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THE
MECHANISTIC CONCEPTION

OF LIFE

THE UNIVERSITY OF CHICAGO PRESS
CHICAGO. ILLINOIS

Bgcnts
THE BAKER & TAYLOR COMPANY

NEW YORK

THE CAMBRIDGE UNIVERSITY PRESS

LONDON AND EDINBURGH

THE
MECHANISTIC CONCEPTION

OF LIFE

BIOLOGICAL ESSAYS

JACQUES LOEB, M.D., PH.D., SO.D.

MEMBER OF THE ROCKEFELLER INSTITUTE
FOR MEDICAL RESEARCH

THE UNIVERSITY OF CHICAGO PRESS
CHICAGO, ILLINOIS

Published July 1912

Composed and Printed By

The University of Chicago Press

Chicago, Illinois, U.S.A.

PREFACE

The essays contained in this volume were written on differ-
ent occasions mostly in response to requests for a popular
presentation of the results of the author's investigations. The
title of the volume characterizes their general tendency as an
attempt to analyze life from a purely physico-chemical view-
point. Since they deal to a large extent with the personal
work of the author, repetition was unavoidable, but in view of
the technical difficulties presented by some of the topics this
may serve to facilitate the understanding of the subject.

The author wishes to thank the editors and publishers who
gave their consent to the reprinting of these essays: Professor
J. McKeen Cattell, of Columbia University, Professor Albert
Charles Seward, of the University of Cambridge, England,
Ginn & Co., of Boston, G. P. Putnam's Sons, of New York and
London, and the J. B. Lippincott Company, Philadelphia.

The Rockefeller Institute

FOR Medical Research

April 4, 1912

TABLE OF CONTENTS

PAGE

I. The Mechanistic Conception of Life - _ - - 3

IL The Significance of Tropisms for Psychologj'- - - 35

IIL Some Fundamental Facts and Conceptions concerning the
Comparative Physiology of the Central Nervous Sys-
tem ----^-----65

IV. Pattern Adaptation of Fishes and the Mechanism of

Vision _-------- 79

V. On Some Facts and Principles of Physiological Morphol-
ogy ---------- 85

VI. On the Nature of the Process of Fertihzation - - 113

VII. On the Nature of Formative Stimulation (Artificial Par-
thenogenesis) - - - - - - - -127

VIII. The Prevention of the Death of the Egg tlii-ough the Act

of Fertilization __--__- 155

IX. The Role of Salts in the Preservation of Life - - - 169

X. Experimental Study of the Influence of Environment on

Animals --------- 195

Index ----------- 229

I. THE MECHANISTIC CONCEPTION OF LIFE

THE MECHANISTIC CONCEPTION OF LIFE^

I. INTRODUCTORY

It is the object of this paper to discuss the question
whether our present knowledge gives us any hope that
ultimately life, i.e., the sum of all life phenomena, can be
unequivocally explained in physico-chemical terms. If on
the basis of a serious survey this question can be answered
in the affirmative our social and ethical life will have to be
put on a scientific basis and our rules of conduct must
be brought into harmony with the results of scientific
biology.

It is seemingly often taken for granted by laymen that
'Hruth" in biology, or science in general, is of the same order
as ''truth" in certain of the mental sciences; that is to say,
that everything rests on argument or rhetoric and that what
is regarded as true today may be expected with some proba-
bility to be considered untrue tomorrow. It happens in sci-
ence, especially in the descriptive sciences like paleontology or
zoology, that hypotheses are forwarded, discussed, and then
abandoned. It should, however, be remembered that modern
biology is fundamentally an experimental and not a descrip-
tive science; and that its results are not rhetorical, but always
assume one of two forms: it is either possible to control a
life phenomenon to such an extent that we can produce it at
desire (as, e.g., the contraction of an excised muscle); or we
succeed in finding the numerical relation between the con-
ditions of the experiment and the biological result (e.g.,

1 Address delivered at the First International Congress of jSIonists at Ham-
burg, September 10, 1911; reprinted from Popular Science Monthly, January,
1912, by coiu-tesy of Professor J. ZSIcKeen CatteU.

4 The Mechanistic Conception of Life

Mendel's law of heredity). Biology as far as it is based on
these two principles cannot retrogress, but must advance.

II. the beginning of scientific biology

Scientific biology, defined in this sense, begins with the
attempt made by Lavoisier and Laplace (1780) to show
that the quantity of heat which is formed in the body of a
warm-blooded animal is equal to that formed in a candle,
provided that the quantities of carbon dioxide formed in both
cases are identical. This was the first attempt to reduce
a life phenomenon, namely, the formation of animal heat,
completely to physico-chemical terms. What these two
investigators began with primitive means has been completed
by more recent investigators — Pettenkofer and Voit, Rubner,
Zuntz and Atwater. The oxidation of a food-stuff always
furnishes the same amount of heat, no matter whether it
takes place in the living body or outside.

These investigations left a gap. The substances which
undergo oxidations in the animal body — starch, fat, and
proteins — are substances which at ordinary temperature are
not easily oxidized. They require the temperature of the
flame in order to undergo rapid oxidation through the oxygen
of the air. This discrepancy between the oxidations in the
living body and those in the laboratory manifests itself also
in other chemical processes, e.g., digestion or hydrolytic
reactions, which were at first found to occur outside the
living body rapidly only under conditions incompatible with
life. This discrepancy was done away with by the physical
chemists, who demonstrated that the same acceleration of
chemical reactions which is brought about by a high tempera-
ture can also be accomplished at a low temperature with the
aid of certain specific substances, the so-called catalyzers.
This progress is connected pre-eminently with the names of
Berzelius and Wilhelm Ostwald. The specific substances

The Mechanistic Conception of Life 5

which accelerate the oxidations at body temperature suffi-
ciently to allow the maintenance of life are the so-called
ferments of oxidation.

