point on a vertical surface
and finds no nourishment there, it does not remain in this
position permanently, as does Cucumaria, but begins to move
or drop down after two days or even less time. Asterina
gibbosa also lives only at the surface of the sea.

Positive heliotropism acts, of course, in a way similar to
negative geotropism. Asterina tenuispina lives near the sur-
face of the sea, as does Asterina gibbosa. It is, however, not
ge< (tropic, but positively heliotropic. I laid a large number of
the two species together in a heap in an aquarium into which
light fell almost horizontally from one side only. Within a
short time the two kinds of animals were clearly separated;
the tenuispime crawled along the bottom toward the soun-e
of light; the gibbosa? distributed themselves in all directions
over the bottom and crawled up \vard upon the vertical walls
without the direction of their movements bring influenced
by the light. In the ocean, in which only those rays of light
which enter vertically are of importance for the spatial

STUDIES IN GENERAL PHYSIOLOGY

orientation of the animals, positive heliotropisin must compel
Asterina tenuispina to move to the surface of the water in
the same way as negative geotropism does Asterina gibbosa.
Since Asterina teuuispina is always positively heliotropic
(and does not alter its heliotropic sense as do the iiauplii of
Balanus), periodic depth-migrations do not occur in it as
they do in the nauplii of Balanus because of the change in
the heliotropic sense of the latter.

3. Preyer briefly mentions in his exhaustive work on the
movements of starfish "the tendency of these animals to
crawl upward" which he noticed in his observations on the
climbing movements of starfish. Preyer writes as follows:

The great tendency of starfish to move upward cannot be
dependent upon lack of oxygen, lack of food, or changes in the
temperature, or currents in the water, nor to a desire for light, for
the upward movements occur also where these conditions are not
at work. Some property of the bottom, or of that particular spot
on the bottom upon which the animal rests, and which has become
unfit for the attachment of the animal or its prolonged residence,
probably compels the animal to climb upward. Yet parasites
which I often found in the ambulacra! feet of Luidia may also
cause them to move upward, in so far as these stimuli may appear
to the animal as originating from the horizontal bottom. 1

The first sentence in this generalization is incorrect, as
light is indeed the factor which compels Asterina tenuispina
to crawl upward. It is shown by the following experiment
that the nature of the bottom upon which the animal rests
does not compel it to move upward. If Asterina gibbosa is
placed in a cubical box having glass sides, the animals leave
the horizontal basal wall and climb up the vertical sides. If
the box is turned through an angle of 90 about a horizontal
axis, the animal leaves the wall, which has now become basal,
and climbs up to, and remains upon, the wall which it left
before when it lay horizontally. The character of the wall was

1 W. PEEYEB, Mittheilungen aus der zoologischen Station zu Neapel, Vol. VII,
p. 96.

GEOTROPISM IN ANIMALS 1 v~

therefore not the cause of the movement of the slai-fisli.
Finally, when Preyer believes that parasites compel the
animals to move upward, it is impossible to see why the
parasites do not compel the animals which are fastened upon
the vertical wall to leave it, as the stimuli caused l>y the
parasites must now seem to the animals to originate from the
vertical wall. Yet in reality Asterina gibbosa (and much
more Cucumaria cueimiis) remains upon the highest point
of the vertical wall. I believe it much more rational to take
into consideration the effect of gravitation, which acts in a
vertical direction, in explaining the upward movements of
certain starfish.

III. THE GEOTROPISM OF HIGHER ANIMALS AND ITS
DEPENDENCE UPON THE INNER EAR

The higher animals are also compelled, within certain
limits, to assume a definite orientation toward the center of
the earth. It is easily seen in many fishes that they orient
themselves toward the center of the earth, both while swim-
ming and while at rest, and that they direct their bellies,
but never their backs, downward. If we try to compel such a
fish to lie upon its back, it endeavors and succeeds, as soon as
liberated, in reassuming its usual orientation. It is not
physically impossible that such a fish should swim with its
back downward, and only physiological factors are present
which compel the fish to direct its belly toward the center
of the earth. Kven \ve are compelled to assume a certain
position with reference to the center of the earth; we dis-
cover it when we bend our heads so that the top of the head
is downward. The force which compels us to assume a defi-
nite orientation toward the vertical is naturally not very
great, yet it has a definite intensity, and it requires an
external stimulus of a certain intensity, or a definite effort of

'

the will, to act against this force in order to overcome it.

186 STUDIES IN GENERAL PHYSIOLOGY

There is a second, better observed and more accurately
studied, effect of gravitation upon higher animals. This has
to do with the axes of the eyes, which are also compelled to
assume a definite position with reference to the horizontal.
If the head of a fish is forcibly brought into a different posi-
tion from that which it occupies normally with reference to
the center of the earth, the eyes tend to reassume, either
entirely or in part, their old orientation. These lasting
changes in the position of the eyeballs with reference to the
axes of the head which accompany a permanent change in
the position of the head are also present, as is well known, in
human beings. They can in this case, as in the case of
animals, be compensated through appositely working optical
conditions or internal stimuli, but a definite force exists in
this case also to compel the axes of the eyes to assume
their proper angle with the horizontal.

Light has nothing to do with these phenomena; they
occur also, as is well known, in the dark, and in persons
totally blind.

We deal rather in these cases, as we know, with the
activities of a definite organ, namely, the inner ear. Schrader
found that frogs from which the inner ear has been removed
upon both sides no longer endeavor to regain their position
when laid upon their backs, 1 and Breuer has corroborated
this observation. 2 According to a paper by Sewall, 3 to
which I do not, however, have access, and with which I am
acquainted only from the abstract which Breuer gives of it,
the compensatory movements of the eyes in sharks and skates
were seriously affected by such lesions of the labyrinth as
led to decided and permanent disturbances in their move-
ments. These experimental facts, indicative of the depend-
ence of geotropic orientation upon the ear, are not numerous

1 SCHEADER, Pfliifjcrs Archiv, Vol. XLI.

2 BREUER, Pflilgers Art-hie, Vol. XL VIII, p. 237.

3 SEWALL, Journal of Physiology, Vol. IV.

GEOTROPISM IN ANIMALS 1ST

compared "with those t>n the dependence of movements upon
the ear. And so it happens that Aubert and Delate'
acknowledge tlie latter relation, but do not, as it seems to
me, acknowledge the former. I myself do not doubt the
belief of Breuer and Maclr that the organs associated with
the permanent changes in the position of the eyeballs, in
movements of the head, lie in the inner ear ; and I think it
probable, in further accord with these authors, that these
organs are more especially the otoliths. I therefore add the
following results, obtained in a large series of experiments
on the inner ear of the shark (Scyllium-canicula), principally
to show that my ideas of the importance of the inner ear in
the geotropism of the higher animals rest upon personal
observations. 3

I. If the otoliths are removed (either with a pair of
forceps or a small sharp spoon) from a shark (Scyllium-
canicula) upon one side, say the left, the following changes
occur in the animal in its orientation toward the center of
the earth:

1. The animal no longer swims, as does the normal, so
that its medium plane is vertical, but it has a tendency to
turn the left or operated side downward at an angle of from
20 to 50, or even more, with the horizontal.

2. The same change in the orientation can be noticed
when the animal is lying quietly upon the bottom of the
aquarium. It then frequently lies resting upon the left
lateral fins, while the right fin often does not touch the
bottom of the dish.

3. When the animal is in the normal primary position

l AfHKRT. Physiologische Studien ill" r <ti<- <>ri, ntintn<i (Tubingen, 18>M.

2M.\< ii. (iriiiKiliiiii-H tier l.fhrc ron /icn Beivegungsempfindungen (Leipzig, 187.")).
p. 110.

3 In an addendum to his paper on the l-'inn'tiiinn of the Ct-nlnil .Yc/vonx S//S/CHI,
etc. M Irann -el i we it,'. l xy ^ ', S i i.i SKI; le -cribes his experiments <>n "I In >e mi -circular
canals of sharks." This author has overlooked the facl that it is the function of the
inner car to call forth compensatory motions and positions, ami so has failed to
make observations cm the geotropic functions of the ear. (See M \i 11 and IJUEUER.)

188 STUDIES IN GENERAL PHYSIOLOGY

its eyes are rotated more or less about the long axis of the
body toward the left, so that the left eye looks downward,
while the right eye looks upward. The (persistent) geo-
tropic movements of the eyeballs upon changing the orienta-
tion of the animal toward the center of the earth still occur,
with this modification, however, that when the whole animal
is turned about its longitudinal axis the amount of the com-
pensatory movement of the eyes is added algebraically to
that caused by the lesion of the ear.

4. Only a slight change is noticeable, usually, in the posi-
tion in which the pectoral fins are kept. The left lateral
fin is turned toward the back while the right is turned
toward the abdomen.

II. If the otoliths of such an animal are removed from
the other side also, all the phenomena described above dis-
appear. Instead, however, now the following abnormalities
appear:

1. The animal is no longer compelled to turn its ventral
side toward the center of the earth. If one carefully attempts
to lay it upon its back the animal does not resist, and when
prevented from falling over, it remains permanently upon its
back. When left to itself, the animal is often found lying
upon its back, or swimming in this position, even when it is
entirely well and vigorous.

2. The permanent alterations in the position of the eye-
balls, when the orientation of the animal toward the center
of the earth is changed, are lacking.

III. If the ant rum is opened and the same injuries are
inflicted as are necessary to remove the otoliths without,
however, injuring these or the nerves of the ant. rum the
phenomena described under I and II do not appear. Nor is
this the case when large pieces are removed from the semi-
circular canals without injuring the ampullae.

IV. If the left auditory nerve of the animal is cut, the

GEOTROPISM IN ANIMALS 1MI

|)lu'iKniiena described under I are observed, only in a greater
degree. If both auditory nerves are cut the phenomena
described under II set in.

A'. It' the otoliths are removed from one side and the
auditory nerve is cut upon the other, the animal behaves as
those described under II; with this difference, however, that
the eyes of the animal are turned about the longitudinal
axis of the animal toward that side where the auditory nerve
is cut.

Also, as regards the so-called forced movements which
are not considered in this paper the animal behaves as if
it had been operated upon only 011 one ear, namely, that whose
auditory nerve is cut.

VI. A shark whose head has been cut off, and which, as
is well known, still swims, is no longer forced to assume a
definite orientation toward the center of the earth. When
laid upon its back it no longer attempts to reassume its
accustomed position with its ventral side downward.

The disturbances in the geotropic orientation which fol-
low the cutting of the auditory nerve have thus far contin-
ued for a month.

ohxcrrafioiix, it scents to me, pro re f/iat fJte c/co-
phenomena oltscrrcd in tltc shark arc dc/x-mlcnf n/ion
flic inner car, and support the assumption of Brener and
Much that thc;i arc called forth It// flic otolilhs.

But we can determine how the inner ear forces an ani-
mal to maintain a certain position with reference to the
horizontal as little as the botanists can explain how geotropic
curvatures are called forth in plants. We can only say. in
harmony with Goltz, 1 or Mach and Breuer. that the animal
comes to rest only under a certain arrangement of strain and
pressure in the peripheral ends of both auditory nerves.
This arrangement exists in the shark when the animal turns

> GOLTZ, Pflugers Archiv, Vol. 111.

190 STUDIES IN GENERAL PHYSIOLOGY

its ventral side toward the center of the earth, and keeps its
longitudinal axis almost horizontal ; but in every other posi-
tion of the animal the altered distribution of strain and pres-
sure upon the ends of the auditory nerves forces the animal
again to assume its customary angle with the horizontal.
This force is maximal when the animal lies on its back. If
one of the auditory nerves is cut the animal is compelled to
assume an oblique position in which the injured side is
directed downward more than usual ; if both auditory nerves
are divided, no force exists to compel the animal to assume
any definite geotropic position. 1

1 will give a single example to illustrate what may be the
further biological significance of geotropism. The reader is
acquainted with the marked difference in the pigmentation
of the ventral and dorsal sides of many animals, especially
that of the fishes. This difference is to a large extent inde-
pendent of light and is determined by conditions which accom-
pany development. In part, however, it is directly dependent
upon light; the back, which is permanently directed toward
the source of light, must become richer in pigment than
the ventral side. Cunningham has, indeed, been able to
show recently that flatfish develop pigment upon their lower
surfaces in an abnormal way when these are illuminated by
the help of mirrors, as strongly as their upper surfaces. 2

1 1 no longer believe that the direction of the waves of sound has any effect,
which I considered possible in my preliminary paper on geotropism.

2 J. T. CUNNINGHAM, Zoologische Anzeiger, Vol. XIV (1891).

VI

ORGANIZATION AND GROWTH 1

I. THE DEPENDENCE OF ORGANIZATION IN ANTENNULARIA
ANTENNINA UPON THE ORIENTATION OF THE ANIMAL
TOWARD THE CENTER OF THE EARTH

1. THE difference between physiological morphology and
the purely descriptive morphology lies in this, that the for-
mer endeavors to control the formation of organs. Its aim
is, therefore, primarily a technical one. Since almost every-
thing is still to be done in this direction, at present those
cases will be especially welcome in which the means of con-
trol of the morphogeiietic processes are very simple and yet
perfect. I have found that in Antennularia antennina, a
Hydroid, it is possible to control the morphogenetic process
with the aid of the position given it with reference to the
center of the earth. By this means we can not only at will
cause another organ to grow in the place of an amputated
one, but we can compel an uninjured existing organ to stop
growing in its old form and form an entirely different organ.

2. In Antennularia antennina (Fig. 37) a straight
unbranched stem SS, about 1-2 mm. in diameter and often
more than 2()cm. long, arises from a mass of roots W. Deli-
cate lateral branches of limited" growth and directed slightly
upward spring from the stem in regular order. Upon the
upper surface of these are polyps and nematophores, which
are indicated only by points in the drawing. In order to
save room, only the lower third of the stem has been drawn.

I have previously pointed out that Antennularians show
a decided geotropisin; that is to say, the growing parts of

'WQrzburg: Gcorg Hertz, 1891. Although dated 1W2, thi- pamphlrt appeared
in 1891.

191

STUDIES IN GENERAL PHYSIOLOGY

these animals are compelled to assume a definite orientation
toward the center of the earth.' If the growing stem of an
Antennularian is put intj any other than a vertical position,
the growing tips bend until the stem again
stands vertically, after which it grows straight
upward. The root, upon the other hand, grows
vertically downward, but not so directly as the
stem. The stem is negatively, the root posi-
tively, geotropic. The stem <il> ( Fig. 38) origin-
ally stood vertically. I tilted it so that it
formed an angle clxi with the vertical; the newly
formed piece he grew vertically upward after
this change in position. I then put the stem
back into its old position, after which growth
continued in a vertical direction c<l. Fig. 39
also shows an upward bending of the stem.
Roots growing downward are shown in W (Figs.
40 and 41 ). While the tip of the growing
stem does not weary of growing vertically
upward whenever its position toward the
horizon is slightly altered, the root is
much more sensitive. It does not bear

FIG. 37

FIG. 39

changes in position very well, and I have been able to
demonstrate positive geotropism only in newly grown

1 Part I, p. 176.

ORGANIZATION AND GROWTH

roots. Light has no deinon-
orientation of Antennularia.

one side only, the animals
and the same occurs also in
uli, however, affect the orien-

.

strable effect upon the
Even when lighted from
continue to grow upward ;
the dark. 1 Contact stim-
tation. When the roots

w

FIG. 40

\

X
X

are brought in contact with a solid body, they attach them-
selves to it and grow over its surface. The stems and branches
do not possess this
form of irritability.

3. With these pre-
liminary remarks, I
shall show how regen-
eration and heteromor-
phosis depend upon
the orientation of the

1 \fl/ /

\

\

\

W

> i w

FIG. 41 FIG. 42 FIG. 43

stem with reference to the hori/on. (th (Fig. 42) represents
a piece cut from the stem of an Antennularian ; a is the

'The specimen grew only slightly in the dark. This may, however, have hap-
1 only by chance.

STUDIES IN GENERAL PHYSIOLOGY

\

apical and /> the basal end (root), ab is suspended ver-
tically in the aquarium so that both cut ends are surrounded
by water; a is directed upward, b downward. Complete
regeneration occurs, a new stem S being formed at <i, while
one or more roots W are formed at b; the former grows

upward, the latter down-
ward.

We repeat the same
experiment with another
piece of stem, only this
time the basal end b is
directed upward, while the
apical end a is directed
downward. A stem S ?'s
formed at the basal end
ir/iich is directed iifi/rard,
while one or more entirely
normal and growing roots
are formed at t/ie ajtieal
end ir lit eh is di reefed
downward (Fig. 43). Deli-
cate lateral branches spring from the new stem which are
directed slightly upward arid carry polyps upon their upper
surfaces. The new branch is an image of the old. In this
way a heteromorphosis is readily brought about in Antennu-
laria.

4. If a piece of the stem of Antennularia is suspended,
not vertically, but obliquely, in the aquarium (Fig. 44), a
stern S which grows vertically upward arises from the upper
cut end ; from the lower end 6 spring roots which grow verti-
cally downward; it matters not whether b is the apical or
the basal cut end. Thus far this experiment shows nothing
new. But branches /Sj and S^, and roots W ; and W ii5 may
arise from any other element of a stem put obliquely. The

FIG. 44

ORGANIZATION AND GROWTH

essential point is again this, that the place of the new growth
is determined by the orientation of the stem toward the cen-
ter of the earth, inasmuch as sf<-nis arise only from the ii)>]>er
surface, tin- roofs only from Hie loirer surface, of ihe element.
That a new stem may arise from the upper surface of any

FIG. 15

element of a stem which is put obliquely is evidenced by the
fact that such stems arise at times from the upper surface of
the lowest elements of the animal. Such a case is shown in
Fig. 4-~>. The stem of an Antennularian was bent so that
the two ends were directed obliquely downward. New stems,
$,, S.,, *S.,, S t , , have formed upon the upper surfaces
of different elements of the old stem. ,S, has grown from

196

STUDIES IN GENERAL PHYSIOLOGY

the upper surface of the lowest element. That stems are
formed upon the upper, while roots are formed upon the
lower, surfaces of the stem, when it occupies any other
than a vertical position, is best shown when stems of an
Aiitennularian are suspended horizontally in the aquarium

so as to be surrounded on all
sides by water. If the experi-
ment is continued for a long
time, other phenomena occur,
especially root formations, a
discussion of which we shall
give later. If this fact is borne
in mind, Fig. 40 may serve to
illustrate such an instance. In
Fig. 41 is shown how a new
stem arises from the basal cut
end and from the upper surface
of a stem lying horizontally.