The work of Lavoisier and Laplace not only marks the
beginning of scientific biology, it also touches the core of the
problem of life; for it seems that oxidations form a part, if
not the basis, of all life phenomena in higher organisms.

III. the 'Griddle of life"

By the ''riddle of life" not everybody will understand the
same thing. We all, however, desire to know how life
originates and what death is, since our ethics must be
influenced to a large extent through the answer to this ques-
tion. We are not yet able to give an answer to the question
as to how life originated on the earth. We know that every
living being is able to transform food-stuffs into living matter;
and we also know that not only the compounds which are
formed in the animal body can be produced artificially, but
that chemical reactions which take place in living organisms
can also be repeated at the same rate and temperature in
the laboratory. The gap in our knowledge which we feel
most keenly is the fact that the chemical character of the
catalyzers (the enzymes or ferments) is still unknown.
Nothing indicates, however, at present that the artificial
production of living matter is beyond the possibilities of
science.

This view does not stand in opposition to the idea of
Arrhenius that germs of sufficiently small dimensions are
driven by radiation-pressure through space; and that these
germs, if they fall upon new cosmic bodies possessing water,
salts, and oxygen, and the proper temperature, give rise to a
new evolution of organisms. Biology will certainly retain
this idea, but I believe that we must also follow out the
other problem: namely, we must either succeed in producing

6 The Mechanistic Conception of Life

living matter artificially, or we must find the reasons why
this is impossible.

IV. . THE activation OF THE EGG

Although we are not yet able to state how life originated
in general, another, more modest problem, has been solved,
that is, how the egg is caused by the sperm to develop into
a new individual. Every animal originates from an egg and
in the majority of animals a new individual can only then
develop if a male sex-cell, a spermatozoon, enters into the egg.
The question as to how a spermatozoon can cause an egg to
develop into a new individual was twelve years ago still
shrouded in that mystery which today surrounds the origin
of life in general. But today we are able to state that the
problem of the activation of the egg is for the most part
reduced to physico-chemical terms. The egg is in the
unfertilized condition a single cell with only one nucleus.
If no spermatozoon enters into it, it perishes after a
comparatively short time, in some animals in a few hours, in
others in a few days or weeks. If, however, a spermatozoon
enters into the egg, the latter begins to develop, i.e., the
nucleus begins to divide into two nuclei and the egg which
heretofore consisted of one cell is divided into two cells.
Subsequently each nucleus and each cell divides again into
two, and so on. These cells have, in many eggs, the tendency
to remain at the surface of the egg or to creep to the surface,
and later such an egg forms a hollow sphere whose shell con-
sists of a large number of cells. On the outer surface of this
hollow sphere cilia are formed and the egg is now transformed
into a free-swimming larva. Then an intestine develops
through the growing in of cells in one region of the blastula
and gradually the other organs, skeleton, vascular system,
etc., originate. Embryologists had noticed that occasionally
the unfertilized eggs of certain animals, e.g., sea-urchins,

The Mechanistic Conception of Life

worms, or even birds, show a tendency to a nuclear or even a
cell division; and R. Hertwig, Mead, and Morgan had suc-
ceeded in inducing one or more cell divisions artificially in
such eggs. But the cell divisions in these cases never led
to the development of a larva, but at the best to the formation
of an abnormal mass of cells which soon perished.

I succeeded twelve years ago in causing the unfertilized
eggs of the sea-urchin to develop into swimming larvae by
treating them with sea-water, the concentration of which was

P C7

Fig. 1

Fig. 2

Fig. 1. — Unfertilized egg of the sea-urchin surrounded by spermatozoa. Only
the heads of the spermatozoa are drawn, since at the magnification used the tails
were not visible.

Fig. 2. — The same egg immediately after the entrance of the spermatozoon.
The egg is surrounded by a larger circle, the fertilization membrane, which is
formed tlxrough the action of the spermatozoon. This formation of a fertilization
membrane can be induced by a purely chemical treatment of the egg.

raised through the addition of a small but definite quantity
of a salt or sugar. The eggs were put for two hours into a
solution the osmotic pressure of which had been raised to a
certain height. When the eggs were put back into normal
sea-water they developed into larvae and a part of these
larvae formed an intestine and a skeleton. The same result
was obtained in the eggs of other animals, star-fish, worms,
and moUusks. These experiments proved the possibility
of substituting physico-chemical agencies for the action of
the living spermatozoon, but did not yet explain how the
spermatozoon causes the development of the egg, since in

8 The Mechanistic Conception of Life

these experiments the action of the spermatozoon upon the
egg was very incompletely imitated. When a spermatozoon
enters into the egg it causes primarily a change in the surface
of the egg which results in the formation of the so-called
membrane of fertilization. This phenomenon of membrane
formation which had always been considered as a phenomenon
of minor importance did not occur in my original method of
treating the egg with hypertonic sea-water. Six years ago
while experimenting on the Californian sea-urchin, Strongylo-
centrotus purpuratus, I succeeded in finding a method of
causing the unfertilized egg to form a membrane without
injuring the egg. This method consists in treating the eggs
for from one to two minutes with sea-water to which a definite
amount of butyric acid (or some other monobasic fatty acid)
has been added. If after that time the eggs are brought back
into normal sea-water, all form a fertilization membrane in
exactly the same way as if a spermatozoon had entered.
This membrane formation or rather the modification of the
surface of the egg which underlies the membrane formation
starts the development. It does not allow it, however, to
proceed very far at room temperature. In order to allow
the development to go farther it is necessary to submit the
eggs after the butyric acid treatment to a second operation.
Here we have a choice between two methods. We can
either put the eggs for about one half-hour into a hypertonic
solution (which contains free oxygen) ; or we can put them for
about three hours into sea-water deprived of oxygen. If the
eggs are then returned to normal sea- water containing oxygen
they all develop; and in a large number the development is as
normal as if a spermatozoon had entered.

The essential feature is therefore the fact that the develop-
ment is caused by two different treatments of the egg; and
that of these the treatment resulting in the formation of
the membrane is the more important one. This is proved

The Mechanistic Conception of Life

Fig. 3

Fig. 4

Fig. 5

Figs. 3, 4, and 5. — Segmentation of the sea-urchin egg, resulting in the
formation of two cells (Fig. 5). The changes from Fig. 3 to Fig. 5 occur in about
one minute or less time. This segmentation occurs after fertilization or after the
chemical treatment of the egg described in the text.

Fig. 6

Fig. 7

Figs. 6 and 7. — The sea-urchin egg divided into four and eight cells
respectively.

The Mechanistic Conception of Life

by the fact that in certain forms, as for instance the star-fish,
the causation of the artificial membrane formation may
suffice for the development of normal larvae; although here,
too, the second treatment increases not only the number of
larvae, but also improves the appearance of the larvae, as
R. Lillie found.

The question now arises, how the membrane formation
can start the development of the egg. An analysis of the

Fig. 8

Fig. 9

Fig. 8. — Blastula. First larval stage of the sea-urchin egg. At the surface
of the cells cilia are formed and the larva begins to swim and reaches the surface
of the water.

Fig. 9. — Gastrula stage. The intestine begins to form and the first indica-
tion of the skeleton appears in the form of fine crystals.

process and of the nature of the agencies which cause it
yielded the result that the unfertilized egg possesses a super-
ficial cortical layer, which must be destroyed before the egg
can develop. It is immaterial by what means this superficial
cortical layer is destroyed. All agencies which cause a
definite type of cell destruction — the so-called cytolysis —
cause also the egg to develop, as long as their action is limited
to the surface layer of the cell. The butyric acid treatment
of the egg mentioned above only serves to induce the destruc-
tion of this cortical layer. In the eggs of some animals this
cortical layer can be destroyed mechanically by shaking the

The Mechanistic Conception of Life

egg, as A. P. Mathews found in the case of star-fish eggs and I
in the case of the eggs of certain worms. In the case of the
eggs of the frog it suffices to pierce the cortical layer with a
needle, as Bataillon found in his beautiful experiments a
year ago.^ The mechanism by
which development is caused
is apparently the same in all
these cases, namely, the de-
struction of the cortical layer
of the eggs. This can be
caused generally by certain
chemical means which play a
role also in bacteriology; but
it can also be caused in special
cases by mechanical means,
such as agitation or piercing of
the cortical layer. It may be
mentioned parenthetically that
foreign blood sera have also a
cytolytic effect, and I succeeded
in causing membrane formation and in consequence the devel-
opment of the sea-urchin egg by treating it with the blood of
various animals, e.g., of cattle, or the rabbit.

Recently Shearer has succeeded in Plymouth in causing a
number of parthenogenetic plutei produced by my method to
develop beyond the stage of metamorphosis, and Delage has
reported that he raised two larvae of the sea-urchin produced
by artificial parthenogenesis to the stage of sexual maturity.
We may, therefore, state that the complete imitation of the
developmental effect of the spermatozoon by certain physico-
chemical agencies has been accomplished.

I succeeded in showing that the spermatozoon causes the
development of the sea-urchin egg in a way similar to that

1 This method does not work with the eggs of fish and is apparently as limited
in its applicability as the causation of development by mechanical agitation.

Fig. 10. — Pluteus stage of Strongy-
locentrotus purpuratus. S skeleton;
D intestine.

12 The Mechanistic Conception of Life

in my method of artificial parthenogenesis; namely, by carry-
ing two substances into the egg, one of which acts like the
butyric acid and induces the membrane formation, while
the other acts like the treatment with a hypertonic solution
and enables the full development of the larvae. In order to
prove this for the sea-urchin egg foreign sperm, e.g., that of
the star-fish, must be used. The sperm of the sea-urchin
penetrates so rapidly into the sea-urchin egg that almost
always both substances get into the egg. If, however, star-
fish sperm is used for the fertilization of the sea-urchin egg,
in a large number of cases, membrane formation occurs
before the spermatozoon has found time to penetrate entirely
into the egg. In consequence of the membrane formation
the spermatozoon is thrown out. Such eggs behave as if
only the membrane formation had been caused by some
artificial agency, e.g., butyric acid. They begin to develop,
but soon show signs of disintegration. If treated with a
hypertonic solution they develop into larvae. In touching
the egg contents the spermatozoon had a chance to give off
a substance which liquefied the cortical layer and thereby
caused the membrane formation by which the further
entrance of the spermatozoon into the egg was prevented.
If, however, the star-fish sperm enters completely into the
egg before the membrane formation begins, the spermatozoon
carries also the second substance into the egg, the action of
which corresponds to the treatment of the egg with the hyper-
tonic solution. In this case the egg can undergo complete
development into a larva.

F. Lillie has recently confirmed the same fact in the egg
of a worm, Nereis. He mixed the sperm and eggs of Nereis
and centrifuged the mass. In many cases the spermatozoa
which had begun to penetrate into the egg were thrown off
again. The consequence was that only a membrane forma-
tion resulted without the spermatozoon penetrating into the

The Mechanistic Conception of Life 13

egg. This membrane formation led only to a beginning but
not to a complete development. We may, therefore, con-
clude that the spermatozoon causes the development of the
egg in a way similar to that which takes place in the case of
artificial parthenogenesis. It carries first a substance into
the egg which destroys the cortical layer of the egg in the
same way as does butyric acid; and secondly a substance
which corresponds in its effect to the influence of the hyper-
tonic solution in the sea-urchin egg after the membrane
formation.