II. CONTINUATION OF THESE
EXPERIMENTS AND HETERO-
MORPHOSIS IN UNINJURED
ORGANS

1. It is not only possible
by the aid of gravity to cause
the growth of another organ
in place of one that has been
lost, but we can also cause an
uninjured organ to grow into
a typical organ of a different kind by making use of the
same force.

If a piece is cut from the stem of an Antennularian, and
this is laid horizontally into the aquarium so as to be sur-
rounded by water, the tiny lateral branches situated on the
under side of the stem begin to grow after some time

FIG. 46

ORGANIZATION AND GROWTH

;i!>out one or two weeks. But they do not grow in the old
direction obliquely toward the tip of the stem, but instead
vertically downward. Nothing of this sort happens to the
branches situated upon the upper surface. The newly formed
organs which grow vertically downward look exactly like

FIG. 47 FIG. 48

roots, and that they are indeed such can be proved physio-
logically. If they are brought in contact with a solid body,
they attach themselves to it and grow over its surface. Be-
sides being positively geotropic, they are also positively
stereotropic a form of irritability possessed only by the
roots, but not by the branches, of Antennularia. Such an
Anteniiularian stem is shown in Fig. 40, in which all the
branches of the lower surface have been transformed into

198 STUDIES IN GENERAL PHYSIOLOGY

roots. Several of the branches on the upper surface in the
neighborhood of the growing stem cd have also become
roots, but this is an exception which we shall discuss later.

2. I cut some long pieces from the stem of an Antennula-
rian, bout them, and hung them into the aquarium, as shown
in Figs. 4."), 4(5. 47. Both ends were oriented in the same way
toward the center of the earth. After some time (Fig. 4tJ)
the branches of the under surface of the nearly horizontal
piece ab began to grow as roots. Some time later the same
occurred in all those branches, the tips of which were
directed downward. In Fig. 47 is shown a second specimen
of the same series of experiments, but drawn six week later.
All the branches from a to b, the tips of which were directed
downward, have grown into roots. By carefully studying
Fig. 45 this phenomenon can again be observed; only I
must call attention to the fact that this drawing was made
very shortly after the beginning of the experiment, and that the
formation of roots had consequently not progressed very far.

Macroscopic-ally the course of the heteromorphosis of a
branch is as follows: The tip of the branch situated upon the
lower side of the stem dies, and the polyps and nematophores
disappear. The new root then sprouts from the free distal end
of the branch, without any operative injury whatsoever hav-
ing been inflicted upon the branch.

3. While I always succeeded under the conditions de-
scribed above, in causing the lateral branches to grow into
roots, I succeeded only once in making them grow into a
stem. This is shown in Fig. 48. A short piece ab of an
Antennularian stem was laid horizontally in the water.
After eorne time the tip of one of the lateral branches c
began to grow vertically upward. No new lateral branches
were formed upon it. A similar negatively geotropic new
growth arose from the aboral cut end at a.

4. The heteromorphoses possible through changes in the

ORGANIZATION AND GROWTH

199

orientation of Antennularia toward the center of the earth
are not exhausted by what has been said in the foregoing.
It is possible to cause the growing tip of the stem to cease
its growth and develop into a root. This is done by invert-
ing the tip. I out long pieces from the stems of Antennu-
larit and hung them vertically
in the aquarium, but in an
inverted position. A rapidly
growing stem was formed at
the upper that is to say, the
basal end. When it had
attained a certain height, I
turned the whole animal about
a horizontal axis through an
angle of 180. Not in a single

c) *'

instance did the tips of the
stems bend upward- -as the
negatively geotropic parts of
plants usually do under the
same conditions but they
eeaxcd. to grow and one or more
roots formed, at their tips.
Fiir. 49 shows a stem at the

O

end of such an experiment;

ah is the piece of stem upon

which the experiment was

started. It was at first fixed

in an inverted position, so that the basal end 6 was directed

upward, and the apical end downward. The new stem be

sprang from the basal extremity, grew vertically upward,

and gave rise to lateral branches which were directed slightly

upward and carried polyps upon their upper surfaces. The

roots W l formed al l>.n\ the base of the new stem, as was usually

the case when the old stem did not stand entirely vertical.

FIG. 49

200

STUDIES T\ GENERAL PHYSIOLOGY

FIG. 50

From the lower apical end sprang the downward-
growing roots W, which remained much shorter
than the roots W l . A complete heteromor-
phosis had therefore been produced, a
root having hern formed in place of the
old tip, and a stem in place of the
root. I now turned the whole animal
about a horizontal axis through an
angle of nearly 180, by which
process the animal was placed
in the position shown in the
drawing. The arrow indi-
cates the direction of the
vertical. The tip of the
stem which was now
directed downward
ceased to grow, but
the stem retained
its vigor and its
normal appear-
ance. Several
- roots IV.

3 '

its

were formed
at its lower end
c, and, besides,
other roots, W
sprang from
lower surface. These
perhaps came in part
or entirely from the lateral
branches. The old roots
at a continued to live, so
that the animal terminated in
roots at both ends.

The same series of phenomena,
differing only in detail, is shown in
Fig. 50. The stem ab was suspended
obliquely in the aquarium, the apical
end b being higher than a. The stem
de sprang from the surface which at that
time had been uppermost, and from an ele-
ment lying fairly low down upon the stem.
Roots W sprang from the point which origi-

ORGANIZATION AND GROWTH 201

nally lay opposite <l<\ and from the lower cut end a of the
stem. When the new stem had attained the length <lc,
I turned the whole animal about its axis, so that b and a
were directed downward, and the specimen was in the posi-
tion indicated in the drawing. The stem <le ceased to jn-ow,

O O

and a delicate root TF 1? growing downward, was formed at
c. The drawing was made soon after the root began to
grow. Besides being formed at e, roots W 5 also grew from
the apical cut end b which was directed downward ; the three
stems S 2 , S 3 , $ 4 , were formed upon the upper surface of
the stem with their corresponding roots TF 2 , W 3 , W 4 .

5. In all these experiments one fact which is, however,
not a specific property of Antennularia constantly repeats
itself : new growths arise much more easily and numerously
from the basal than from the apical end. I have observed
the same fact in Actinia mesembryaiithemum, and A. von
Heider mentions it in connection with Cladocora. 1

In general, too, the formation of roots in Antennularia is
dependent upon the formation of a stem'. If a stem is
formed at any point, roots also form after some time, at first
upon the under surface of the old stem, then upon the upper
surface in the immediate neighborhood of the new stem,
and finally at the lower end of the new stem itself (Fig. 40).
It is self-evident how such phenomena complicate the
influence of orientation upon the place where new organs are
formed.

0. I cannot say precisely how the position of the new
organs formed in Antennularia is determined by the orienta-
tion of the animal toward the center of the earth. Naturally
the effect of gravitation cannot be different in animate from
what it is in inanimate nature, and there would be no reason
for being disappointed should it be found that the apparently
mysterious rflVrt of gravitation upon organization in Anteii-

1A. VON HEIDEK, Wiener Sitzungsberich/c, Vol. LCCCIV, Part I (1881), p. 664.

202 STUDIES IN GENERAL PHYSIOLOGY

nularia consists, for example, in simple hydrostatic effects.
It would also be entirely useless to ask whether gravity is
"the cause" of organization in Antennularia ; naturally it is
only one of the conditions determining organization. Under
ordinary conditions, however, it is of paramount importance. 1

I add these remarks since it seems to me that the work
of Ptiilger on the etl'ect of gravitation upon the division of
cells is misunderstood in some of its points, and its value to
the further development of physiological morphology is not
appreciated as it should be.

7. I shall in conclusion describe an experiment which I
found in the botanical literature on the transformation of
organs in plants through external forces. The reader may
see from it that plants and animals are not essentially differ-
ent in regard to the physiology of organization.

I choose as an illustration Noll's 2 experiments on
Bryopsis, which perhaps represent the most successful
experiments ever made on the control of organization in
plants through external forces. According to Noll, the

so-called "leaf -barbs" (leaves) of Bryopsis nmcosa arise near the
tip of the upright stem. The}' are hollow tubes which spring 1 from
the main stem in two rows at an angle of about 45. The roots
arise from the lower part of the plant and grow away from the
light into the ground, where they become attached to the particles
of earth much as do the root hairs of the higher plants. The
stems and leaves do not show this reaction to contact stimuli.

The morphological differentiation of the organs seems to
be much less marked in Bryopsis than in Antennularia,
for, according to Noll, "by examination of a single organ
doubt can easily arise as to whether the fragment originates
from a root, a stem, or a leaf tubule." In a normal Antennu-

!In his Grundriss der Naturlehre, MACH defines cause and effect as follows:
" Die auffalleudste Bedinguug der eingetretenen Veranderung pflegt man die
Ursache, die Veranderung selbst die Wirkung zu nennen."

-F. NOLL, Arbeiten ties botanischen Instituts in Wiirzburg, Vol. Ill (Leipzig,

1888).

ORGANIZATION AND GROWTH 203

laria it is an easy matter to diagnose the organ from which
a fragment lias originated. If such a Bryopsis is laid hori-
zontally upon the ground, "the growing stern becomes erect
at its tip, the leaf tubules again grow obliquely upward at
their usual angle, and the root tubules grow downward into
the dark earth." Noll then tried "whether a root could be
produced from the tip of the stem, and a new stern from the
root tubules." He suspended plants from which the roots
had been cut in glass tubes in an inverted position, that is,
with the tip downward. When the experiment was brought
to a close after some time,

the tip in a number of plants had greatly increased in length, but
instead of growing upward it had grown downward into the sand,
was bent, and intimately connected with the grains of sand ; in
short, it had been transformed into a typical root tubule. Even
the few leaves which had been formed had also grown into roots.
Another series of these specimens did not show this transformation ;
in these the tips turned upward at an acute angle and returned in
their old direction ; they remained stems.

In regard to the latter point the behavior of Antennularia
is essentially different. When the tip is directed downward,
its growth as a stem ceases immediately ; never under these
circumstances does it bend so as to grow in its old upward
direction.

Further growth can now take place only in so far as the
tip continues to grow as a root. In NolPs experiments a
tubule which grew upward arose at the upper basal end, and
in some cases showed a beginning barbule formation.
According to Noll, light is the essential factor in the control
of the morphogenesis.

III. THE ARTIFICIAL PRODUCTION OF A VARIETY OF
ANTENNULARIA ANTENXINA

The main stem of Antennularia aiitemiina is uiibranrhed.
In the hundreds of specimens that passed through my hands

204 STUDIES IN GENERAL PHYSIOLOGY

in Naples I did not find a single exception to this rule.
When a piece was cut from the stem of Antennularia anten-
niiia and a new stem grew out of it, either through regenera-
tion or through heterouiorphosis, the newly formed stem
had, under ordinary circumstances, all the characteristics of

the old it grew upward with-
out branching.

Under certain external con-
ditions, however, an entirely
different kind of stem was
regenerated. At first a short
stem grew straight and verti-
cally upward, but from this soon
sprang a new branch having the
diameter of the old. Since
every branch one after another
gave rise to other new lateral
branches, which after a time no
longer grew perfectly vertically
upward, forms of Antennularia
with many ramifications
originated. Faithful represen-
FIG -> l tations of three such forms are

given in Fig. 51. They all arose from stems lying absolutely
horizontal, or nearly so, two at the apical end and one at
the basal end. No branches were formed.

I have, moreover, made experiments to decide whether
pieces of the old stems which under abnormal conditions
gave rise to variations would produce unbranched or branched
antennina stems when brought back to normal conditions.
Under these conditions they formed only unbranched anten-
nina stems.

Under the abnormal conditions, however, not all the stems
branched; this happened only in part of the specimens.

ORGANIZATION AND GROWTH

I shall have to continue the experiments before I can
discuss the conditions which determine the formation of
these abnormalities.

IV. ON THE INTERNAL CAUSES OF ORGANIZATION IN
TUBULARIA MESEMBRYANTHEMUM

1. Iii most animals at a cut edge with a definite orienta-
tion only one kind of organ originates. If the tail of a
lizard or a salamander is amputated, only a tail and not a
head is regenerated in its place, and a lobster develops a new
pincer in place of the one lost, and never anything else. 1
Nor can it be said that only the higher animals show this
inflexibility in organization. As shown by all the experi-
ments which have been made upon it for a hundred and fifty
years, Hydra behaves thus; even in Infusoria Nussbaum
found such a relation to exist between the new growth and
the orientation of the wound." In plant physiology the
instances in which complete control of organization through
external forces is possible are very scarce. In the majority
of these experiments, also, it seems that internal, and at
present unknown, factors essentially determine the position
of the organs. If we wish to control these internal con-
ditions in animals, we must try to obtain further information
concerning them through experiments. In order to give the
reader a clearer idea of the role of these internal causes in
animal morphogenesis I will remind him of the experiments
upon Cerianthus rneinbranaceus, given in the first volume of
my Physiological Morphology. 9 The oral pole a of Cerian-
thus differs markedly morphologically from the aboral pole b
(Fig. ~>'2). To be brief, I will mention only one difference-
that tentacles arise from the oral pole. If a rectangular piece

1 This is no longer correct. Herk-t has shown that in Crustaceans in the place

of an eye a new eye or a new tentacle can he produced at desire. | r.10,'5]

2 M. NUSSBAUM. Archiv filr mikroskopische Anatomic, Vols. XXVI and XXIX.
8 See Part I, p. 115.

206

STUDIES IN GENERAL PHYSIOLOGY

i-tlcf is cut from the body of an Actinian, tentacles are
formed upon only one of the four cut edges, namely, upon
the oral surface ef. From this and similar experiments
which may be read in the original it follows that the place
where the new tentacles are formed in a fragment of Cerian-
thus is determined by the orientation which
the fragment had in the intact animal; just as
in every fragment of a broken magnet the
position of the poles is determined by the
orientation of the piece in the unbroken
magnet. From this analogy which, how-
ever, goes no further the term "polarity"
has recently been applied to the organization of
such animals. According to this idea, Cerian-
thus would be a typical "polarized animal."

Now, Cerianthus is not a satisfactory sub-
ject for investigation of the causes of "polar-
ity ;" Tubularia mesembryanthemum, a Hydroid, adapts itself
much better to these experiments. 1 Tubularia ends in a root
at its aboral end, and in a polyp at its oral end. If a piece
is cut from the stem, and both cut ends are surrounded by
water, a heteromorphosis results, as I have shown in a former
paper. A polyp arises from both cut ends, and a bioral animal
is obtained. Yet the formation of polyps is different at the
two ends in one particular: it is always more rapid at the
oral than at the aboral pole. In so far as this fact renders
it possible to determine which was the oral and which the
aboral end in the uninjured stem, it can still be considered
as an expression of polarity. The following experiments
deal with the internal conditions which determine the differ-
ence in time between the formation of the polyps at the oral
and at the aboral ends in Tubularia.

The starting-point of these experiments is J. Sachs's

l See Part I, p. 115.

ORGANIZATION AND GROWTH 207

theory of organization. 1 Sachs assumes that "with differ-
ences in the forms of organs are connected differences in the
substances composing them," and that "from principles
which hold for all sciences \ve must assume that the differ-
ences must be derived causally from the differences in
chemical constitution" (p. 4'2~)). "We shall have to assume
the existence of just as many specific formative substances
as there are different forms of organs in the plant." The
specific morphogenetic substances are affected through
external conditions, especially gravity and light, "so that in
eel-tain cases the sjHitial arrangement of the different organs
is determined thereby." 1 The monstrous formation of an
organ at the place where normally another organ should
exist as in the case of heteromorphoses Sachs attributes
to an absence of the specific substances necessary for the
formation of the normal organ at this place, and the presence
instead of the specific formative substances of another organ
(p. 4r>4-). Sachs also explains why any regeneration what-
soever of roots or stems occurs in plants deprived of them.
"Why is it that the simple removal of a piece calls forth a
regeneration of organs at places where this never would
have occurred without some disturbing influences such as
the removal of the piece? 1 ' (p. 470). The answer is as
follows :

I assume that as long as a green-leafed plant with an upright
stem is nourished and growing, the specific formative substances
for the root flow- from the assimilating leaves to the system of roots
at the lower end of the stem, while the substances forming the stein
flow in a similar manner upward toward the growing points of the
Mem and the branches. It' a piece, is cut out of the stem or the
root, the cut surface in itself offers a barrier to the further move-
ment. The specific formative substances contained in it will, in
consequence, collect in the neighborhood of the two cut surfaces.
Those substances leading to the formation of roots will collect at what

i J. Suns, Ai-lii'ilt'H f/c.s' liiitiiiuxr/ii ii 1 iixt/ 1 ii tn nt H"i< rziiur(j. Vol. II

ISM;;, pp. ir,-. tisy.

208 STUDIES IN GENERAL PHYSIOLOGY

has thus far been the lower end of the piece, while those substances
leading to the formation of stems will collect at the upper end.
Since their flow is stopped here, which would have been impossible
in the uninjured plaut, they give rise to the roots and stems at the
corresponding ends. In a piece of leaf capable of regeneration both
kinds of formative substances will be moving simultaneously
toward the basal end to flow toward the stem; stopped
by the cut surface, they collect there to form stems
and roots simultaneously. (P. 470.)

a. Finally, "the different formative substances are
produced only in limited quantities" (p. -468).

Corresponding to this theory we assume that
there are present in the stems of a Tubularian
~d specific substances for the formation of polyps
which gather at both cut ends and thus bring
about the formation of polyps. These substances
must, however, cctcris paribus, collect more
quickly, or in sufficient amounts, sooner at the
oral than at the aboral cut end; and the cause of
this difference is to be determined by experiment.
3. It might at first be thought that the specific
substances necessary for the formation of polyps exist from
the beginning in greater amount at the oral end of a Tubu-
larian stem than at the aboral, and that a polyp is, in conse-
quence, formed more quickly at the oral than at the aboral
end. To ascertain whether this be true I amputated the
root and the polyp of Tubularians and cut the remaining
stem ab (Fig. 53) in half by a transverse incision between c
and d. All four cut ends were surrounded by water. If
now the substance necessary for the formation of polyps
were present in larger amounts in the oral half ac than in
the aboral piece bd, the former should form polyps sooner
than the latter. Yet polyps were formed at about the same
time upon both pieces, only the oral cut ends a and d
developed polyps much sooner than c and 6. I have already

ORGANIZATION AND (|RO\YTH

described tliis experiment in the first volume of my
Pltyxi<>l<>!ii{-<tl Morphology. During my last stay in Naples
1 repeated it, and found that in one case the polyps at a
were formed about twenty-four hours earlier than at <l.
But this difference is not great enough to explain the differ-
ence in time in all cases between the formation of polyps at
the oral and at the aboral end, which not infrequently
amounts to one or more weeks.

4. Another possibility is conceivable. The substances
necessary for the formation of polyps are formed in the
stems of the Tubularia, and move in both directions, but
the movement occurs first in the direction from root to oral
pole (upward), and more easily than in the reverse direction.
If this assumption were correct, then when we divide the
stem (ib as above, the polyp at the oral cut end (/ of the
aboral piece bd ought to be formed distinctly earlier than at
the aboral end c of the oral piece c, because the formative
substance of the polyp has to move in the first case in the
direction from root to polyp (upward) ; in the second, in the
reverse direction.

I picked out eight healthy specimens of Tubularia from
a colony, amputated root and polyp, and divided the remain-
ing pieces ab by a transverse incision between c and d. The
pieces ac were fixed in an inverted position, the pieces bd in
an upright position in narrow glass tubes standing vertically,
so that only the ends c and d were surrounded by water and
could form polyps. After three days two polyps were formed
on the d ends; upon the following day all the d ends carried
polyps. On this day the formation of polyps only just
began on the c ends in two of the specimens; not until four
days later was regeneration complete in all the specimens.

In a second experiment I used nine Tubularian stems.
After three days all the d ends had polyps, while regenera-
ation had only begun in two cends; only three days later

210 STUDIES IN GENERAL PHYSIOLOGY

had regeneration begun in all of these specimens. In
other experiments, also, the same results were obtained.
That the orientation of the Tubularian stem toward the
center of gravity has no effect I have mentioned in the
preceding paper iv; and I have corroborated this finding
by careful control experiments. I suggested in paper iv
that regeneration perhaps occurs more rapidly at the oral
pole because a distinct difference in the diameter of the
lumen usually exists between the two extremities. The
experiments cited above show that this suggestion is incorrect ;
there is no difference in diameter at a transverse section,
and yet regeneration always occurs much earlier at the one
cut end than at the other.

5. If the Tubularian stems used in the experiment are
very turgescent, and highly pigmented, the difference in
time between the formation of the oral and the aboral polyps
may be very slight, and amount to only a few days. Very
often one finds pale Tubularise which form polyps within the
usual time at the oral pole, in which, however, the polyps at
the aboral pole are formed only after one or more weeks.
I made the following experiment upon such animals : I fixed
eight animals (I) taken from one colony vertically, in the
sand, but with their oral ends downward, in order to prevent
the formation of polyps at these ends. At the same time I
suspended seven other animals (II) of the same colony also
vertically and in an inverted position in the aquarium, but
so that both ends were surrounded by water. These animals
had to form polyps at both cut ends. All animals were kept
in the same aquarium, and were therefore exposed to the
same temperature. After three days the first few new polyps
were formed at the oral pole of those Tubularians (II) of
which both ends were surrounded by water. On the next
day all of these had formed polyps at the oral cut end. The
same day, however, polyps were formed in five of the animals

ORGANIZATION AND GROWTH 211

of the other group (I)- at the ahoral end; and on the day
following, regeneration was complete in all of the animals of
this group. In Group II, on the other hand, in which the
animals had formed polyps at the oral pole, regeneration did
not set in at the ahoral cut end until five days later; and not
until six days after this was regeneration complete. The
formation of polyps at the aboral pole was therefore com-
plete eleven days sooner in the animals of Group I, in which
tin' formation of polyps was prevented at the oral pole, than
in the animals of Group II, in which polyps were formed at
the oral pole. By prereiiliinj t/ie formation, of polyps at
the oral pole, flic formation of polij}>s at the alioral can
therefore be accelerated, I have repeated this experiment
some ten times, but always with the same result ; usually the
delay in the formation of polyps at the aboral end caused by
allowing polyps to develop at the oral end was even greater
than that here given. This simple experiment, which can
easily be repeated, and, according to my experience, with
certainty, may be thus explained in the light of Sachs's
theory. The specific formative substances for the polyps are
present only in limited amounts, these being sufficient for
the formation of only one polyp in the Tubularian stem at
the moment that it is cut. When both poles are subject to
the same external conditions, these substances reach the oral
cut end first. If, however, the formation of polyps is ren-
dered impossible at this end, they wander to the other end. 1
If the formation of polyps is not prevented at the oral end, a
polyp can form at the aboral end only after a sufficient
amount of the formative substance for the polyps has been
formed anew.

('). The question now arises as to whether in the stem of
Tubularia such a migration of substance toward the cut end
(.an, indeed, be observed to precede the regeneration of a

lit is possible tli;it in -MIIII- (MM--; of compensatory hypertrophy, fur example iu
the testicles, specific formative substances for the oruan- play a Mmilar rOlu.

212 STUDIES IN GENERAL PHYSIOLOGY

polyp. If a Tubularian stem is examined immediately after
the polyp has been cut off, separate red pigment granules are
found distributed quite regularly throughout the stem. On
the following day a denser aggregation of these granules is
observed in the vicinity of the cut ends, and after two or
three days they are often so numerous that the cut end
seems saturated with red. Soon thereafter the poly}) is
formed into which the red granules are collected. This
occurs also in the formation of a polyp at the aboral end.
I will not, of course, assert that these pigment granules are
the formative substances of the polyps in the sense of Sachs's
hypothesis. It is rather to be assumed that, since pigment
granules have no active motion, movements probably occur
in the protoplasm of the Tubularian stem, at first in the
direction from the aboral end to the oral, and later, when the
oral polyp has been formed, in the reverse direction. It
might also be that in some cases in which external stimuli
have an influence upon organization these stimuli bring
about or modify such protoplasmic streaming. The move-
ment of protoplasm in plants under the influence of external
stimuli has been observed directly by Wortmann. 1 The
migration of the pigment granules toward the cut ends
actually observed, and the connection between this migration
and the formation of polyps, probably indicate that one is
justified in believing that organization in Tubularia and
also other animals is associated with the migration of
formative substances.

7. For the time being we may, therefore, believe that a
heteromorphosis can be brought about whenever the specific
formative substances can wander in different directions in
the animal body; while in the case of animals possessing
"polarity'' this migration is possible in only one direction.
When heteromorphosis occurs in Tubularia, but the oral

i WORTMANN, Botanische Zeitung (1887).

ORGANIZATION AND GROWTH

polyp is formed sooner than the aboral, we should have to
assume that the substances necessary for the formation of
new polyps move in both directions in the Tubularian stem,
but this movement occurs more rapidly, or earlier, in the
direction from root to polyp, than in the opposite direction.
8. I shall in conclusion mention the fact that under one
condition the polyp may occasionally be formed sooner at
the aboral pole than at the oral, even when both are subject
to the same external conditions. This can happen when the
formation of polyps is retarded, or made almost impossible,
through external conditions. According to my former obser-
vations, this can also happen when the pieces of Tubulariau
stem used for the experiment are very short.

V. IRRITABILITY AND ORGANIZATION IN TUBULARIA

We saw that the physical condition which determines the
orientation of Antennularia in space has an effect also upon
the position of its organs; the same relation also exists be-
tween irritability and organization in Tubulariaiis. Tubu-
laria mesembryanthemum is neither heliotropic nor geo-
tropic; /////// il (jrariti/ <ilxo hare no effect upon the ornjin
of neir (</( t it* in Tubnlai-id. On the other hand I have found
that the animal is highly stereotropic; that is, the orienta-
tion of its organs is very clearly affected by contact with the
surface of "solid bodies. The root of Tubularia is positively
stereotropic, that is, when brought in contact with the sur-
face of a solid body it attaches itself to it and grows over
the surface. Possibly through the slight friction a sub-
stance is secreted which causes the root to stick. The polyp,
however, is negatively stereotropic. When brought in con-
tact with a solid body, it turns away from it until it is again
surrounded by water, after which it grows in the direction
determined by the orientation of the polyp. The name
Stimulus irhich (Irlcnitinrs tin- orirn/al ion of Tnhnld n'a in

214 STUDIES IN GENERAL PHYSIOLOGY

space is also of importance for its organization. For it
is possible to control organization in Tubularia by contact
stinmli to such an extent that by their help we can pre-
scribe in a large measure whether a root or a polyp shall
grow from the aboral cut end of a Tubularian. If the
aboral end of a piece of Tubularian stem is surrounded by
water, so that it does not rub against the surface of
solid bodies, almost without exception a polyp is formed
upon it instead of a root; heteromorphosis occurs instead
of regeneration. If, however, the same end is brought into
permanent contact with the surface of a solid body if,
for example, the stem is laid horizontally upon the bottom
of the vessel a root is often formed. But to accomplish this
it is necessary to keep the dish quiet, and care must be taken
that the same side of the stem remains in contact with the
solid body ; otherwise it does not attach itself. If the stem
does not attach itself, a polyp is soon formed, just as a polyp
is formed at the tip of a Tubularian root which has been
freed from the bottom of the vessel, as soon as it is sur-
rounded by water. Of course, it is in every case necessary
that the general conditions of growth of the animal, which
we shall discuss later, be fulfilled. I remember that only
once in hundreds of individual experiments did I see a root
grow without contact with a solid body. 1 But a factor came
into play here which still brings this case under the
general grouping. I had cut a section from the stem of
a Tubularian, and had laid it horizontally upon the bot-
tom of a glass vessel. A root sprang from the aboral end
which attached itself to the bottom of the vessel. Simul-
taneously with this root arose a second, which curved up-
ward somewhat, and which continued to grow as a root with-
out contact with a solid body. The exception was probably

1 In recent experiments on Tubularia crocea I have frequently observed the for-
mation of roots at the aboral end, even if the latter was not in contact with a solid
body. [1903J

ORGANIZATION AND GROWTH 215

only apparent. It may be that I had to do with a bifur-
cation of a root which had been produced through contact
stimuli, of which the one branch continued to grow without
contact. But, as before, I did not succeed in causing the
growth of a root at the oral pole in even a single instance;
it may be that this is impossible in Tubularia by any of the
means which I have employed thus far. In the light of my
previous experiments on Antennularia and Aglaophenia, I
need not emphasize that this is only a special characteristic
of Tubularians.

If a root is formed at one extremity of a Tubularian,
and this has grown vigorously for some time, its tip not
infrequently becomes faintly red, and this coloration deepens
rapidly ; in such cases a polyp forms at the tip of the root,
which then immediately bends away from the substratum
upon which the root has grown and grows almost perpendic-
ularly upward, We then have a colony of two polyps which
are attached to the base upon which they grow by a common
root. The difference between such an animal and the
heteromorphic bioral animal lies in the fact that the latter
lacks an inherited organ, the root. In its place a polyp has
been formed.

VI. EXPERIMENTS ON ORGANIZATION IN CIONA INTESTINALIS

1. The animals in which I have thus far been able to
produce heteromorphoses stand low in the animal scale ; we
have dealt only with Hydrozoa. It might bethought that
these "colony-forming" animals are an exception to the gen-
eral rule, and that only in them a movement of formative
substances can take place in both directions in the sense of
Sachs's theory. Such a view is supported by the fact that
"polarity" is very pronounced in a series of Actinia?, and
that, in spite of my efforts, I have not succeeded in bringing
about heteromorphoses in them. The fact that a solitary

211)

STUDIES IN GENERAL PHYSIOLOGY

Hyclroid, Hydra, should conduct itself, so far as known, as
a strictly "polarized" animal, might render it still more
probable that heteromorphosis is simply a characteristic of
"colony-forming" Hydrozoa. To settle this question I made

experiments upon Ciona in-
testinalis, a solitary Ascidian.
The Ascidians, as is well
known, stand near the verte-

d, ~*^? '*'&.

brates in the animal scale.

In Fig. 54 is shown in
almost natural size the exter-

FIG. 54 FIG. .M FIG. r,6

nal form of a Ciona intestinalis. a is the oral opening, 6
the excretory opening, c the foot, which, like the foot of the
Hydrozoa, attaches the animal to the surface of solid bodies.
At the outer edge of both tubes are situated the ocellse,
which are just visible to the naked eye (Fig. 55). The fol-
lowing experiments deal with the formation of these ocellse.
If the animal behaved like a purely "polarized animal," such
as Cerianthus, then, when a lateral incision cba (Fig. 56) is
made into it, ocellaB should be formed only along the lower
cut edge 6c, just as the tentacles are formed only along the
lower cut edge of Cerianthus. But the opposite occurs in
Ciona. After a short time ocellaB (indicated by points in the
drawing) are formed along both edges of the wound (cib and

ORGANIZATION AND GROWTH

217

FIG. 57.

Ic). This occurs no matter whether the oral or the anal tube
is incised. Fig. 55 illustrates such a case; an incision
was made into the oral tube; a week later ocellre had
formed alonjr both ed^es of the wound. The foot is not

O O

indicated in the drawing. Ciona iiitestirialis, therefore,

behaves not so much as Cerianthus, but
more like certain Hydroids, in which we
have to assume a movement of formative
substances in two directions. I do not
think that anyone will be inclined to
believe that the general causes underly-
ing the formation of organs are deter-
mined by the position which the animal
occupies in the animal scale.
2. I have still to describe what occurs after the ocelli are
formed. All the elements of the cut edge begin to grow in
length, and a new tube springs from the cut edge. The ani-
mal of Fig. 55 had assumed the form
shown in Fig. 57 four weeks later. A new
third tube had been formed at a. This
continued to grow in length, and attained
not only the size of the normal tube, but
usually became even longer than this. 1

If* several incisions are made simulta-
neously into the same animal, several new
tubes may be formed at once. The ani-
mal in Fig. 58 has four such tubes.

VII.

FIG. 58

EXTIRPATION AND REGENERATION OF THE CENTRAL
NERVOUS SYSTEM IN CIONA INTESTINALIS

1. Much more remarkable than that just described is
another phenomenon of regeneration in this animal namely,

'Dr. P. Miiitfazzini made experiments upon Ciona intestinalis similar to those
which I have described, and also found that a third tube is formed when the mantle
of Ciiina is in<'ised. While at work upon paper, vi I obtained his published account
of the-e experiments: P. MlNGAZZIM, Molleti no llcltu S,,i-/,/,i ,U

(Napoli, 1891).

218 STUDIES IN GENERAL PHYSIOLOGY

the regeneration of the central nervous system. Scarcely
another animal is so well adapted to the study of this prob-
lem as Ciona. The brain of Ciona consists of a snow-white
ganglion, somewhat larger than the head of a pin, situated
in the angle (/ near the surface between the two tubes a mid
b (Fig. 54 j. The mantle of Ciona is very transparent, so
that the ganglion can be removed with certainty with-
out seriously injuring the other organs of the animal.
The only difficulty encountered in the experiments is that
the animal contracts when touched, and that the ganglion
is then no longer visible. I overcame this difficulty in the
following way: The root of a Ciona which had been kept in
the aquarium was carefully detached from its base. The
animal was then quickly removed from the water and laid
upon a dark glass plate. As long as the animal was not
touched, it remained relaxed. I then brought the tip of a
pair of scissors behind the ganglion, and by a rapid cut sev-
ered the connection between the ganglion and the pedal part
of the body. Without removing the scissors, I seized the
ganglion with forceps, drew it out, and removed it by a
second snip of the scissors.

The Ascidian usually bears the extirpation of the ganglion
well, and after the operation behaves in a way not expected
from our general conceptions of animal physiology.

2. The first phenomenon worthy of note is that after
extirpation of the central nervous system the rcjle.rcs />rr-
sixt. Since Ciona is a sessile animal, the reactions of the
uninjured animal to external stimuli are limited to simple
contraction and stretching out of the body. This contraction
is brought about by a highly developed muscular system.
If the aquarium is slightly shaken, the animal contracts
quickly, to relax only when everything is again at rest.
Such a contraction of the whole animal also often follows
when it is carefully touched with the point of a needle. If

ORGANIZATION AND GROWTH 219

tht> t i|> of one of the tubes is touched, this only may contract.
It is generally believed that when this occurs the external
stimulus which brings about a change in the sensory nerve
endings is conducted to the central organ, here to be trans-
formed into a motor impulse which causes the muscles to
contract. These ideas harmonize with the facts. But it is
also generally believed that without the central nervous sys-
tem the reflexes are no longer possible. This idea, as the
following experiments will show, is not true in Ciona intes-
tiiialis. When the ganglion has been removed from a Ciona,
it at first remains fully contracted. After some time, how-
ever, in favorable cases, as early as the next day the animal
again relaxes. If a drop of water is allowed to fall into the
basin of water, the entire Ciona contracts rapidly, just as an
animal whose central nervous system is intact.