The question arises as to how the destruction of the corti-
cal layer can cause the beginning of the development of the
egg. This question leads us to the process of oxidation.
Years ago I had found that the fertilized sea-urchin egg can
only develop in the presence of free oxygen; if the oxygen
is completely withdrawn the development stops, but begins
again promptly as soon as oxygen is again admitted. From
this and similar experiments I concluded that the spermato-
zoon causes the development by accelerating the oxidations
in the egg. This conclusion was confirmed by experiments
by O. Warburg and by Wasteneys and myself in which it was
found that through the process of fertilization the velocity
of oxidations in the egg is increased to four or six times its
original value. Warburg was able to show that the mere
causation of the membrane formation by the butyric acid
treatment has the same accelerating effect upon the oxidations
as fertilization.

What remains unknown at present is the way in which
the destruction of the cortical layer of the egg accelerates the
oxidations. It is possible that the cortical layer acts like
a solid crust and thus prevents the oxygen from reaching
the surface of the egg or from penetrating into the latter
sufficiently rapidly. The solution of these problems must
be reserved for further investigation.

14 The Mechanistic Conception of Life

We therefore see that the process of the activation of
the egg by the spermatozoon, which twelve years ago was
shrouded in complete darkness, is today practically com-
pletely reduced to a physico-chemical explanation. Con-
sidering the youth of experimental biology we have a right
to hope that whd.t has been accomplished in this problem will
occur in rapid succession in those problems which today still
appear as riddles.

V. NATURE OF LIFE AND DEATH

The nature of life and of death are questions which
occupy the interest of the layman to a greater extent than
possibly any other purely theoretical problem; and we can
well understand that humanity did not wait for experimental
biology to furnish an answer. The answer assumed the
anthropomorphic form characteristic of all explanations of
nature in the prescientific period. Life was assumed to begin
with the entrance of a '^life principle" into the body; that
individual life begins with the egg was of course unknown to
primitive or prescientific man. Death was assumed to be
due to the departure of this ''life principle'' from the body.

Scientifically, however, individual life begins (in the case
of the sea-urchin and possibly in general) with the accelera-
tion of the rate of oxidation in the egg, and this acceleration
begins after the destruction of its cortical layer. Life of
warm-blooded animals — man included — ends with the cessa-
tion of oxidation in the body. As soon as oxidations have
ceased for some time, the surface films of the cells, if they
contain enough water and if the temperature is sufficiently
high, become permeable for bacteria, and the body is
destroyed by micro-organisms. The problem of the begin-
ning and end of individual life is physico-chemically clear.
It is, therefore, unwarranted to continue the statement that
in addition to the acceleration of oxidations the beginning of

The Mechanistic Conception of Life 15

individual life is determined by the entrance of a meta-
physical ''life principle" into the egg; and that death is
determined, aside from the cessation of oxidations, by the
departure of this ''principle" from the body. In the case of
the evaporation of water we are satisfied with the explanation
given by the kinetic theory of gases and do not demand that
— to repeat a well-known jest of Huxley — the disappearance
of the "aquosity" be also taken into consideration.

VI. HEREDITY

It may be stated that the egg is the essential bearer of
heredity. We can cause an egg to develop into a larva
without sperm, but we cannot cause a spermatozoon to
develop into a larva without an egg. The spermatozoon
can influence the form of the offspring only when the two
forms are rather closely related. If the egg of a sea-urchin
is fertilized with the sperm from a different species of sea-
urchin, the larval form has distinct paternal characters. If,
however, the eggs of a sea-urchin are fertilized with the sperm
of a more remote species, e.g., a star-fish, the result is a sea-
urchin larva which possesses no paternal characters, as I
found and as Godlewski, Kupelwieser, Hagedoorn, and
Baltzer were able to confirm. This fact has some bearing
upon the further investigation of heredity, inasmuch as it
shows that the egg is the main instrument of heredity, while
apparently the spermatozoon is restricted in the transmission
of characters to the offspring. If the difference between
spermatozoon and egg exceeds a certain limit the hereditary
effects of the spermatozoon cease and it acts merely as an
activator to the egg.

As far as the transmission of paternal characters is con-
cerned, we can say today that the view of those authors was
correct who, with Boveri, localized this transmission not only
in the cell nucleus, but in a special constituent of the nucleus,

16 The Mechanistic Conception of Life

the chromosomes. The proof for this was given by facts found
along the lines of Mendelian investigations. The essential
law of Mendel, the law of segregation, can in its simplest
form be expressed in the following way. If we cross two
forms which differ in only one character every hybrid resulting
from this union forms two kinds of sex-cells in equal numbers;
two kinds of eggs if it is a female, two kinds of spermatozoa
if it is a male. The one kind corresponds to the pure paternal,
the other to the pure maternal type. The investigation of
the structure and behavior of the nucleus showed that the
possibility for such a segregation of the sex-cells in a hybrid
can easily be recognized during a given stage in the formation
of the sex-cells, if the assumption is made that the chromo-
somes are the bearers of the paternal characters. The proof
for the correctness of this view was furnished through the
investigation of the heredity of those qualities which occur
mainly in one sex; e.g., color blindness which occurs pre-
eminently in the male members of a family.

Nine years ago McClung published a paper which solved
the problem of sex determination, at least in its essential
feature. Each animal has a definite number of chromosomes
in its cell nucleus. Henking had found that in a certain
form of insects {Pyrrhocoris) two kinds of spermatozoa exist
which differ in the fact that the one possesses a nucleolus
while the other does not. Montgomery afterward showed
that Henking's nucleolus was an accessory chromosome.
McClung first expressed the idea that this accessory chro-
mosome was connected with the determination of sex. Con-
sidering the importance of this idea we may render it in his
own words:

A most significant fact, and one upon wliich almost all investi-
gators are united in opinion, is that the element is apportioned to
but one-half of the spermatozoa. Assuming it to be true that the
chromatin is the important part of the cell in the matter of heredity,

The Mechanistic Conception of Life 17

then it follows that we have two kinds of spermatozoa that differ from
each other in a vital matter. We expect, therefore, to find in the off-
spring two sorts of individuals in approximately equal numbers, imder
normal conditions, that exhibit marked differences in structure. A
careful consideration will suggest that notliing but sexual characters
thus divides the members of a species into two well-defined groups,
and we are logically forced to the conclusion that the peculiar chromo-
some has some bearing upon the arrangement.