If, therefore, a normal Ciona and one without the central
nervous system are kept in the same vessel, both have the
same reflex irritability qualitatively. It is nevertheless pos-
sible to differentiate clearly between the reaction of the nor-
mal and the brainless Ascidian. In ike latter the thresh-
old of stimulation is much hiylier than in flic former. To
determine this I utilized a procedure which I have employed
in a series of comparative experiments on the irritability of
the lower animals, and which mi^ht be used with advantage

' O O

in human beings. I allow a drop of water to fall from a
pipette upon the organ to be stimulated, which lies a certain
distance (varying according to circumstances) beneath the
surface of the water. If the same pipette is always used,
the weight of the drop is nearly always the same, so that the
height which the drop must fall just to bring about a reac-
tion is a convenient measure of the threshold of stimulation.
In what follows I shall give the height of the fall of a drop
of water which just sufficed to bring about a contraction of
the entire Ciona. The normal and the brainless r.nimals

220 STUDIES IN GENERAL PHYSIOLOGY

were kept in the same vessel, and at the same distance
below the surface of the water. Under a are given the
height of the fall in normal, under h in brainless, animals.
The figures are given only to show how constant is the differ-
ence in the threshold of stimulation between the two ani-
mals; the absolute height of the threshold of stimulation
cannot be determined from them.

a b

8 mm. 65 mm.

1 75

10 80

80

In two other animals I obtained the following values.

a b

6 mm. 22 mm.

8 20

The temperature was 13 C.

This difference in irritability may be due to the fact that
the wave produced by the drop of water stimulates the motor
nerves or muscles directly, and that the threshold of stimula-
tion is higher for these than for the sensory nerve endings.

3. It can be shown by a different experiment that another
path for the conduction of stimuli must exist in the brain-
less animal than in the uninjured, where stimuli no doubt
travel over the nerves for the most part. If an incision is
carefully made in the tube of a Cioiia whose ganglion has
been removed, not only the injured portion, but the entire
animal, contracts, just as when the same stimulus is applied
to an uninjured animal. This also may occur when one
point of the oral edge is carefully touched with a needle. I
imagine that in a brainless Ciona the motor nerves or muscles
lying nearest the point of incision are stimulated mechani-
cally, that these contract, and that the jarring or pulling

ORGANIZATION AND GROWTH 221

associated with this contraction stimulates the neighboring
nerves or muscles, thus causing the latter to contract. In
this way a conduction of stimuli is brought about without the
presence of a central nervous system, the effect of which is
tin- same as when a central nervous system is present. There
is so little difference between the latent period of stimulation
and the time the body remains contracted after the stimula-
tion, in the normal and in the brainless animal, that it can-
not be determined by mere observation unaided by special
apparatus.

What occurs here in the entire animal happens in a lim-
ited portion of an earthworm when it is cut across trans-
versely and the two pieces are sewed together. As Benedikt
Friedlander has shown, both pieces are still able to perform
co-ordinated movements of locomotion. 1 I have repeated
the experiments of Friedlander upon leeches, and have
observed the same series of phenomena in them.

Two years ago I made some observations upon a marine
Planarian, Thysanozoon brocchii, which are similar in cer-
tain respects to those upon Cioiia, into a discussion of which,
however, I cannot enter here. 2

4. Since I had received the impression that Ciona is helio-
tropic, I tried to see whether a Ciona robbed of its nervous
system would also react to light by heliotropic bendings.
The object of my experiments was thwarted by an unwished
for, but perhaps interesting, result. In the course of four
weeks ll the. <uii>n<ils (which had not died) had regenerafcil
a new hrain. I repeated the experiment, because I did
not believe my first results. But I can no longer doubt

i BENEDIKT FRIEDLANDER, Kiolut/isriicx f'cntntlhl., Vol. VIII (1889-t.

-Tin- Darwinian^ would probably call the reaction of a Ciona inte>t inali- to
contact the consequence of a " protect i w- in-i inct.'' Thr-n "protective in-tinct-."
so far as I can see, are said to con.-ist in this, that the animal has by natural selec-
tion acquired, in the course of the customary million years, certain cerebral con-
trivances which an; now inherited from one generation to another. But in theca-e

of ('ion. i t he-e he red it a ry " in-l incts " cannot we'll \>:- located in any special portion
of the brain, for they cont inue to exist after the removal of this oryau.

222 STUDIES IN GENERAL PHYSIOLOGY

their correctness. The growth of the new ganglion is fairly
rapid. Instead of one large ganglion, several small ones
were formed. For these reasons I have not followed further
the question as to whether Ciona is at all heliotropic. Dr.
P. Mingazzini has also observed the regeneration of the
brain of Ciona after its removal; he suggests that the new
ganglion is formed from the ectoderm, as in the embryo.

VIII. THE RELATION BETWEEN REGENERATION AND THE
CONCENTRATION OF THE SKA-WATER IN TUBULARIA

1. Now that we have seen that it is possible to alter the
inherited form of an animal by substituting one organ for
another, we shall pass to experiments on the general physi-
ological factors which underlie regeneration and growth in
animals. These experiments, as the preceding ones, had to
be made with the simplest experimental accessories, and in
addition they were not completed when I was compelled to
leave Naples. Since they can be continued only at the sea-
shore, I shall here give only those experiments which have
been carried to a certain degree of completion.

If the polyp of a Tubularian is amputated, a new polyp
regenerates. What general physiological conditions must
be fulfilled in order that this may occur? Since the stem
begins to grow as soon as the polyp is formed, the second
question arises: What conditions influence growth? The
following experiments are intended to answer these ques-
tions.

I shall first show how the absorption of water affects
regeneration. The reader will know that from the point of
view gained by a study of osmosis, the living protoplasm of
plant cells is characterized by its permeability to water and
its total or partial impermeability to many substances dis-
solved in the water. There is no reason for believing that
animal protoplasm behaves any differently in this respect

ORGANIZATION AND GROWTH 223

from vegetable protoplasm. If a Tiibularinn stem is trans-
iVnvd from pure sea-water to sea-water that has been diluted
by the addition of distilled water, water must enter the cells,
and these must in consequence become more turgid. On
the other hand, if the Tubularian is placed in sea-water, the
concentration of which has been raised through evaporation,
the absorption of water by the animal must fall below the
normal, and water must finally pass from the cells into the
solution. The following experiments show how changes in
the concentration of the sea-water affect regeneration. The
temperature in all these experiments varied between 12 and
15 C., and was always the same for the same series of ex-
periments. The number of animals in the individual dishes
of the same series of experiments was also nearly equal ;
every dish contained 300 c.c. of sea- water.

2. I distributed a large number of fresh, healthy stems
of the same colony of Tubularia into three salt solutions.
The first of these consisted of sea-water to which 33^ per
cent, of its volume of distilled water had been added; the
second, of ordinary sea-water; and the third, of sea- water
which had been evaporated to 75 per cent, of its volume. In
order to have all the solutions exactly alike except in con-
centration, the two former were heated to the boiling-point,
filtered, and, like the third, shaken for some time in the air.
After two days nine of the twelve stems in the dilute sea-
water had regenerated the polyps which they had lost; in
the normal sea-water only six of the sixteen stems had
regenerated ; in the concentrated sea-water regeneration had
not taken place in a single stem. Six hours later regenera-
tion was complete in all the Tubularians in the dilute sea-
water, and on the following day this was also the case in the
ordinary sea-water. Three days later a change occurred at
the cut end of one of the animals in the concentrated salt
solution which looked like a beginning of regeneration, but it

224 STUDIES IN GENERAL PHYSIOLOGY

went no farther. I now divided the concentrated sea-water
and the animals contained in it between two vessels, and
added enough distilled water to one of them to restore it to
the concentration of normal sea-water. After a few days
all the animals in this vessel regenerated their polyps in an
entirely normal manner, but no regeneration whatsoever
occurred in the other. I have repeated this experiment sev-
eral times, always with the same result. While, therefore,
a great lowering of the concent r;it inn of the sea-water (!'!.,
per cent.) not only does not inhibit regeneration, but may
accelerate it, an increase in the concentration to a like
amount suffices to prevent regeneration altogether, or almost
entire! \. without, however, injuring the power of regenera-
tion, provided the animal docs not remain too long in the
concentrated solution.

3. In order to determine the limit of concentration in
which regeneration is possible, I evaporated normal sea-water
to ( .>0, '^0. To, TO. '>0. and "() per cent, of its volume. The
solutions were filtered and shaken for some time in the air.
Regeneration took place in the first three solutions, but not
in the last three. Regeneration, however, did not occur
simultaneously in the first three solutions, but the more
slowly, the more concentrated the solution. After lying for
several days in the TO per cent, solution, the Tubularia3 had
not lost their power of regeneration, but in the stronger
solutions even this had suffered. When the Tubularians
were brought back from the stronger solutions into normal
sea-water, they no longer formed polyps. The tissues of
the stem were shrunken and often separated from the peri-
derm. Upon the other hand, when I added as much as 50
per cent, of distilled water to ordinary sea-water, a distinct
inhibition of the process of regeneration could not once be
noted; only upon the addition of 100 per cent, of distilled
water did regeneration cease. If still more distilled water,

ORGANIZATION AND GROWTH 225

about 200 per cent., is added to the sea-water, a remarkable
phenomenon occurs: large pieces of protoplasm escape
from the Tubulariae without losing their form. They are
surrounded by a transparent membrane which is formed per-
haps through their contact with the sea-water. I saw a
piece 12 cm. long escape from a Tubularian in this way.

4. That an increased absorption of water by the tissues
favors regeneration when it does not exceed a slight amount ;
while a greater absorption is harmful, might seem contradic-
tory. But it is an entirely general and well-known fact that
when water enters cells in too large quantities it acts as a
poison. Hoppe-Seyler, for instance, attributes the death of
frozen plants which are thawed out too rapidly to the fact
that "in freezing, the water separates to a large extent from
the solids and collects in crystals. When thawed out rap-
idly, the particles of solid matter lying nearest these crystals
are flooded with water."

5. The pieces regenerating in the highly dilute solutions
often show changes which indicate the great turgidity of
the tissues, in consequence of the abnormally great absorp-
tion of water. Before the polyp is formed, globular excres-
cences appear at the cut ends, and the new polyps are thicker
and much more nearly globular than the normal polyps.
The opposite phenomena are noted in the strongly concen-
trated salt solutions, in which polyps remain exceedingly
small.

('). In conclusion I wish to give a few figures on the rela-
tion between regeneration and the absorption of water. Tn
order not to repeat what has already been said, I shall take the
figures from experiments in which the concentration of
the sea-water was increased by the addition of sodium
chloride, or decreased by the addition of fresh water (ti<-rino
water). According to F<rrh hammer, the amount of salt

1 Hopri.-SiiYLER, rhytiioloyische Chentic (Berlin, 1877), p. 30.

220

STUDIES IN GENERAL PHYSIOLOGY

contained in the sea-water taken from the ocean in the
vicinity of Naples is about 3.S p ( -r cent., of which about 3
per cent, is sodium chloride. 1 The following table shows
how with an increase in the amount of NaCl regeneration is
inhibited more and more, and finally prevented altogether.
The figures in the first horizontal line of the table indicate

O

how many grams of NaCl were added to each 100 c.c. of
sea- water. In the first vertical line are indicated the num-
ber of days that have elapsed since the introduction of the
Tubulariio into the solutions. In the horizontal lines after
each date are given the number of polyps regenerated up to
that date. The stems were all taken from the same colony,
and each solution contained twelve stems.

TABLE I

Date

Normal
Sea-Water

0.6 K .
NaCl

1 Og.

NaCl

1.3 K.

NaCl

i.; g .

NaCl

Second iluv

4

1

o

o

Third dav

10

n

u

o

Fourth dav

10

8

o

Fifth dav

12

10

5

1

o

Sixth dav

1-2

11

5

1

In order not to make the table too long, I will add that
no regeneration whatsoever occurred in the most concentrated
of these solutions that in which 1.0 g. of sodium chloride
had been added to each 100 c.c. of sea-water. The retardation
of regeneration was well marked, even when only 1 per cent.
NaCl was added to the sea-water, and even in such a solu-
tion not all the animals regenerated. The repetition of these
experiments yielded the same results.

The concentration in which no regeneration occurs is
therefore reached when 1.0 g. of NaCl is added to 100 c.c. of
sea-water. This value corresponds to a concentration obtained
by evaporating 100 c.c. of sea-water to 09 c.c., if the entire

1 ROTH, Allgemeine und Chemische Geologic, Vol. I, p. 524.

ORGANIZATION AND GROWTH

227

amount of salt contained in the sea-water is counted as NaCl.
Our previous experiments had, indeed, shown that the con-
centration limit for regeneration in Tubulaii;e is reached
when 100 c.c. of sea-water is evaporated to 70 c.c.

The second table gives the results of an experiment which
shows the difference between the effect of increasing and
decreasing the concentration of the sea-water. Each solu-
tion contained six animals. The percentages indicate the
volume of normal sea-water, -f- indicating an increase,
indicating a decrease, in the amount of water contained in it.
The figures indicate the percentage by which the original
volume has been increased or decreased.

TABLE II

Date

-26%

-21%

-8%

Normal
Sea-W't'r

+60%

+50 %

+70%

+100%

Third dav

1

2

4

4

1

Fourth day ....
Fifth day

3

5
6

6
6

6
6

6
G

6
6

5

6

4
6

I continued my observations for two weeks, but no regen-
eration occurred in the most concentrated of these solutions
( 26 per cent.) beyond the three animals mentioned in the
table.

7. The facts of this chapter may be shortly summed up
in this: an absorption of water is essential to regeneration in
Tubularia (and probably all animals). If the absorption of
water is limited by keeping Tubularia in concentrated sea-
water, regeneration is retarded, and finally completely inhib-
ited, by even a slight increase in the concentration. Regen-
eration is, however, not only not retarded but, it' anything,
accelerated, if the Tubularians are put into diluted sea-water.
Only when the dilution exceeds a certain limit and the
tissues are flooded with water a retardation of the regenera-
tion occurs. While a decrease of 30 per cent, in the orginal

228 STUDIES IN GENERAL PHYSIOLOGY

volume of water contained in ordinary sea- water renders
regeneration impossible, the same effect is not obtained until
125 per cent, of fresh water has been added.

i

IX. THE RELATION BETWEEN LONGITUDINAL GROWTH AND THE
CONCENTRATION OF THE SEA-WATER IN TUBULARIA

1. One searches modern text-books of physiology in vain
for a chapter on growth; it scarcely exists even by title.
So far as I have been able to judge from the literature,
observations on the physiological conditions necessary for
growth in animals have been exceedingly few. I will give
here what I have been able to fiml.

The oldest observations on the growth of animals are
probably made by Bonnet, who was encouraged to do so
through Hales's work. 1 Bonnet measured the growth of
worms. The choice of material for these experiments was
unfortunate, as the length of these animals is subject to
great variations because of the contractions of the body.
Bonnet measured with calipers "la plus grande longueur du
ver,"' and made these values the basis of his conclusions.
He cut a worm into two, a second into four, a third into
eight parts, etc., and tried to see whether the growth of the
parts differed. His measurements showed "qu'il n'y a pas
de difference considerable entre le progres que font dans le
nierne temps des moities et des quarts et ceux de huitiemes
et de dixiemes" (p. 214); and further, u que la derniere
portion est celle de toutes qui, en temps egal, prend le moins
d'accroissement, et apres elle, celles qui la precedent imine-
diatement." It seems strange that the work of Bonnet stimu-
lated no one to repeat his experiments upon more suitable
material and with a better guarantee of accuracy.

So far as I can determine, further experiments on the
conditions for the longitudinal growth of animals have been

i CHAELES BONNET, GEuvres d 'histoire naturelle et de philosophie, Vol. I, "Trait6
d'Insectologie " (Neuchatel, 1779), p. 193.

N i/ \TION AND (iuo\YTH '2'2 ( .)

math' only l>y Semper. 1 Semper started witli the idea ex-
pressed by H. Spencer in his /Y/m-//>/rx of Iliolot/i/: "All
\vlio have had experience in fishing in the Highland lochs
know that when 1 the trout are numerous they are small, and
where they are comparatively large they are comparatively
few." Semper studied the effect of the quantity of water
upon the longitudinal growth of Lyrnnaeus stagnalis. Elodea
canadensis furnished in abundance served as food. The
stiff shells of the Lymnaei allowed an accurate measurement
of the amount of growth. Semper found that when the
food was equally overabundant the Lymnaei grew the more
rapidly, within certain limits, the greater the amount of water
in which they were contained. With an increase in the volume
of water from 100 c.c. to 500 c.c. the longitudinal growth
increased rapidly. As soon as the volume of water amounted
to 5,000 c.c., a further increase had no effect upon growth.
After sixty-five days in one experiment, a shell

in 100 c.c. of water was 6 mm. long

in 250 " " 9 "

in 600 " " " 12 "

in 2,000 " " " 18 "

Semper is inclined to believe that the dependence of the
growth upon the amount of water present is attributable to
the presence of a substance in the water "without which the
other conditions for growth, even when present, can have no
effect upon growth." In his book on The Condifioiix of Ilic
"Existence of Animals, he describes further experiments in
this direction:

The salts in the water, the presence of which can bo demon-
strated chemically, cannot be the cause of the stunted development.
With the friendly aid of a chemist, Professor Hilger of Erlangen,
I repeated my experiments with distilled water, or with water in

i C. SEMPER, " Ueber di Wachstamsbedingungen <\\~< Lymnaeus stagnalis,"

Ai-lnit' it aUSdem //. tootom. Inxtitut in \\'in-:/:iir</. Veil. I i \Ynr/l>mx', IsTI i. p. l.'iT;
aud I>ic iKiturulixritcit Existenzbedingungen >''/ '/'//<> / . I'.H-I I i Leipzig, 18SO), i>. r. ~>.

230 STUDIES IN GENERAL PHYSIOLOGY

which the substances contained naturally (calcium carbonate,
magnesium sulphate, etc.) were present to the point of saturation.
The course of the experiments always showed that the salts con-
tained in the water, and the presence of which had been demon-
strated chemically had no particular effect.

In spite of this, Semper concludes

that some substance must be present in the water, probably in
exceedingly small amounts, which, through its solution in the water.
and its osmotic behavior toward the skin of the animal, is absorbed
by the latter in definite, though perhaps small, amounts.

It seems to rue that an important factor has not been
recognized in these experiments. An excess of food (Elodea
canadensis) was probably present in all the dishes, but it
was not observed whether all the animals ate the same
amounts a factor upon which tin- results depend altogether.
When I was raising butterflies I noticed Low readily young
caterpillars lose their appetites (especially immediately after
hatching). In such cases the growth of the caterpillars of
the same brood varies according to the amount of food they
take up. It is quite possible that, if this point is taken into
consideration, the experiments of Semper find a simple
explanation.

I undertook my own experiments on growth to determine,
first of all, whether the mechanics of growth is the same in
animals as in plants. In plants it is believed that water
enters the cells osrnotically, that the cell-walls are stretched
in consequence, and that through further changes this
stretching of the cell-walls becomes permanent, and remains
even when the intra-cellular pressure has again fallen off.
If the water furnished a plant is decidedly reduced, longi-
tudinal growth is diminished and finally stopped entirely.
I have described an experiment in the preceding volume on
experimental morphology which shows that those parts of
the animal which have lost their turgidity are just as little
, able to grow as wilted plants.

ORGANIZATION AND GROWTH

Wlu> 1 1 a transverse incision <ihc (Fig. 59) is made
transversely in a Ceriauthus membranaceus fairly near
the oral pole, the tentacles T l situated above the incision
collapse like the wilted portions of the plant. When I made
such a transverse incision in a Cerianthus the tentacles of
which were growing actively, the tentacles
above the incision ceased to grow, while the
remaining tentacles went on. 1 Cerianthus,
however, could not be used for a detailed
study of the dependence of animal growth
upon cell turgor. Longitudinal growth can-
not be measured accurately in contractile
animals. Moreover, in the experiment on
Cerianthus described above the amount of
water absorbed by the organs cannot well
be controlled experimentally. And, finally,
the growth of Cerianthus is relatively slow.
I therefore chose a more suitable animal upon
which to experiment, namely Tubularia, and
a different method of varying the amount of
water absorbed the osmotic. I am obliged to Dr. Wort-
mann for calling my attention to the plasmolytic experiments
of de Tries.

The stem of Tubularia is surrounded by a rigid periderm,
and an increase in length can be measured as accurately in
this animal as in the rigid stem of a plant. I changed the
concentration of the sea-water through the addition of sodium
chloride or fresh water. The temperature was always the
same for all the animals in the same series of experiments;
the amount of liquid in each vessel was always 300 c.c. ; and
tin 1 number of animals in each vessel was also nearly always
the same.

-. As is well known, Tubularia grows in length only when

!See "Heteromorphosis," Part I, p. 155.

Fit;. :.'.'

232 STUDIES IN GENERAL PHYSIOLOGY

a polyp is present; or, what is perhaps more correct, Titbii-
Inria r/ro/rs in length pcrioa'icallij at its oral cinl. crcri/
hi-i/iiiiiiiii/ irif// flic formation of a ncir polyp, and
irlicn tl/is organ dro)>s off". It is therefore necessary
that all the animals of a series of experiments which are to
be compared with each other should be in the same phase of
growth. Since this does not necessarily occur naturally, I
cut off the polyps of all of the animals when a series of
experiments was started. In all these then began a new
period of growth, in which a polyp was first formed, and
after which tin- stem grew in length (the growing part of a
stem being situated close behind the tip of the polyp).
I waited until the polyps had dropped off in all the speci-
mens, and then I knew that the period of growth was at an
end. I then compared the longitudinal growth in the indi-
vidual specimens which had been subject to different condi-
tions. Since growth always occurs with the formation of a
new polyp, it follows, without further comment, that the
concentration limits for the regeneration of the polyps are
also the concentration limits for the growth of the Tubularian
stem.

3. I cut pieces having about the same length and thick-
ness from the stems of a large number of individuals and
distributed them equally into various dishes containing sea-
water of different concentrations. Every vessel contained
seven to nine animals. After eight days, in which time they
had formed new polyps and grown vigorously, the Tubu-
lariaus were removed and the amount of new growth was
measured. The following table shows the increase in the
linear growth of the individual Tubulariaus. The figures of
the first horizontal line show the amount of salt, in per cent.,
contained in the different solutions used ; in the vertical line
under each of these figures is given the increase in the length
of the individual Tubularians.

ORGANIZATION AND GROWTH

233

TAHLK III

3.8*

.-,.1

1 s

II

1.1

1 Normal

3.2*

i :.

i .;i

1.6X

1 ::

Soa-Wat'n

nun.

mm.

nini.

nun.

mm.

mm.

mm.

nun.

mm.

mm.

1

2

3

11

10

13

19

5

1

1

o

4

5

5

1

13

5

0.5

0.5

3

2

3

5

6

8

4

0.5

1

ii.5

>

6

1

7

3

1

2

3

3

5

11

5

6

3

2

5

12

3.5

2

3

4

5

4

8

ti

4

o

Av...0.3

1.1

3.2

3.8

4.4

C

10.6

4.4

0.3

We obtain a better view of these results when we present
them graphically. In the curve shown in Fig. 60 of the text
the different amounts of salt contained in each 100 c.c. of the

3
FIG. 60

various solutions are represented on the axis of abscissas,
the increase in the length of the animals on the ordinates.
\Yc set' that the growth is nil in a 1.3 per cent, salt solution;
that it is just perceptible in a 1.6 per cent, solution; that it
increases very rapidly with an increase in the concentration,
attaining a maximum in a 2.5 p ( >r cent, solution, and then
decreases slowly with a farther increase in the concentration.
M n. r/ii/iil linear (jroirfli docs not occur in ordinary sea- water,
hnl in n/iirlccdl// <lilnf<'<{, scd-irafcr. The meaning of the
curve is very simple: the cells of Tubularia must be turgid
in order to grow, and this is possible only as long as the

234 STUDIES IN GENERAL PHYSIOLOGY

concentration of the solution in which they are contained
does not exceed a certain limit; this limit is attained when
1.6 g. of NaCl is added to 100 c.c. of sea-water when the
solution contains 5.4 per cent. salt. As the concentration
decreases, the Tubularian stem must absorb more and more
water from the solution, and we therefore find linear growth
to increase with a decrease in the amount of salt in the solu-
tion, until the limit is reached where the poisonous effects
of the large amount of water show themselves. From this
point on a further increase in the amount of water contained
in the animal must cause growth to fall rapidly to zero.
This accounts for the rapid fall in the curve between the
values 1.9 and 2.5 of the abscissa.

A second experiment, performed under entirely similar
conditions, yielded the same result as the above. This
is shown by Table IV.

TABLE rv

Amount f Salt Average Growth

in the Solution in Nino JJays

.">.!: - 0.5 mm.
4.8 4

4.4 - 7

4.1 12

3.8 (normal sea-water) - - 12.6

3.2 14.3
2.2 - 15

1.9 10.5

As in the preceding experiment, linear growth increases
in this case also with a decrease in the concentration, attain-
ing a maximum, not in ordinary sea-water but in a more
dilute salt solution about 2.2 per cent. Beyond this point
growth falls rapidly. In other experiments also which,
however, I omit here because I failed to measure the in-
crease in the length of all the specimens, and therefore cannot
tabulate them I was able to show that the relation be-

ORGANIZATION AND GROWTH

tween growth and the amount of water in the solution was in
all points essentially the same as detailed here. We found in
a preceding chapter that the polyp is regenerated later in a
very highly concentrated or a very dilute solution than in a
solution the concentration of which lies between these two
extremes. We have therefore to determine in how far this
circumstance compels us to make corrections in the experi-
mental results given above. Between the concentrations
4.42.5 per cent, the difference in time between the forma-
tion of polyps in the different concentrations is so slight
that they need not be considered. So far as the very con-
centrated solutions, 5.1 and 4.b' per cent., are concerned, I
have made experiments which I have continued for weeks
and months, and have found that the absolute increase in
length during this time is practically zero, even though the
animals formed new polyps repeatedly during this time.

4. I have not made any measurements on the increase in
the thickness of Tubularia. Yet the effect of the concen-
tration of the salt solution upon the diameter of the newly
formed stem was very apparent even without measurements.
The new stems formed in the more concentrated solutions.
Those in which about 1 g. of NaCl had been added to each
100 c.c. of sea-water were markedly thinner than the old ones
which had been grown in ordinary sea-water. On the other
hand, in the diluted sea- water the thickness of the new stems
was not only not less than that of the old stems, but even
greater.

5. With an increase in the concentration of a salt solu-
tion the amount of oxv^en dissolved in it decreases, and, as

v O

we shall see later that this is an important factor in regenera-
tion and growth, we must determine whether differences in
concentration influence the growth of Tubularia through their
effect upon the amount of oxygen dissolved in the solutions
and, if so, how much. The one direct measurement of the

236 STUDIES IN GENERAL PHYSIOLOGY

relation between the amount of oxygen dissolved in a sodium-
chloride solution and its concentration that I have been able
to find in the literature is the following: According to the
experiments of Fernet 1 if I understand the term "titre de
solution" correctly, the absorption coefficient

of a 5.42 per cent. XaCl solution at 16 C. = 0.0284
of a 0.72 per cent. XaCl solution at 14.1 C. = 0.0293.

Tin- corresponding coefficients for distilled water are accord-
ing to Bunsen 2 0. 02941) and <Ui:jn:>,0. The effect of con-
criitration upon gas absorption is therefore so slight that
it may be neglected in our experiments. Since data such as
these are but few in number, I wish to add a few on the ab-
sorption of carbon dioxide.

Professor Zuntz, who brought Fernefs work to my notice,
was so kind as to inform me that, according to his experi-
ments, a saturated NaCl solution absorbs about one-third as
much CO as distilled water. The figures of Fernet about

& '

correspond with these. According to this author, the ab-
sorption coefficient of CO 3

in a 6.25 per cent. XaCl solution at 11.2 C. = 93.T>,
in distilled water (according to Bunsen) at 11 C. = 1.1336, differ-
ence = 0.2001;

in a 2.22 per cent XaCl solution at 14.1 : C. = 0.9463,
in distilled water (according to Bunsen) at 14.1 C. = 1.0291; differ-
ence =00828;

in a 0.83 per cent. XaCl solution at 16 D C. =0.9591,
in distilled water (according to Bunsen) at 16 C. = 0.9753; differ-
ence = 0.0162.

The decrease in the absorption coefficient of CO 3 with an
increase in the concentration of the NaCl solution from to
6 per cent, is about 0.2. This decrease about corresponds
with that caused by an increase in the temperature from 10

'FERNET, Annales des sciences natu relies, 4th Series, "Zoologie," Vol. VIII
(lsr>7). See also ZUNTZ, " Blutgase und resp. Gaswechsel," HERMANN'S Handbuch
der Physiologic, Vol. IV.

2 BUNSEN, Gasometrische Methoden, 2d ed., 1877.

ORGANIZATION AND GROWTH 237

to 16 C. Now, I have found that a rise of temperature of
from 10 to 10 not only does not diminish growth, but in-
creases it, in spite of the fact that the Tubularias need more
oxygen when the temperature is raised. It can therefore
not be assumed that a decrease in the amount of oxygen
contained in the sea-water brought about by the addition of
soil in in chloride in varying amounts up to 1.6 y. to each
100 c.c. of water determines the decrease in linear growth.
Professor Zuntz, to whom I appealed in the absence of more
extensive experiments on the effect of concentration upon
gas absorption, does not believe that so slight a difference in
the amount of dissolved oxygen as was observed in the solu-
tions employed in these experiments need be considered in
my results, since animals get along well in summer, when the
temperature is high and the demand for oxygen is corre-
spondingly increased.

X. SOME REMARKS ON THE EXPERIMENTS OF SCHMANKEWITSCH

1. The proof which has been given in the preceding
chapter that, with changes in the amount of water absorbed,
the growth of animals is changed in the same way as the
growths of plants, enables us, I believe, to give a physical
explanation of some of the wonderful experimental results
obtained by Schmankewitsch in the artificial conversion of
the genera Artemia mulhausenii, salina, and Branchipus. 1

In 1871, during a flood,

the dam which separates the less salty water of the upper part of
the Kujalnik Limaii from the lower part, which is filled with salt
precipitated from its own waters, broke through, diluting- the
water of the lower part to 8 Beaum6, and causing a large numbi-r
of Artemia salina to appear in it, which had evidently been brought
down from the upper part of the Kujalnik and the salt-water pools
in its vicinity. In the course of the following year the couceutra-

1 W. J. SCHM.VXKEWrrsril, Xrifsi-liriftfilr wixfifiisrlm/'flirli,- XiMtlniiic, Vol. XXV,
Supplement (1875); ibid.. Vol. XXIX (1S87).

238 STUDIES IN GENERAL PHYSIOLOGY

tiou of the water slowly rose to 25 Beaum6, after which the salt
again began to be precipitated. 1

In the course of this time progressive changes occurred in
Arteniia salinn, so that the Artemia present in the year 1874
had the characteristics of the species A. mulliausenii. These
changes in detail are the following: (1) The adult animals
of A. mulhausenii are not so large as the adult animals of
A. salina. (2) Artemia salina has caudal bristles and caudal
appendages, which are lacking in Artemia uiulhausenii ; as
the coneeiitration of the salt water increased, the caudal
bristles became progressively smaller. (3) The surface of
the gills is longer and narrower in Artemia salina than in
Artemia mulhausenii. Ludwig (in Leunis's $////o/W.s) gives
only the tirst two points. According to this author, the
length of A. mulhausenii is 08 mm. ; that of Artemia salina,
8-10 mm. Schmankewitsch was able to convert A. salina
into A. miilhausenii by increasing the amount of salt in the
aquarium.

2. By growing Artemia in salt water that was gradually
diluted, Schmankewitsch obtained a variety having the
characteristics of the genus Branchipus Schaeff. The differ-
ences are very slight. Artemia has eight, Branchipus nine,
footless terminal segments; and, what is of importance to us,
Branchipus ferox attains a greater length, the less concen-
trated the salt water in which it lives.

3. If we do not allow ourselves to be influenced by the
nomenclature of the systeniatist, the experiments and obser-
vations of Schmankewitsch show ihat the effect of the con-
centration of the s<tlt shows itself most distinctly in the.
longitudinal growth of the entire animal and of some of its
organs; ami tin's always in such a way that iritk an increase
in the concentration of the solution the longitudinal groir///

1 Beaume's hydrometer is graduated, according to Wullner, so that the point to
which it sinks in water is marked 0; that, to which it sinks in a solution of flftf-cn
p.irts of so limn chloride and 85 parts of water, 15. Water of 8" Boaum6 therefore
contains about 9 per cent, salt; that of ii" Beaume, about 23 per cent.

ORGANIZATION AND GROWTH 239

of the entire (iiiin/dl or the intlirithinl ori/tms is decreased.
The result is therefore similar to that obtained in the pre-
vious experiments; wherefore I believe that the influence of
concentration upon the conversion of the genera Artemia
into Branchipus is to a large extent nothing but an expression
of the dependence of animal growth upon the absorption of
\v;iter. Schuiankewitsch also mentions that in the same
length of time the animals in the dilute salt solution
(Branchipus) grow more rapidly than those in the concen-
trated salt solution (Artemia): "At the same temperature,
the growth of specimens of Artemia salina in highly concen-
trated sea water is less than a third as great as the growth
of Branchipus ferox in the less concentrated sea water." But
that we are, indeed, justified in this case in substituting for
the conceptions of "adaptation" and ''change of species"
toward which Schmankewitsch takes a more critical view
than most Darwinians the osmotic dependence of longitu-
dinal growth upon the concentration of the salt solution, is
well shown by a fact, which is especially emphasized by
Schmankewitsch, that the "change" persists in the suc-
ceeding generations only as long as the animals remain in
the sea-water of the altered concentration as should be the
case if we are dealing merely with osmotic effects during the
period of growth. Since not only the absorption of water,
but also the secretion of water, must be considered i-n such
experiments as have been detailed here, it is to be expected
that the growth of all animals and all organs is not atfecte:!
to the same extent by changes in the concentration of the
salt solution.

4. Schmankewitsch interprets the effect of the concent ra-
tion of the salt solution upon changes in the characteristics
of the jinimals in a different way; he attributes it solely to
the fact that with the increase in the concentration the
amount of air dissolved in the salt solution is decreased, and

240 STUDIES IN GENERAL PHYSIOLOGY

its specific gravity increased. He seems to have overlooked
the osmotic effects. The following indicates Schmanke-
witsch's (physiologically untenable) conception of the effect of
the air dissolved in the salt water: "If the salt water is not
diluted very gradually, the specimens of Artemia salina die
of exhaustion, attributable probably to the increase of oxida-
tions due to the greater amount of air in the more diluted
sea- water." In this case we probably have to do with the
fact that in the rapid dilution of the salt water the proto-
plasm of the cells of Artemia is suddenly flooded with water; 1
it is an effect similar to that brought about by rapidly thawing
out frozen organs. I will not deny, of course, that the
decrease in the amount of oxygen dissolved in a salt solution
can also inhibit growth when it exceeds a certain limit. This
is shown very clearly by the following experiments.