I must here also point out a fact that does not seem to have the
recognition it deserves; viz., that if there is a cross-division of the
chromosomes in the maturation mitoses, there must be two kinds of
spermatozoa regardless of the presence of the accessory chromosome.
It is thus possible that even in the absence of any specialized element
a preponderant maleness would attach to one-half of the spermatozoa,
due to the ''qualitative division of the tetrads."

The researches of the following years, especially the
brilliant work of E. B. Wilson, Miss Stevens, T. H. Morgan,
and others, have amply confirmed the correctness of this
ingenious idea and cleared up the problem of sex determina-
tion in its main features.

According to McClung each animal forms two kinds of
spermatozoa in equal numbers, which differ by one chromo-
some. One kind of spermatozoa produces male animals,
the other female animals. The eggs are all equal in these
animals. More recent investigations, especially those of
E. B. Wilson, have shown that this view is correct for many
animals.

While in many animals there are two kinds of sperma-
tozoa and only one kind of eggs, in other animals two kinds
of eggs and only one kind of spermatozoa are formed, e.g.,
sea-urchins and certain species of birds and of butterflies
(Abraxas). In these animals the sex is predetermined in the
egg and not in the spermatozoon. It is of interest that,
according to Guyer, in the human being two kinds of sperma-
tozoa exist and only one kind of eggs; in man, therefore, sex
is determined by the spermatozoon.

The Mechanistic Conception of Life

How is sex determination accomplished ? Let us take
the case which according to Wilson is true for many insects
and according to Guyer for human beings, namely, that there
are two kinds of spermatozoa and one kind of eggs. According
to Wilson all unfertilized eggs contain in this case one so-called

Figs. 11-16 (after E. B. Wilson). — Diagrammatic presentation of sex deter-
mination in an insect (Protenor). a a are the nuclei of im^fertUized eggs. Each
contains one sex cliromosome marked X; the other six dark spots are the chromo-
somes which are supposed to transmit hiereditary characters not connected with
sex. b and c represent the two different types of sperm; b containing a sex
chromosome X, c being without such a chromosome.

d represents the constitution of the egg nucleus after it is fertilized by a
spermatozoon of the type b containing a sex chromosome. This egg now has two
sex chromosomes and therefore wiU give rise to a female, e represents a fertilized
egg after a spermatozoon of the type c (without a sex chromosome) has entered it.
This egg contains after fertilization only one sex chromosome X and hence will
give rise to a male.

sex chromosome, the X-chromosome. There are two kinds
of spermatozoa, one with and one without an X-chromosome.
Given a sufficiently large number of eggs and of spermatozoa,
one-half of the eggs will be fertilized by spermatozoa with
and one-half by spermatozoa without an X-chromosome.
Hence one-half of the eggs will contain after fertilization two
X-chromosomes each and one-half only one X-chromosome

The Mechanistic Conception of Life 19

each. The eggs containing only one X-chromosome give
rise to males, those containing two X-chromosomes give rise
to females — as Wilson and others have proved. This seems
to be a general law for those cases in which there are two
kinds of spermatozoa and one kind of eggs.

These observations show why it is impossible to influence
the sex of a developing embryo by external influences. If,
for example, in the human being a spermatozoon without an
X-chromosome enters into an egg, the egg will give rise to a
boy, but if a spermatozoon with an X-chromosome gets into
the egg the latter will give rise to a girl. Since always both
kinds of spermatozoa are given off by the male it is a mere
matter of chance whether a boy or a girl originates; and
it agrees with the law of probability that in a large popula-
tion the number of boys and girls born within a year is
approximately the same.^

These discoveries solved also a series of other difficulties.
Certain types of twins originate from one egg after fertiliza-
tion. Such twins have always the same sex, as we should
expect, since the cells of both twins have the same number of
X-chromosomes.

In plant lice, bees, and ants, the eggs may develop with
and without fertilization. It was known that from fertilized
eggs in these animals only females develop, males never.
It was found that in these animals the eggs contain only one
sex chromosome; while in the male are found two kinds of
spermatozoa, one with and one without a sex chromosome.
For Phylloxera and Aphides it has been proved with certainty
by Morgan and others that the spermatozoa which contain no
sex chromosome cannot live, and the same is probably true
for bees and ants. If, therefore, in these animals an egg is

1 It is stated that the number of males bom exceeds that of the females by a
slight percentage. If this statement is correct it must be due to a secondary-
cause, e.g., a greater motility or greater duration of life of the male spermatozoon.
Further researches will be needed to clear up this point.

20 The Mechanistic Conception of Life

fertilized it is always done by a spermatozoon which contains
an X-chromosome. The egg has, therefore, after fertilization
in these animals always two X-chromosomes and from such
eggs only females can arise.

It had been known for a long time that in bees and ants
the unfertilized eggs can also develop, but such eggs give
rise to males only. This is due to the fact that the eggs of
these animals contain only one X-chromosome and from eggs
with only one chromosome only males can arise (at least in the
case of animals in which the male is heterozygous for sex).

The problem of sex determination has, therefore, found a
simple solution, and simultaneously Mendel's law of segrega-
tion also finds its solution.