XI. THE NECESSITY OF OXYGEN IN REGENERATION

When sea-water was boiled, and Tubularian stems intro-
duced into it after it had cooled, no regeneration of polyps
occurred; if, however, I shook the boiled water for a time
thoroughly with air, regeneration followed. Water there-
fore must contain a certain minimal concentration of oxygen
in order to render regeneration possible.

But it can also be shown that the end of a Tubularian
which is to regenerate must be surrounded by water contain-
ing a definite amount of oxygen, and that it is not sufficient
to have merely the remaining portions of the Tubularia
taking up oxygen. The bottom of an aquarium was covered
with fine sand. I set glass tubes 5cm. long and 3-4mni. in
diameter vertically in the sand. The upper end of the tube
(Fig. 61) was drawn to a point a, which was just suf-
ficiently fine to allow the stem of a Tubularian to pass

1 Recent experiments which I have made point to the possibility that the rapid
diffusion of salts from the animal when it is brought into more dilute solution*
is, in some animals, the cause of death. [1903]

ORGANIZATION AND GROWTH

241

through. I inserted into the tube a Tubularian from which I
had previously cut off the root and polyp, so that the one
end I) was in the tube and the other end c was freely sur-
rounded by water in the aquarium. A polyp was formed
nearly always upon the free end c, but only exceptionally
upon the end b in the tube. The oxygen dis-
solved in the water within the glass tube did
not suffice to render regeneration possible at
the end b.

That regeneration only was inhibited, while
the power of regeneration was preserved, is
evidenced by the fact that if, after not too
long a time, I brought the end b back into
fresh sea-water, regeneration occurred.

This also explains some of the facts mentioned
in paper iv (Part I, pp. 123 and 124). I
mentioned that when one end of a Tubularian
is fixed in the sand, or in a narrow cleft between
two slides, no regeneration occurs at this end
(even though death does not occur at least for some time).
At that time I was inclined to attribute the result to mechan-
ical factors (pressure), but I believe now that we were prob-
ably dealing with lack of oxygen.

When I suspended Tubularia3 in the aquarium in such a
way that the one end was very close to the surface of the
sand, but did not touch it, and arranged my apparatus so
that only a small current of water entered the surface of the
aquarium, and kept the whole free from movement, no
regeneration followed at this end. The cause for this is as
above. According to Jacobscn, the layer of water just above
the mud bottom of the ocean is poor in oxygen. 1

It may be that a movement of protoplasm toward the end
of a stem is possible only when this cut end is contained in

i JACOHSKN. Annalen der < ln-mic un<l I'harmazic, Vol. CLXVII (1873).

'Sand

FIG. 61

242 STUDIES IN GENERAL PHYSIOLOGY

water which is rich in oxygen ; in other words, that we are
dealing with a case of "chemotropism."

XII. THE RELATION OF REGENERATION AND GROWTH IN
TUBULARIA TO SOME OF THE INORGANIC SUBSTANCES
CONTAINED IN THE SEA-WATER, ESPECIALLY POTASSIUM

1. The salts dissolved in sea- water may be of importance in
regeneration and growth, not only through their osmotic
effect, but also through their effect upon the metabolism of
Tnbularians. We have already become acquainted with
their osmotic effect upon regeneration and growth. We will
now investigate whether any of the substances dissolved in
sea-water are indispensable for regeneration and growth in
Tubularia. In the following experiments the weight of the
salts always refers to the dry salt after the water of crystal-
lization has been removed. One thousand parts of sea-
water, according to the analysis of Forchheiiner, contain the
following inorganic substances:

Sodium chloride - 30.292

Potassium chloride 0.779

Magnesium chloride - 3.240

Calcium sulphate 1.605

Magnesium sulphate 2.638

Silicates, calcium phosphates, and residue - 0.080

2. I added 11.3 g. of NaCl to 300 c.c, of fresh water (Serino
water) and the same amount to 300 c.c. of distilled water.
The distilled water was thoroughly shaken in the air after the

*,

addition of the salt. The amount of salt in each of the
solutions about equals that contained in ordinary sea-water.
But while the Tubularians in a control dish of 300 c.c. of
normal sea-water regenerated rapidly, no regeneration
occurred in the animals of the same colony which were put
into the pure NaCl solutions.

I now tried 300 c.c. of each of the following solutions:

ORGANIZATION AND GROWTH

I II III

3.3 g. Nat'l 3.3 g. NaCl 1.6 g. N< 1 1

100 c.c. fresh water 0.03 g. KC1 I .', g. M gS( ) ,

100 c.c. fresh water 0.03 KC1

100 c.c. fresh water

No regeneration whatsoever occurred in the first solution;
small and not entirely normal polyps formed in the second,
but no growth occurred; normal regeneration and growth
occurred in the third solution.

3. It might be possible from these experiments that the
MgSO 4 is -essential for regeneration and growth, while KC1
is only secondary. I therefore experimented with the follow-

ing solutions :

I II

2.3 g. NaCl 2.3 g. NaCl

1.0 g. MgSO 4 1 g. MgSO 4

100 c.c. fresh water 0.03 g. KC1

100 c.c sea- water

No regeneration whatsoever occurred in the first solution ;
but regeneration and growth were normal in the second.
After six days I divided the first solution and the animals
contained in it into two vessels, adding 0.05 g. of KC1 to
one of them, while I left the other unaltered. The animals
contained in the dish to which I had subsequently added
the KC1 regenerated and grew in a normal way, while not
even a suggestion of regeneration was apparent in the other.
The presence of potassium in sea-water /s therefore -neces-
sary for rei/i-iicrd/ioit.

4. It had still to be decided which constituent of MgSO 4
is essential for regeneration in Tubularia. I substituted
X;i,SO 4 for MgSO 4 , and experimented with 800 c.c. of each
of the following solutions :

I II

2.5 g. NaCl 2.5 g. NaCl

0.8 g. N;i _ SO 4 O.S g. Na 2 SO t

100 c.c. fresh water O.OU g. KC1

!<)() c.c. froh water

244 STUDIES IN GENERAL PHYSIOLOGY

No regeneration whatsoever occurred in the first solution;
in the second small abnormally formed polyps with tiny
tentacles, which soon dropped off, developed. No growth
occurred.

5. Whether other substances can be substituted for NaCl
I was not able to investigate at this time. If we omit this salt
from our conclusions, our experiments show that a small
amount of potassium must be contained in solution if the
polyps are to regenerate, but that for the formation of normal
polyps and for normal growth magnesium is also required.
Besides XaCl (for which substitutes may perhaps be found).
these two substances suffice for regeneration and growth in
Tubularia.' In only one respect did the parts regenerated in
my artificial solutions differ from those obtained in normal sea-
water the periderui of the former remained soft, while that
of the latter became hard. The importance of potassium
in the formation of cells has been emphasized by Hoppe-
Seylt-r :

Even though definite compounds of potassium with organic
substances which might be considered essential to life-processes
are unknown, the presence, without exception, of potassium in all
organisms from the lowest to the highest compels us to assume that
compounds of this metal play a necessary role in all general devel-
opmental processes. The organs of the invertebrates also always
contain potassium, and those parts of plants contain the greatest
amount of potassium which have the greatest developmental pow-
ers. The power of organisms to separate from and retain the
potassium of the liquids which nourish them is shown especially
well by the fact that fresh-water streams and lakes and sea-water
contain only very small amounts of potassium beside much larger
amounts of sodium, calcium, and magnesium, yet organisms grow-
ing in them contain much potassium, which they have been able to
collect only from these waters which are so poor in potassium, and
from nowhere else. 2

1 This is not correct. The Serino water contains Ca which is also necessary for
regeneration. [1903]

- Physiologische Chemie, p. 61.

ORGANIZATION AND GROWTH 245

XIII. FURTHER OBSERVATIONS ON THE EFFECT OF DIFFERENT
SALTS UPON REGENERATION AND GROWTH IN TUBULARIA

I. According to Hoppe-Seyler, potassium has a poisonous
effect upon higher animals when introduced in too large
amounts into the food. I have already shown that after tin-
addition of 1.3 per cent. NaCl to sea -water, regeneration is
still possible, but not growth. With this as a starting-point,
I investigated whether regeneration is still possible after the
addition of 0.0 g., 1.0 g., 1.3 g., and 1.0 g. of KC1 to each 100
c.c. of ordinary sea-water. An opaque precipitate was imme-
diately formed at both cut ends when I introduced Tubu-
lariaii stems into the two most concentrated of these solutions.
In not one of them did regeneration occur. After eight
days I returned a number of these animals which had been
in the potassium-chloride solutions to normal sea-water, in
order to determine whether they were dead, or whether
regeneration had only been inhibited in them. The animals
returned from the 0.0 per cent, and 1.0 per cent. KC1 solu-
tions to normal sea- water regenerated and grew in this; the
remainder, however, were dead.

I now added to each 100 c.c. of sea-water 0.10 g. and
0.33 g. of KC1. In the weaker of these solutions all the ani-
mals regenerated, but much more slowly than the control
animals of the same colony kept in ordinary sea-water. In
the second solution only four of the nine animals regenerated.
Growth was also much diminished. While longitudinal
growth in the animals kept in the normal sea-water amounted
to 11 mm. upon the average, it amounted to only 1 mm. in
the same time and at the same temperature after the addition
of 0.10 per cent. KC1. No measurable growth occurred in
the second solution. The addition of 0.33 g. of KC1 to 100
c.c. of sea- water therefore suffices to prevent growth entirely,
while regeneration is not stopped until 0.0 per cent. KC1 is

246 STUDIES IN GENERAL PHYSIOLOGY

added. In doses of 1.3 g. to 100 c.c. of sea-water it is fatal
to Tubularia.

2. KNO 3 inhibited growth when 0.6 g. was added to 100
c.c. of sea-water ; regeneration was prevented by the addition
of 1 g. of KNO 3 to 100 c.c. of sea-water. NaNO 3 had a
weaker effect; regeneration still followed the addition of 1.3
g. to 100 c.c. of sea-water; but when 1.6 g. of this substance
was added, regeneration did not occur. An experiment on
growth gave the following average results; ten specimens
were used in each case ; the experiment lasted eleven days,
and the temperature was about 16 C.

Average Growth

In normal sea-water 7.3 mm.

Addition of 0.6 per cent. NaXOs 1.1

Addition of 1.0 per cent. XaXOs - 0.3

The greater effect of the potassium over the sodium is less
apparent in this experiment than in the chlorine compounds
of this metal.

3. The poisonous action of NH 4 C1 upon Tubularia is very
striking. An opaque precipitate is formed at both ends of a
Tubularian stem when only 0.06 g. of NH 4 C1 is added to 100
c.c. of sea-water. A quantity 0.03 g. to each 100 c.c. of
sea-water suffices not only to inhibit all regeneration and all
growth, but renders these life processes forever impossible;
for when the animals are returned from such a solution to
normal sea-water, they no longer regenerate and grow.

4. In conclusion I wish to direct the reader's attention
to Fig. 62, which shows the regeneration and growth of
three Tubularian stems taken from the same colony.
They had been put into different salt solutions for the
same length of time at the same temperature, ab is in
all cases the original piece cut from the animal. Fig. A
remained in ordinary sea- water. The piece bd had grown at
the oral end 6, the piece ac at the aboral end a; the polyps
were very sturdy. The diameter of the new stem is nearly

ORGANIZATION AND GROWTH

247

equal to that of the old. Fig. B remained in sea-water to
which 1.3 per cent, NaCl was added. Frail polyps have
formed at each end with only four to six tentacles. The
growth is practically zero, and the diameter of the newly
grown parts is much less than that of the old stems. C re-
mained in sea-water to
which 0.3 per cent. KC1
was added. At the oral
end b a deformed polyp
without tentacles has been
formed, while no growth
whatsoever has taken place.
The drawing has been
somewhat enlarged, and
the piece ab of Fig. A is
not shown in full length in
order to save room; it was
in reality as long as that
shown in Figs. B and C.

XIV. THE RELATION OF RE-
GENERATION AND GROWTH
TO THE QUANTITY OF SEA-
WATER

FIG. :;i

It is self-evident that
for the regeneration and

growth of Tubularian stems the quantity of sea- water is of
importance only in so far as it enables the stems to obtain
from it a sufficient amount of the inorganic substances
necessary for regeneration and growth. As soon as this is
the case, variations in the amount of sea-water should have
no effect upon these processes.

In the experiments detailed thus far the amount of water
contained in cadi vessel always amounted to exactly ^00 c.c.

248 STUDIES IN GENERAL PHYSIOLOGY

This time I distributed the individual animals of a colony of
Tubulariae into different vessels, containing 10, 20, oO, 100,
and 200 c.c. of sea-water. Each vessel contained six Tubu-
laiian stems, the roots and polyps of which had been ampu-
tated at the beginning of the experiment. The vessels were
relatively Hat, and only lightly covered with a glass plate to
shut out particles of dust that might be carried in through
the air. Oxygen could therefore readily reach the sea- water.
The tirst polyps were formed after three days five in each
of the vessels containing 100 and 200 c.c. of sea- water, and
two in each of the others. Two davs later all of the animals

*

had formed new polyps. The inhibition of regeneration was
therefore only slight in the vessels containing but a small
amount of sea-water. On the eighth day after the beginning
of the experiment I measured the growth of the individual
specimens, which was as follows:

Iu 10 c.r. of Sea-Water In 200 c.c. of Sr.i-Water

8.0mm. 7.0 nun.

12.0 8.0

6.5 i;j.()

5.5 9.0

7.0 10.0

4.0 3.0

Average 7.1mm. 8.3mm.

I obtained about the same average values in the rest
of the vessels. Ten c.c. of sea-water therefore contain suffi-
cient inorganic material for normal regeneration and normal
growth, and variations in the quantity of sea-water above
this limit have no effect upon these processes. I have not
made experiments with less than 10 c.c., as these barely
suffice to cover the Tubularian stems. The result of these
experiments is free from the complication which enters
into Semper's experiments, in which the animals devoured
an uncontrolled (and possibly uncontrollable) amount of
vegetable food.

ORGANIZATION AND GROWTH 249

XV. SOME CASUISTIC REMARKS ON HETEROMORI'MOSIS

1. It was my intention to analyze the conditions under-
lying heteromOrphosis in other forms as carefully as I have
done in the case of Tubularia and Antennularia. La<-k <>f
time, however, rendered this impossible, so I was compelled
to postpone these experiments. I wish, however, to add a
few casual observations.

During the winter of 1889-90 I had already observed that
in the aquarium the stems of Gonothyrea often grew into
roots even when not injured externally. I thought at the
time that lack of light and oxygen lay at the basis of these
phenomena, but did not mention this fact, as I wished to
make it the starting-point of new experiments. For the
reasons given above, I did not succeed in mastering organi-
zation in this animal in the time at my disposal, and so
have again postponed further work upon these experiments.

I have already called attention to the tendril-like bend-
ings of the roots of Aglaophenia pluma in paper iv. These
curvatures, dependent apparently upon internal causes, play
perhaps a much more important role than I at first antici-
pated. They probably are responsible for the fact that the
orientation of the organs, even those at a distance, does not
occur with the same regularity as in Antennularia. My
first experiments were made in very intense light, and it is
possible that this is the determining factor in bringing
about the downward growth of the adventitious roots in
Aglaophenia. Considering the complexity of the conditions
determining organization, experiments upon this animal
with the klinostat might prove fruitful.

I found in Sertularia that new growths which had the
form of roots, but were positively heliotropic, formed a polyp
at their tips after they had attained a certain length, and
then remained positively heliotropic. According to Sachs,
certain substances are not only necessary in the formation of

250 STUDIES IN GENERAL PHYSIOLOGY

certain organs, but the specific reactions of an organ toward
light and gravity are also dependent upon the nature of its
substances. It might be believed, therefore, that the polyp-
forming substances also determine positive heliotropisin,
while the root-forming substances determine negative heliot-
ropism. These circumstances might therefore explain the
apparent paradoxes in the reaction of Sertularia to light. I
hope to be able to study this question experimentally, and
therefore will not enter into any further theoretical dis-
cussion.

2. I mentioned in the previous volume of these studies
that Bonnet and Dalyell had found that an organ of another
kind may occasionally grow in place of one that has been
lost. Dr. A. von Heider, of Gratz, called my attention to the
fact that he, too, had observed and described such a case. 1 I
will give his description in full here :

I have often had the opportunity of testing in Cladocora the
great powers of reproduction which Coelenterates in general are
known to possess. Without discussing the rapid healing of wounds
and the renewal of wornout portions of the body, the following case
seems worthy of description. I cut off by a rapid incision, and as
near the rim of the shell as possible, the polyp of a Cladocora,
which was protruding a great distance beyond its shell, and
allowed the animal to go on living in the aquarium. As early as
the next day the tentacles of the animal, which had been robbed of
its calcareous support, were entirely unfolded, the transverse
wound at the opposite end had puckered to a conical scar, and the
polyp moved over the bottom of the vessel by means of its ten-
tacles. \Yhen examined with a lens some weeks later, the aboral
end of the animal was completely healed and possessed of a plate
running parallel to the oral plate, at the periphery of which were
tiny elevations corresponding to the tentacles of the oral plate. In
the course of two months these developed into full-grown ten-
tacles. In the center of this new plate of tentacles was a round
opening, the newly formed mouth, so that an entire oral plate had
been formed at the cut end of the polyp, which differed in external

1 A. VON HEIDEE, Wiener Sitzungsberichte, Vol. LXXXIV, Part I (1881).

ORGANIZATION AND GROWTH

appearance in noway from the old oral plate. A slight swelling 1 of
the body-wall showed the position of the original cut in this double
polyp. The position of the latter also showed that the body itself
had grown aborally.

XVI. SUMMARY OF THE MORE IMPORTANT RESULTS

I. The orientation of organs and the place where they
originate can be controlled in Antemmlaria antennina at will
through the following circumstances:

I. The stems are negatively, the roots positively, geo-
tropic and positively stereotropic.

"2. The place where the organs form is determined by the
orientation of the animal* toward the center of the earth, so
that branches arise only on the upper surface of a stem ; or, if
the latter is in an absolutely vertical position, only from that
cut end which is directed upward. The opposite holds for
the roots, with this addition, however, that in the region
where new stems originate new roots may at times also be
formed upon the upper surface of the old stem.

3. If a growing but uninjured stem of Antermularia
antennina is suspended with its tip downward, the stem
ceases to grow as such, but roots may arise from the tip.

4. When a stem is placed horizontally or obliquely, the
branches which are directed downward may grow as roots,
even when they are not injured and not in contact with solid
bodies.

II. If a piece is cut from the stem of a Tubularian, the
regeneration of the polyp at the oral end may retard the for-
mation of a polyp at the other. By suppressing the for-
mation of the oral polyp one can accelerate considerably the
formation of the polyp at the aboral end.

III. If an incision is made into one of the tubes of a
Ciona intestinalis, ocelhu are formed at both edges of the
wound.

IV. If the entire brain of a Ciona inl.-st inalis is extir-

252 STUDIES IN GENERAL PHYSIOLOGY

pated, the reflexes are preserved, and only the threshold of
stimulation for their production is raised.

V. The brain of such an animal is regenerated in the
course of a few weeks.

VI. Growth and regeneration in Tubularia is, as in plants,
dependent upon the amount of water absorbed. Growth is
increased by an increase in the amount of water absorbed;
while it is decreased through a diminution in the amount of
water absorbed. Growth is practically zero in sea-water
containing 5.1 per cent, salt, though regeneration of polyps
is still possible; when the water contains 5.4 per cent, salt,
regeneration also is impossible. "With a decrease in the
concentration of the sea- water, growth becomes progressively
greater, until it attains a maximum in water containing 2.5
per cent. salt. If the concentration is further diminished,
growth decreases rapidly until a concentration of 1.3 per
cent, is reached, when neither regeneration nor growth any
longer takes place. The temperature was about 15 C. in
these experiments.

VII. When the pressure of oxygen is very low, regenera-
tion no longer takes place ; it is also necessary that the end
at which regeneration is to occur be constantly surrounded by
water containing a sufficient concentration of oxygen.

VIII. The salt solution in which Tubularia is to regener-
ate and grow must contain potassium and magnesium ; yet
potassium must be present only in small amounts. The
addition of 0.33 g. of KC1 to 100 c.c. of sea-water prevents
growth ; an addition of 0.6 g. to 100 c.c. of sea-water pre-
vents regeneration also.

IX. The amount of sea-water has no noticeable effect
upon growth in Tubularia so long as the animals are sur-
rounded by a sufficient amount. f

VII
EXPERIMENTS ON CLEAVAGE 1

1. IN tin- second part of my Untersuchungen zur physio-
hcii Morphologic* I showed that regeneration and
growth in animals are, as in plants, a function of the amount < >i'
water contained in the cells. When I increased the amount of
water in the cells of Hydroids by bringing these organisms
into more diluted sea-water than that in which they usually
live, the rate of growth increased with the decrease of the con-
centration of the sea-water. When I diminished the amount
of water in the tissues of Hydroids by bringing these animals
into a more concentrated solution than the normal sea-water,
the rate of growth diminished too. We know that seedlings
of plants need water in order to develop. It is the same in
the animal egfiT, as recent investigations concerning the

OO ' O CU

development of sea-urchins, starfish, arthropods, and fish
showed me. If we reduce the amount of water contained in
the egg of the sea-urchin by bringing it into more concen-
trated sea-water, the process of segmentation is retarded
only as long as the increase in the concentration is small.
As soon as the concentration is greater, however, the 1'ertil-
i/t-d egg does not segment at all. In one case the eggs had
been fertilized at 9:40 A. M. A few minutes after the impreg-
nation, one part () of the eggs were put into sea-water to
which 1 g. of NaCl to 100 c.c. had been added. A second
part (1) were put into sea-water to which I had added Log.
of XaCl to 100 c.c. A third part (r) were brought into sea-
water, the concentration of which was increased by the addi-
tion of 2 g. of XaCl to lOOc.c.; and a fourth part (J)

1 Journal of Mornho/"'///, Vol. VTI (1592), p. li.Vi.
i Wurzburu, is'.'2. Part I, p. 191.

253

254 STUDIES IN GENERAL PHYSIOLOGY

remained in normal sea-water. At 10:50 nearly all the eggs
which had remained in normal sea- water were in the two-cell
stage, while none of the eggs in the other solutions were yet
segmented; in part (.) the first egg was segmented at 10:55;
in (b) the first segmentation took place at 11:45 nearly an
hour later than in normal sea-water; and in (c) no segmenta-
tion at all took place. That the amount of water and the
ii ilra-cellular pressure in these experiments varied with the
concentration could be seen from the form of the cleavage
spheres. In normal sea-water, and still more in sea-water
which was a little diluted by the addition of 10-20 per
cent, of fresh water, the first two cleavage spheres were nearly
perfect hemispheres. In sea-water of higher concentration
the first two cleavage spheres became ellipsoidal in shape,
approaching the sphere more the higher the concentration
was. When I added more than 2 g. of NaCl to 100 c.c. of
sea- water, in a few hours plasmolysis took place, and the
surface of the protoplasm began to shrink irregularly. But
by bringing the eggs back into normal sea-water the normal
form was restored in a few minutes.

2. Further investigations concerning this subject led me
to another series of facts, which, as I believe, give the physio-
logical explanation of some of the phenomena of cleavage.
In my investigations concerning the regeneration and growth
of Hydroids, I found that a salt solution which is just con-
centrated enough to prevent regeneration and growth by no
means kills the Hydroids, or even annihilates the power of
growth and regeneration. Hydroids which had been in
such a solution for several days when brought back into normal
sea-water began to regenerate and to grow. When I made
the same experiments on fertilized eggs, the results were the
same. A salt solution which is just concentrated enough to
prevent segmentation does not annihilate the power of seg-
mentation at once. But when I brought such eggs back

EXPERIMENTS ON CLEAVAGE -~>~>

into normal sea-water, I found that the manner of s

tion changes in a remarkable way, according to the time the

eggs had been in the concentrated sea-water.

3. I fertilized eggs of sea-urchins at 9:30 in the morn-
ing, and at 9:43 a part of these eggs were put into sea-water
to which 2 g. of NaCl to 100 c.c. had been added. The rest
of the eggs remained in normal sea- water. I will call the
sea-water to which 2 g. of NaCl to 100 c.c. had been added
the concentrated solution, and the eggs which had been
exposed to it the plasmolyzed eggs. At 10:20, before any
segmentation even in the normal sea-water had taken place,
I took a lot of eggs out of the concentrated solution and
brought them back into normal sea- water. At 10:33 these
eggs began to segment. The segmentation was a normal
one, as only segmentation into two cells took place. At the
same time segmentation had taken place in nearly all of the
normal eggs. The only difference between the normal eggs
and the plasmolyzed eggs was that the former at 10:33 were
nearly all segmented, while of the latter only a small part had
undergone segmentation. Ten minutes later, however, every
second one of the plasmolyzed eggs was segmented, mostly
into two, exceptionally into four, segments. But now the
situation began to change. By this time the normal eggs
began to reach the four-cell stage, and now many of the
plasmolyzed eggs which had not yet segmented into two cells
began to segment into three or four cells at once, without
going through the two-cell stage at all. The cleavage took
place in this way, that at the same time, or shortly after each
other, spherical projections appeared on the surface of tin-
egg, which at first were coherent, but which soon, at the same
time or in quick succession, were separated. This kind of
segmentation seems to be identical with that which O. and
R. Hertwig observed under other circumstances, and have
described as Knospenfurchung? The further segmentation

I O. AND R. HERTWIG, .7o(i//x<-//<- Zr/tsrlirift, Vol XX (1S87).

25l> STUDIES IN GENERAL PHYSIOLOGY

was the same in the plasmolyzed and in the normal

At 11 o'clock I brought a second lot of eggs back from
the concentrated solution into normal sea-water. These eggs
did not show the slightest trace of segmentation. At 11:22
the eggs began to segment, but in hardly any case did the
eggs divide into two, but nearly all of them segmented into
more cleavage spheres at once. The number and size of the
cleavage spheres were not quite regular. There were mostly
about four spheres in one egg; sometimes, however, five to
eight. The size of the single cleavage spheres of the same
egg varied, the smallest spheres being about the size of a
cleavage sphere of the eight-cell stage, the largest that of a
two-cell stage. At 11:44 the first segmentation was finished,
and from now on the segmentation was perfectly regular.
At 11:40 the normal eggs were in the eight-cell stage.

At 2:40 I brought another lot of eggs from the concen-
trated solution back into normal sea-water. Not one egg
showed segmentation. At 2:50 the segmentation began.
Just as in the 11 o'clock lot, hardly one egg segmented into
two cleavage spheres. But while most of the eggs of the 11
o'clock lot segmented into from four to eight cells, most of
the eggs segmented now into from eight to sixteen cleavage
spheres at once. The number and size of the cleavage
spheres varied again in the different eggs, but the striking
feature this time was the prevalence of cleavage spheres of
the size of the sixteen-cell stage. The normal eggs by this
time were into the morula stage. At 4:05 another lot of
eggs was brought back from the concentrated solution into
normal sea-water. Not one egg had segmented. Twenty
minutes later, however, nearly all the eggs were in cleavage.
But this time they did not divide into sixteen, but into many
more segments at once. I think that most of the eggs
showed about thirty cleavage spheres. Of course, in this

EXPERIMENTS ON CLEAVAGE

lot, just as in the foregoing lots of the same kind, 1 found
cleavage spheres of very ditl'erent sixes in the same egg. At
<> :-")() 1 repeated the same experiment, taking out a lot of
eggs from the eoncentrated solution, and bringing them hack
into normal sea-water. Not one egg showed any trace of
segmentation, but in a very short time about twenty min-
utes the eggs segmented at once into a great number of
small cleavage spheres, the smallest and most numerous having
tlu 1 size of a cleavage sphere of about the sixty-four-cell stage.
I repeated this experiment about twenty times, always with
the same result, which in a few words may be expressed as
follows: If ico bring impregiKded eggs info sea-irater of a
certain higher concentration, no segmentation takes place;
hut if /re bring them back info normal sea-water, theydiride
in about In-cut // minutes directly into 'nearly, but not <]/tile,
so many clearage spheres as they irould contain l>y that
time if they had remained in normal sea-water all the time.
It must be added, however, that the normal eggs in this
experiment are always ahead of the plasmolyzed eggs in
regard to their stage of segmentation, and that their advance
becomes the more obvious the farther they develop.

Eggs, after having been in the concentrated solution from
twelve to twenty-four hours, do not segment at all if brought
back into common sea- water. All these experiments are the
more satisfactory the better the material is.

4. I varied these experiments by sometimes bringing the
impregnated eggs into the concentrated solution immediately
after impregnation, and sometimes later. The result remained
the same, on the whole, and I will not dwell upon the details
if these experiments. But the following fact may l>e of inter-
est : I impregnated eggs in normal sea-water, and left them
then- until 'hey were all in the two-cell stage. Then I
brought them into the concentrated solution. The cleavage
^topped directly. After having been there for three hours,

258 STUDIES IN GENERAL PHYSIOLOGY

I brought them back into normal sea-water; and now every
cleavage sphere divided at once into more than two pieces,
sometimes into eight or even more.

."). I concluded from the foregoing experiments Unit in tin-
concentrated solution a segmentation of the nuclei >n/<///f

fake />ldcc iriflionf (in// xc</nicnf<i/ ion of flic }>ro/o/)/<ixi.
Eggs which had been impregnated in normal sea-water were
brought into the concentrated solution and watched care-
fully. No segmentation of the protoplasm took place; but
the nucleus divided, indeed, into two, and then further divi-
sions followed. I tried, moreover, to see whether the proto-
plasm of such eggs, if brought back into normal sea-water,
divided into as many cleavage spheres as there were nuclei
preformed. I saw, indeed, that every nucleus becomes the
center of one of these projections, which later on become
cleavage spheres. Dr. Conklin was kind enough to stain
some of the eggs which had been in the concentrated solu-
tion for some time and which showed 110 trace of segment a-
t i< >n. Some of these stained eggs showed very distinctly from
four to about thirty distinct nuclei. In other eggs the seg-
mentation of the nucleus was not so perfect. The nucleus,
extremely enlarged, seemed to consist of several parts, which,
however, were still connected. These eggs had been killed
at a time when the eggs of the same lot which had remained
in normal sea- water all the time were in about the sixty-four-
cell stage.

6. Fol and O. and R. Hertwig found that in the case of
polysperrnia the egg at once divides into about as many cells as
there are asters. We know that for the segmentation of the
protoplasm it does not make any difference whether the nuclei
are derived from the male pronuclei exclusively, as in the
case of the impregnation of an enucleated egg ; or from the
conjugated nuclei, as in the normal case ; or from both conju-
gated nuclei and male pronuclei together, as in. some cases

EXPERIMENTS ON CLEAVAGE 259

described by Fol. In my experiments the eggs were impreg-
nated under normal conditions, and cases of polyspermia were
very rare indeed. Nearly all of the eggs which remained in
normal sea-water segmented quite normally. But I thought
of the possibility that new spermatozoa might enter the
impregnated egg in the concentrated solution. I knew that
such a supposition was in contradiction with all known facts,
but these facts are still meager. If a polyspermia in my
experiments took place, it could happen only in the concen-
trated solution, as here the increase of the number of the
nuclei was observed. But I found that the spermatozoa were
perfect!} paralyzed as soon as they were brought into the con-
centrated solution; that is, in the sea- water to which 2 g. of
NaCl to 100 c.c. had been added. I could show, moreover,
that in this concentrated solution no impregnation is effected.
I brought unfertilized eggs into this concentrated solution and
added spermatozoa. When I brought them back into nor-
mal sea- water, it took more time from that moment until seg-
mentation began than it took in normal eggs and in normal
sea-water from the moment of impregnation to the moment
of segmentation. The spermatozoa contained in the concen-
trated salt solution became active again a few minutes after
being brought back into normal sea-water and then entered
the eggs. Polyspermia in this case could be observed, but
not as a rule. Most of these eggs segmented into two cells.
But it was astonishing how soon the spermatozoa lost their
power of impregnating under these circumstances. Sperma-
tozoa which had been in the concentrated solution only a
few hours, when brought back into normal sea- water fertil-
ized only a thousandth part, or still less, of the normal eggs;
while spermatozoa of the same animal which had remained
in normal sea- water fertilized at the same time practically all
the eggs of the same female. When I tried to fertilize eggs
in normal sea-water which had been in the concentrated

200 STUDIES IN GENERAL PHYSIOLOGY

solution for a few hours with spermatozoa that had been
under the same conditions, only about one egg in a million
began to show some trace of segmentation, and as a rule this
segmentation remained in statu luiwmU, but was not accom-
plished. All these observations are totally different from the
phenomena described above. Eggs which had been fertilized
in normal sea-water, and which were put into the concen-
trated solution, after being brought back into normal sea- water
for from ten to twenty minutes segmented without any excep-
tion, and were able to develop into normal blastulae and plutei.
Eggs of this kind were still able to develop into normal
larvse after having been in the concentrated solution for four
to six hours. But eggs which l>cj\>rc impregnation had been
put into the concentrated solution together with spermatozoa,
and which four to six hours later were brought back into nor-
mal sea-water, reached only the first stages of segmentation,
if they segmented at all, and then stopped developing. I
never got a living larva from these eggs. From all these
facts I conclude that, the continual increase of the nuclei of
the impregnated eggs in the concentrated solution was due,
not to polyspermia, but simply to segmentation of the nucleus.
In these experiments bacteriological precautions are neces-
sary, as the water of the aquarium is liable to contain quan-
tities of spermatozoa.