In many insects and in man the cells of the female have
two sex chromosomes. In a certain stage of the history of the
egg one-half of the chromosomes leave the egg (in the form
of the '^ polar-body") and it keeps only half the number of
chromosomes. Each egg, therefore, retains only one X or
sex chromosome. In the male the cells have from the begin-
ning only one X-chromosome and each primordial sperma-
tozoon divides into two new (in reality into two pairs of)
spermatozoa, one of which contains an X-chromosome
while the other is without such a chromosome. What can be
observed here directly in the male animal takes place in
every hybrid; during the critical, so-called maturation divi-
sion of the sexual cell in the hybrid, a division of the chromo-
somes occurs, whereby only one-half of the sex-cells receive
the hereditary substance in regard to which the two original
pure forms differ.

That this is not a mere assumption can be sho"v\Ti in those
cases in which the hereditary character appears only, or pre-
eminently, in one sex as, e.g., color blindness which appears
mostly in the male. If a color-blind individual is mated with
an individual with normal color vision the heredity of color

The Mechanistic Conception of Life 21

blindness in the next two generations corresponds quantita-
tively ^vith what we must expect on the assumption that the
chemical substances determining color vision are contained
in the sex chromosomes. In the color-blind individual some-
thing is lacking which can be found in the individual with
normal color perception. The factor for color vision is obvi-
ously transmitted through the sex chromosome. In the next
generation color blindness cannot appear, since each fertilized
egg contains the factor for color perception. In the second
generation, however, the theory demands that one-half of the
males should be color blind. In man these conditions cannot
be verified. T. H. Morgan has found in a fly (Drosophila) a
number of similar sex-limited characters which behave like
color blindness, e.g., lack of pigment in the eyes. These
flies have normally red eyes. Morgan has observed a muta-
tion with white eyes, which occurs in the male. When he
crossed a white-eyed with a red-eyed female all flies of the
first generation were red-eyed, since all flies had the factor
for pigment in their sex-cells; in the second generation all
females and exactly one-half of the males had red eyes, the
other half of the males, however, white eyes, as the theory
demands.

From these and numerous similar breeding experiments of
Correns, Doncaster, and especially of Morgan, we may con-
clude with certainty that the sex chromosomes are the bearers
of those hereditary characters which appear pre-eminently in
one sex. We say pre-eminently, since theoretically we can
predict cases in which color blindness or white eyes must appear
also in the female. Breeding experiments have sho^\TL that this
theoretical prediction is justified. The riddle of Mendel's law
of segregation finds its solution through these experiments and
incidentally also the problem of the determination of sex which
is only a special case of the law of segregation, as ]Mendel
already intimated.

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The Mechanistic Conception of Life 23

The main task which is left here for science to accomplish is
the determination of the chemical substances in the chromo-
somes which are responsible for the hereditary transmission of
a quality, and the determination of the mechanism by which
these substances give rise to the hereditary character. Here
the ground has already been broken. It is knowTi that for the
formation of a certain black pigment the cooperation of a
substance — tyrosin — and of a ferment of oxidation — tyrosinase
— is required. The hereditary transmission of the black color
through the male animal must occur by substances carried in
the chromosome which determine the formation of tyrosin or
tyrosinase or of both. We may, therefore, say that the solu-
tion of the riddle of heredity has succeeded to the extent that
all further development will take place purely in cytological
and physico-chemical terms.

While until twelve years ago the field of heredity was the
stamping ground for the rhetorician and metaphysician it is
today perhaps the most exact and rationalistic part of biology,
where facts cannot only be predicted qualitatively, but also
quantitatively.

VII. THE HARMONIOUS CHARACTER OF THE ORGANISMS

It is not possible to prove in a short address that all life
phenomena will yield to a physico-chemical analysis. We have
selected only the phenomena of fertilization and heredity, since
these phenomena are specific for living organisms and without
analogues in inanimate nature; and if we can convince our-
selves that these processes can be explained physico-chemically
we may safely expect the same of such processes for which
there exist a-priori analogies in inanimate nature, as, e.g., for
absorption and secretion.

We must, however, settle a question which offers itself
not only to the layman but also to every biologist, namely, how
we shall conceive that wonderful " adaptation of each part to the

24 The Mechanistic Conception of Life

whole " by which an organism becomes possible. In the answer
to this question the metaphysician finds an opportunity to put
above the purely chemical and physical processes something-
specific which is characteristic of life only: the ''Zielstrebigkeit,"
the ''harmony" of the phenomena, or the "dominants" of
Reinke and similar things.

With all due personal respect for the authors of such terms
I am of the opinion that we are dealing here, as in all cases of
metaphysics, with a play on words. That a part is so con-
structed that it serves the ''whole" is only an unclear expression
for the fact that a species is only able to live — or to use Roux's
expression — is only durable, if it is provided with the automatic
mechanism for self-preservation and reproduction. If, for
instance, warm-blooded animals should originate without a
circulation they could not remain alive, and this is the reason
why we never find such forms. The phenomena of "adapta-
tion" cause only apparent difficulties since we rarely or never
become aware of the numerous faultily constructed organisms
which appear in nature. I will illustrate by a concrete example
that the number of species which we observe is only an infinitely
small fraction of those which can originate and possibly not
rarely do originate, but which we never see since their organiza-
tion does not allow them to continue to exist long. Moenk-
haus found ten years ago that it is possible to fertilize the egg
of each marine bony fish with the sperm of practically any
other marine bony fish. His embryos apparently lived only
a very short time. This year I succeeded in keeping such
hybrid embryos between distantly related bony fish alive for
over a month. It is, therefore, clear that it is possible to cross
practically any marine teleost with any other.

The nmnber of teleosts at present in existence is about
10,000. If we accomplish all possible hybridizations 100,000,000
different crosses will result. Of these teleosts only a very small
proportion, namely about one one-hundredth of 1 per cent,

The Mechanistic Conception of Life 25

can live. It turned out in my experiments that the heterogene-
ous hybrids between bony fishes formed eyes, brains, ears, fins,
and pulsating hearts, blood and blood-vessels, but could live
only a limited time because no blood circulation was established
— in spite of the fact that the heart beat for weeks — or that the
circulation, if it was estabhshed at all, did not last long.

What prevented these heterogeneous fish embryos from
reaching the adult stage? The lack of the proper "domi-
nants"? Scarcely. I succeeded in producing the same type
of faulty embryos in the pure breeds of a bony fish {Fundulus
heteroclitus) by raising the eggs in 50 c.c. of sea-water to which
was added 2 c.c. 1/100 per cent NaCN. The latter substance
retards the velocity of oxidations and I obtained embryos
which were in all details identical with the embryos produced
by crossing the eggs of the same fish with the sperm of remote
teleosts, e.g., Ctenolahrus or Menidia. These embryos, which
lived about a month, showed the peculiarity of possessing a
beating heart and blood, but no circulation. This suggests
the idea that heterogeneous embryos show a lack of ''adapta-
tion" and durability for the reason that in consequence of the
chemical difference between heterogeneous sperm and egg the
chemical processes in the fertilized egg are abnormal.

The possibility of hybridization goes much farther than we
have thus far assumed. We can cause the eggs of echinoderms
to develop with the sperm of very distant forms, even mollusks
and worms (Kupelwieser) ; but such hybridizations never lead
to the formation of durable organisms.

It is, therefore, no exaggeration to state that the number of
species existing today is only an infinitely small fraction of
those which can and possibly occasionally do originate, but
which escape our notice because they cannot five and reproduce.
Only that limited fraction of species can exist which possesses
no coarse disharmonies in its automatic mechanism of preserva-
tion and reproduction. Disharmonies and faulty attempts in

26 The Mechanistic Conception of Life

nature are the rule, the harmonically developed systems the rare
exception. But since we only perceive the latter we gain the
erroneous impression that the ''adaptation of the parts to the
plan of the whole" is a general and specific characteristic of ani-
mate nature, whereby the latter differs from inanimate nature.
If the structure and the mechanism of the atoms were
knowTi to us we should probably also get an insight into a world
of wonderful harmonies and apparent adaptations of the parts
to the whole. But in this case we should quickly understand
that the chemical elements are only the few durable systems
among a large number of possible but not durable combinations.
Nobody doubts that the durable chemical elements are only
the product of blind forces. There is no reason for conceiving
otherwise the durable systems in living nature.

VIII. THE CONTENTS OF LIFE

The contents of life from the cradle to the bier are wishes
and hopes, efforts and struggles, and unfortunately also dis-
appointments and suffering. And this inner life should be
amenable to a physico-chemical analysis? In spite of the
gulf which separates us today from such an aim I believe that
it is attainable. As long as a life phenomenon has not yet found
a physico-chemical explanation it usually appears inexplicable.
If the veil is once lifted we are always surprised that we did
not guess from the first what was behind it.

That in the case of our inner life a physico-chemical explana-
tion is not beyond the realm of possibility is proved by the fact
that it is already possible for us to explain cases of simple mani-
festations of animal instinct and will on a physico-chemical
basis; namely, the phenomena which I have discussed in former
papers under the name of animal tropisms. As the most
simple example we may mention the tendency of certain ani-
mals to fly or creep to the light. We are dealing in this case
with the manifestation of an instinct or impulse which the

The Mechanistic Conception of Life 27

animals cannot resist. It appears as if this blind instinct which
these animals must follow, although it may cost them their life,
might be explained by the same law of Bunsen and Roscoe,
which explains the photochemical effects in inanimate nature.
This law states that within wide limits the photochemical effect
equals the product of the intensity of light into the duration
of illumination. It is not possible to enter here into all the
details of the reactions of these animals to light; we only wish
to point out in which way the light instinct of the animals
may possibly be connected with the Bunsen-Roscoe law.

The positively hehotropic animals — i.e., the animals which
go instinctively to a source of light — have in their eyes (and
occasionally also in their skin) photosensitive substances which
undergo chemical alterations by light. The products formed
in this process influence the contraction of the muscles — mostly
indirectly, through the central nervous system. If the animal
is illuminated on one side only, the mass of photochemical
reaction products formed on that side in the unit of time is
greater than on the opposite side. Consequently the develop-
ment of energy in the symmetrical muscles on both sides of the
body becomes unequal. As soon as the difference in the masses
of the photochemical reaction products on both sides of the
animal reaches a certain value, the animal, as soon as it moves,
is automatically forced to turn toward one side. As soon as
it has turned so far that its plane of symmetry is in the direction
of the rays, the symmetrical spots of its surface are struck by
the light at the same angle and in this case the intensity of light
and consequently the velocity of reaction of the photochemical
processes on both sides of the animal become equal. There
is no more reason for the animal to deviate from the motion in
a straight line and the positively heliotropic animal will move
in this line to the source of light. (It was assumed that in
these experiments the animal is under the influence of only
one source of light and positively heliotropic.)

The Mechanistic Conception of Life

In a series of experiments I have shown that the heliotropic
reactions of animals are identical with the heliotropic reactions
of plants. It was knowTti that sessile heliotropic plants bend

Fig. 18

Fig. 19

Figs. 18 and 19. — Positive heliotropism of the polyps of Eudendrium. The
new polyp-bearing stems all grow in the direction of the rays of light which is
indicated by an arrow in each figure. (From nature.) These animals bend in
the same way to the light as the stems of positively heliotropic plants kept under
similar conditions.

their stems to the source of light until the axis of symmetry of
their tip is in the direction of the rays of light. I found the
same phenomenon in sessile animals, e.g., certain hydroids
and worms. Motile plant organs, e.g., the swarm spores of
plants, move to the source of light (or if they are negatively

The Mechanistic Conception of Life

heliotropic away from it), and the same is observed in motile
animals. In plants only the more refrangible rays from green to
blue have these heliotropic effects, while the red and yellow
rays are little or less effective; and the same is true for the
heliotropic reactions of animals.