7. From the above I believe to have shown that by bring-
ing fertilized eggs of sea-urchins into more concentrated sea-

O ' '

water we added 2 to 2.4 g. of NaCl to 100 c.c. of sea-water
-the segmentation of the nucleus proceeds, although more
slowly than under normal conditions, while no segmentation
of the protoplasm is possible. The fact in itself is of some
technical value, as it enables us to separate two processes
which nature generally produces together, or which hitherto
we had not the power to separate at desire. In regard to
our knowledge of segmentation, we see from this that the

EXPERIMENTS ON CLI:\V\<;E '_!i'>L

physiological conditions for segmentation of the nucleus arc
different from the physiological conditions of the segmenta-
tion of the protoplasm. \Ve now can he positive in this
regard, as under the same conditions the nucleus continues

O

segmenting, while the protoplasm does not show the slightest
trace of segmentation. But these experiments allow us to
go one step farther and to make clear one element in the
complex called segmentation, namely, the physiological cause
for the segmentation of the protoplasm. We saw that in the
concentrated solution the protoplasm did not segment, while
as soon as it was brought back into the normal sea-water it
segmented at once into about as many cleavage spheres as
nuclei were formed. All further inferences depend upon
our knowledge of the effect of salt solutions on protoplasm.
I have investigated this point myself, and have caused others
also to take up this question. The result of all investigations
hitherto carried on is as follows: Raising the concentration
of the salt solution in which an animal or a tissue lives has
the same effect as lowering the temperature; lowering the
concentration has qualitatively and quantitatively the same
effect as raising the temperature. I will mention two cases
to illustrate this. First, one example to show the parallelism
of the mentioned effect of the temperature and the concentra-
tion in qualitative regard. I have recently succeeded in
making animals belonging to different classes lame of
Polygordius, Copepods, etc. positively heliotropic by bring-
ing them into low temperatures, and making them negative] v
heliotropic by raising the temperature of the water. In water
from to about 10 larvae of Polygordius, for instance, are
exclusively positively heliotropic. In water above 25 they
are exclusively negatively heliotropic. But by adding a
certain amount of NaCl to normal sea- water I was able to
make them just as well positively heliotropic, and by adding
a certain amount of fresh water to the normal sea-water i

262 STUDIES IN GENERAL PHYSIOLOGY

could make them negatively heliotropic. The same was the
case in Copepods, only the absolute figures differ, as was to
be expected. By ~brin<juuj lirin;/ tissues into a solution of
In'ifher concentration, we reduce the irritability hij reducing
the (inionnt of water continl in f/icni. By reduction
of irritability we mean that the effect determined by the
same cause is quantitatively less. That explains how the
segmentation of the protoplasm is generally determined,
why in a solution of a certain concentration no segmenta-
tion of the protoplasm takes place, and why when brought
back into normal sea-water the protoplasm segments at
once into about as many spheres as there are nuclei pre-
formed. The se(/)nenfation of the protoplasm is the effect of
a stimulus irlticli the nucleus (t}>]>liex fo the protoplasm, mid
which makes the }-<>to]>lsiti close around flic nucleus. If
we bring the fertilized egg in the concentrated salt solution

o ~ ~

(2 g. of Nad to 100 c.c. of sea-water), the nucleus divides,
and every nucleus applies the stimulus to the protoplasm
with which it is in contact. But the protoplasm of the egg,
on account of its containing too little water, is in the condi-
tion of a cooled-off muscle, which does not answer to the
stimulation of the nerve, and no segmentation of the proto-
plasm takes place. But as soon as we bring the egg back
into normal sea-water, the protoplasm takes up water very
fast and regains its irritability ; and now, of course, it answers
to the stimuli from the nuclei, and closes around every
nucleus of segments. If we add a smaller dose of NaCl-
namely 1.3 g. of NaCl to 100 c.c. of sea-water the irrita-
bility is only a little less than it is normally, and the whole
effect is that the reactions of the protoplasm are somewhat
slower and retarded. Of what kind the stimulus is, and from
which part of the nucleus it is exercised, we cannot tell. From
other facts I am inclined to believe that this stimulus is a
chemical one, and caused by certain substances produced in

EXPERIMENTS ox CLKA v.\<; i:

the nucleus which also may be effective if separated from the
nucleus.

s. The physiological causes of the segmentation of the
nucleus are not directly touched by these experiments. But
two points ought to be mentioned : first, that the segmenta-
tion of the nucleus in the concentrated solution ( k 2 g. of NaCl
to 100 c.c. of sea-water) was retarded, and at last ceased
entirely after from twelve to twenty-four hours; secondly,
that the segmentation of the nucleus was extremely irregular
when the protoplasm did not take part in segmentation. We
see in these facts some of the influences which the proto-
plasm exercises on the segmentation of the nucleus. This
influence may be exercised in this way, that by the high
intra-cellular pressure which normally exists in the cleavage
spheres these spheres press and flatten each other. The form
of the cell, however, determines, as Sachs showed long ago,
the orientation of the plane of division, and, as Hertwig
believes, in such a way that the longitudinal axis of the
K<'rii*i>iit<l<'J is put in the longest diameter of the cell.
Therefore we ought to expect that, within certain limits, with
increasing intracellular pressure, Sach's law of the rectangu-
lar division of the planes of cleavage would become more
obvious. I found, indeed, that in normal, or still more in
somewhat diluted, sea-water, where the turgor, and conse-
quently the flattening of the cleavage spheres, was the great-
est. Sach's law was the most exactly realized. Therefore this
geometrical regularity in the segmentation of the nucleus
which is so striking under normal conditions must disappear
at once if the protoplasm dees not take part in segmentation.

!>. Our observations concerning the dependence of irrita-
bility of the protoplasm upon the water contained in the tis-
sues add one more fact to those given already to explain the
importance of water for all processes of growth and develop-
ment. If we reduce the amount of water in a regenerating or

204 STUDIES IN GENERAL PHYSIOLOGY

growing tissue, we not only retard or prevent these processes
by reducing the volume of the cells and the mechanical
effects of the intracellular pressure, but we reduce also the
irritability of the protoplasm. This irritability, as we saw,
plays an important role in the process of cleavage, and as
regeneration and grow r th is a function of processes of cleav-
age, we at once understand why regeneration and growth
must be retarded or accelerated by bringing Hydroids into
more concentrated, or more diluted, sea-water. But if this
inference' is right, our experiment holds good for the process
of cleavage not only in eggs, but in cells in general.

The experiments which are mentioned in this paper were
all made on sea-urchins (Arbacia).

The chief result of these investigations is, shortly, as fol-
lows.

If we reduce the irritability of the protoplasm of the egg
by reducing the amount of water contained in it, the nucleus
can segment without segmentation of the protoplasm. If we
increase again later the amount of water, and consequently
the irritability of such an egg, the protoplasm at once divides
into about as many cleavage spheres as there are nuclei pre-
formed. The segmentation of the protoplasm in the egg,
and probably in every cell, is only the effect of a stimulus
exercised as a rule by the nuclei.

VIII

THE ARTIFICIAL TRANSFORMATION OF POSITIVELY
HELIOTROPIC ANIMALS INTO NEGATIVELY HELIO-
TROPIC AND VICE VERSA 1

THE new facts contained in the following pages deal
chiefly with the task of rendering positively heliotropic
animals negatively heliotropic, and rice versa. I think also
that I have discovered a difference in positively and in nega-
tively heliotropic animals with regard to the liberation of
energy. As both series of observations may give us some
clue in regard to the nature of heliotropic phenomena in
general, I have briefly repeated here the description of the
simple facts of heliotropism, and have prefaced it with a
short theoretical explanation. A later part in this paper
treats of the behavior of animals, which, though not helio-
tropic, still react to the light by movements. These I shall
term photokinetic (unt&rschiedsempfindlicK). In the con-
cluding part of this paper are given the results of some further
experiments bearing 011 the causes of depth-migration and
depth-distribution in marine animals.

I. THE SIMPLE FACTS OF HELIOTROPISM

1. All former authors who have studied the behavior of
animals toward light have, without exception, been of the
opinion that animals " preferred" either light or darkness,
and correspondingly either sought the light places in spare
or shunned them. Five years ago I showed that there is a
large number of animals which are oriented by the light.
and in such a way that they are forced to place their axes
or planes of symmetry in the direction of the rays of light.

i PflQgers Archiv, V.>1. LIV (ISD'ii, p. si.

265

266 STUDIES IN GENERAL PHYSIOLOGY

This leaves still two possibilities: the oral, or the aboral,
pole maybe turned to the source of light. When the former
is the case, the animals are called positively heliotropic;
when the latter is the case, negatively heliotropic. In the
case of sessile animals orientation was brought about by the
light without any complicating secondary phenomena, and
when light fell upon them from one side only, heliotropic
curvatures resulted just as in plants. Spirographis spallan-
zaiiii gave rise to positively heliotropic curvatures; while the
stolons of Sertularia gave rise to negatively heliotropic cur-
vatures under certain conditions. If, however, the animals
are able to move freely, a complicating feature appears, in-
asmuch as the animals execute progressive movements, and
these take place in the direction of the rays of light, as the
median plane of the animals is brought into this direction.
If the animals are positively heliotropic, progressive move-
ments must occur taininl the source of light. If the
animals are negatively heliotropic, they must move away
from the light. The difference between this idea and that
of former authors is recognized immediately. According to
my idea, the fact whether the animals go toward the light or
away from it, is a consequence of their orientation by the
light a fact which former authors overlooked. Moreover, the
direction of the progressive heliotropic movements lies in
the direction of the rays of light another fact which had been
universally overlooked. The former conception, that certain
animals seek the "light," while others seek the "darkness,"
is completely refuted by the fact, which I discovered, that
positively heliotropic animals can be forced to go in the
direction of the rays of light from sunlight into the shade,
and to remain there; while negatively heliotropic animals
can be compelled to move in the direction of the rays of
light, from the shade into direct sunlight, and remain there.
A few experiments will better illustrate the nature of helio-

TRANSFORMATION OF HELIOTROPIC ANIMALS iv>7

tropic phenomena than long discussions, and as negatively
heliotropic animals are very rare indeed, much rarer than
I formerly assumed--! will illustrate the more simple facts
of heliot ropisin in such an animal and on one which I have
had the opportunity of studying in America.

2. The larv;B of Limu-
lus polyphemus the
horseshoe crab are ener-
getically negatively helio-
tropic for some time after
they have escaped from
the egg. If these animals,
which live for months
without food in a small
vessel of sea water, are
near a window,

brought

they collect during the

day in a narrow zone on the room side of the vessel. If the
vessel is carefully turned through an angle of 180, so that
the animals are brought to the window side, they at once
return in perfectly straight lines to the room side of the
dish. The animals are clumsy in their walking movements,
and tumble over very easily a fact which must of course
be considered,

It can easily be shown that the movements of the animals
follow the direction of the rays of light. Let AB in Fig.
63 represent the horizontal section of a window through
which direct sunlight falls obliquely. SS^ are the horizontal
projections of the sun's rays. The circle is the section of
the vessel in which the animals are contained. At the be-
ginning of the experiment the larva? are at C. Immediately
after being exposed to the light they begin to migrate, not.
however, in the direction CD, perpendicular to the plane of
the window, but in the direction of the sun's rays

268

STUDIES IN GENERAL PHYSIOLOGY

Nor do the animals collect at D on the room side of the dish,
but rather at E. The movements, therefore, occur in flic
direction of 1li<> niijs of liyltf. If the experiment is to be
demonstrated to others, a shadow may be thrown into the
vessel by a rod, in which case one can see directly that the

animals move parallel to
the shadow.

Attention need scarcely
be called to the fact that
if rays of light strike the
animal simultaneously
from various directions,
and the animal is able to
move freely in all direc-
tions, the more intense rays
FIG. 64 w ill determine the direc-

tion of the progressive movements.

That it makes no difference to the negatively heliotropic
Limulus larva? whether they go from regions of less intense
light to regions of greater intensity that is to say, from
the "dark" into the "light" 1 -but that only the direction
of rays of light determines the direction of the progressive
movements, is shown by the following experiment. Let AB
in Fig. 04 again be the plane of the window ; SS 1 the hori-
zontal projection of the sun's rays falling into the room
obliquely from without and above. The horizontal part of
the window frame casts the shadow CD upon the table. The
strip CD will, of course, be illuminated by reflected daylight.
I placed the vessel ef containing the Limulus larvae upon
the table so that the window side e of the dish lay in the
shadow, while the room side / of the dish was in the sunlight.
At the beginning of the experiment the larvae were collected
in the shadow on the side of the dish nearest the window.
They at once began to move to the room side in the path of

TRANSFORMATION OF HELIOTROPIC ANIMALS 209

the dotted line cj\. In the shadow the animals were oriented
by the diffused light, and as the rays fell into the dish sym-
metrically from both right and left, the animals at first
moved in a line perpendicular to the plane of the window,
but as soon as they came out of the shade into the direct
sunlight, they did not turn about, nor did they even hesitate,
but followed in the direction of the sun's rays to/ 15 where
thev remained. The animals went thus from the "dark"

ti

into the "light."

To overcome the objection that the animals "love the
light," I made a third experiment, in which the conditions
remained just as in the experiment described above, except
that I placed the dish near the window in such a way that
the room side was in the shade and the side next the window
in direct sunlight. The animals which were on the window
side at the beginning of the experiment moved, as before, in
the direction of the sun's rays out of the sun into the shade,
where they remained.

I wish to emphasize the fact that the animals remained
permanently on the room side of the dish, under all con-
ditions, no matter whether this part was in the sunlight, in
diffuse daylight, or in twilight.

These facts show, first, that the larvse of Lirnulus move
in the direction of the rays of light, away from the source
of light ; and, secondly, that they do so even when by so
doing they pass from shade into direct sunlight (or n'cc
versa).

I call those animals which are oriented by light heiio-
tropic, no matter whether, besides this, they execute pro-
gressive movements or not. But 1 wish to point out that not
every animal that is sensitive to light is also heliotropic. As
we shall see below, aside from the heliotropic, there is
another reaction to light, which does not consist in a direct
orientation of the animal.

270 STUDIES IN GENERAL PHYSIOLOGY

II. ON THE THEORY OF HELIOTROPISM

Every attempt to formulate a theory of heliotropism is
handicapped by our ignorance of the nature of the changes
which are produced by the light in the illuminated tissues.
If we acknowledge this gap, then the rest of the heliotropic
etl'ects of light upon animals may, perhaps, be understood as
follows: Let us imagine any number of sections made
parallel to the three principal axes of a bilaterally symmetri-
cal, heliotropic animal. Of these elements into which the
animal has been divided, always two which occupy symmetri-
cal positions with reference to the median plane of the
animal possess equal irritability. Every other two elements,
however, possess unequal irritability, and generally the
irritability of the oral end is greater than that of the aboral
end. Corresponding elements on the dorsal and ventral
sides have unequal irritabilities. I imagine the importance
of this distribution of irritability for the orientation of the
animals to be as follows: If the light strikes one side of the
animal, changes occur in the illuminated tissues, which at
present are unknown. In consequence. <i cli<tn<jc occurs in
flic fenxion <>f fhc ii/uxclex (or the contractile elements which
act like muscles), which may be of two kinds: the light
either brings about an increased tension of the muscles on
that side of the animal which is exposed to the light (or of
those muscles which turn the animal toward this side) ; or
the opposite occurs, and the light brings about a decrease in
the tension of these muscles and a preponderance of the
tension of their antagonists. The first takes place, as I
assume, in positively heliotropic animals; the second, in
negatively heliotropic animals. These assumptions explain
the orientation of animals by light. Let SS t (Fig. 65) be
parallel rays of light ; a the oral, b the aboral end of a helio-
tropic animal. At the beginning of the experiment the
animals move in a straight line in the direction ba. The

TRANSFORMATION OF HELIOTROPIC ANIMALS '271

FIG. 65

tension of the muscles turning the animal to the riglil
and to the left is then the same. As soon, however, as
the rays of light SS l strike the right side, the tension of
the muscles which turn the animal toward the light side
either becomes (1) greater or (2) less; and this difference in
the tension of the symmetrically situated
muscles will in either case be greater at
the more irritable, oral end a of the animal
than at the less irritable, aboral end b. In
the former case the animal will be forced
to assume the position 6a 15 and, further
more, under the same conditions, to bring
its median plane into the direction of the
rays of light; it is positively heliotropic.
In the latter case it will be forced to assume
the position ba.,', it is negatively heliotropic.
As soon as the plane of symmetry coincides with the direction
of the rays of light, symmetrically situated points on the
body of the animal are struck at the same angle by equally
strong rays of light, and the animal can then no longer be
driven either to the right or to the left by the light, and
consequently continues to move in the direction of the rays
of light. As soon, however, as the animal is again disturbed
in its movements in. this direction, through some other
external or internal stimulus, symmetrically situated points of
the animal are again stimulated unequally by the light. In
consequence there is a corresponding change in the tension
of the symmetrical muscles, and as a result of this the
animal is again brought into its proper orientation.

I wish, however, particularly to emphasize the fact that
the progressive movement O f heliotropic animals in the
direction of the rays of light is a fact which can be directly
observed MIK! demonstrate'!, and is not a mere hypothesis.

The question further arises whether facts are indeed at

272 STUDIES IN GENERAL PHYSIOLOGY

hand to show that the phenomena of the liberation of-energy
produced by the light show different characteristics in posi-
tively and negatively heliotropic animals to correspond with
this theory. Negatively as well as positively heliotropic
animals execute progressive movements under the influence
of light, independently <>f their orientation, and a difference
could be expected only in the efforts which the animal must
make to execute the given progressive movements. It might
be thought that in positively heliotropic animals light brings
about a condition of the muscles or the nervous system in
which the liberation of energy is made easier, while in nega-
tively heliotropic ;mim;ds a condition of the muscles is
brought about by the light in which the liberation of energy
is made more difficult. I have given some observations in
sec. 4 of this paper which seem to sustain such an assumption.
Before doing this, however, I wish to acquaint the reader with
a series of new facts which deal with the transformation of
positive heliotropisui into negative, and vice versa.

III. ON THE TRANSFORMATION OF POSITIVE HELIOTROPISM
INTO NEGATIVE HELIOTROPISM, AND THE REVERSE

1. In my earlier papers I was able to describe such ani-
mals only as were constantly positively or negatively helio-
tropic. Later. Groom and I described some observations at
Naples on the behavior of the iiauplii of Balanus perforatus,
and certain other marine animals, which were at times nega-
tively heliotropic. and at other times positively heliotropic. 1
We found that the intensity of the light determines the sense
of heliotropism in these animals. Above a certain intensity
light makes these animals negatively heliotropic, and this
the more quickly the greater the intensity of the light.
By lamplight the animals were always positively heliotropic.

I have made further experiments 011 pelagic animals at

i " Der Heliotropismus der Nauplien von Balanus perforatus," Biologisches
Centralblutt, Vol. X.

TRANSFORMATION' OF HELIOTROPIC ANIMALS 273

Woods Hole on tin- artificial transformation of positive lieliot-
n >| )isiii into negative heliotropisiu, and rice verxa. I obtained
the best results in the lame- of Polygordius in the early
stages of development. These appeared in count less num-
bers for about two weeks in June in the surface dredging
near the coast of Woods Hole, and I was able to collect my
material iu abundance and in good condition.

Immediately after the larvae had been caught they were
always negatively heliotropic. When they were left undis-
turbed, they