It has been shown by Blaauw for the heliotropic curvatures
of plants that the product of the intensity of a source of light
into the time required to induce a heliotropic curvature is a

Fig. 20. — Positive heliotropism of a marine worm (Spirographis). (From
nature.) The light fell into the aquarium from one side only and the worms all
bent their heads toward the source of light, as the stems of positively heliotropic
plants would do under the same conditions.

constant; and the same result was obtained simultaneously
by another botanist, Froschl. It is thus proved that the Bunsen-
Roscoe law controls the heliotropic reactions of plants. The
same fact had already been proved for the action of light on our
retina.

The direct measurements in regard to the applicability of
Bunsen's law to the phenomena of animal heliotropism have not
yet been made. But a number of data point to the probability
that the law holds good here also. The first of these facts is
the identity of the light reactions of plants and animals. The
second is at least a rough observation which harmonizes mth
the Bunsen-Roscoe law. As long as the intensity of light or
the mass of photochemical substances at the surfaces of the

30 The Mechanistic Conception of Life

animal is small, according to the law of Bunsen, it must take a
comparatively long time until the animal is automatically
oriented by the light, since according to this law the photo-
chemical effect is equal to the product of the intensity of the
light into the duration of illumination. If, however, the inten-
sity of the light is strong or the active mass of the photochemical
substance great, it will require only a very short time until the
difference in the mass of photochemical reaction products on
both sides of the animal reaches the value which is necessary
for the automatic turning to (or from) the light. The behavior
of the animals agrees with this assumption. If the light is
sufficiently strong the animals go in an almost straight line to
the source of light; if the intensity of light (or the mass of
photosensitive substances on the surface of the animal) is
small the animals go in irregular lines, but at last they also
land at the source of light, since the directing force is not
entirely abolished. It will, however, be necessary to ascertain
by direct measurements to what extent these phenomena in
animals are the expression of Bunsen-Roscoe's law. But we may
already safely state that the apparent will or instinct of these
animals resolves itself into a modification of the action of
the muscles through the influence of light; and for the meta-
physical term 'Svill" we may in these instances safely substi-
tute the chemical term "photochemical action of light."

Our wishes and hopes, disappointments and sufferings have
their source in instincts which are comparable to the light
instinct of the heliotropic animals. The need of and the
struggle for food, the sexual instinct with its poetry and its
chain of consequences, the maternal instincts with the felicity
and the suffering caused by them, the instinct of workmanship,
and some other instincts are the roots from which our inner life
develops. For some of these instincts the chemical basis is at
least sufficiently indicated to arouse the hope that their analysis,
from the mechanistic point of view, is only a question of time.

The Mechanistic Conception of Life 31

ix. ethics

If our existence is based on the play of blind forces and only
a matter of chance; if we ourselves are only chemical mechan-
isms— how can there be an ethics for us? The answer is,
that our instincts are the root of our ethics and that the instincts
are just as hereditary as is the form of our body. We eat,
drink, and reproduce not because mankind has reached an
agreement that this is desirable, but because, machine-like,
we are compelled to do so. We are active, because we are com-
pelled to be so by processes in our central nervous system;
and as long as human beings are not economic slaves the instinct
of successful work or of workmanship determines the direction
of their action. The mother loves and cares for her children,
not because metaphysicians had the idea that this was desirable,
but because the instinct of taking care of the young is inherited
just as distinctly as the morphological characters of the female
body. We seek and enjoy the fellowship of human beings
because hereditary conditions compel us to do so. We struggle
for justice and truth since we are instinctively compelled to see
our fellow beings happy. Economic, social, and political con-
ditions or ignorance and superstition may warp and inhibit
the inherited instincts and thus create a civilization with a
faulty or low development of ethics. Individual mutants may
arise in which one or the other desirable instinct is lost, just as
individual mutants without pigment may arise in animals; and
the offspring of such mutants may, if numerous enough, lower
the ethical status of a community. Not only is the mechanistic
conception of life compatible \vith ethics : it seems the only con-
ception of life which can lead to an understanding of the source
of ethics.

II. THE SIGNIFICANCE OF TROPISMS FOR

PSYCHOLOGY

THE SIGNIFICANCE OF TROPISMS FOR PSYCHOLOGY^

A mechanistic conception of life is not complete miless it
includes a physico-chemical explanation of psychic phenomena.
Some authors hold that even if a complete physico-chemical
analysis of these phenomena were possible today it would leave
the 'Hruly psychical" imexplained. We do not need to enter
into a discussion of such an objection since we are still too far
from the goal. We are at least able to show for a limited group
of animal reactions that they can be explained unequivocally on
a purely physico-chemical basis, namely, phenomena which the
metaphysician would classify under the term of animal 'Svill."

Through the writings of Schopenhauer and E. von Hart-
mann I became interested in the problem of vrill. When as a
student I read Munk's investigations on the cerebral cortex
I believed that they might serve as a starting-point for an
experimental analysis of will. Munk stated that he had
succeeded in proving that every memory image in a dog's
brain is locahzed in a particular cell or group of cells and that
any one of these memory images can be extirpated at desire.
Five years of experiments with extirpations in the cerebral
cortex proved to me without doubt that Munk had become the
victim of an error and that the method of cerebral operations
may give data concerning the path of nerves in the central
nervous system but that it teaches little about the dynamics of
brain processes.

A better opportunity seemed to offer itself in the study
of the comparative psychology of the lower animals in which

1 Lectiire delivered at the Sixth International Psychological Congress at
Geneva, 1909. (After a translation in Popular Science Monthly by Miss Grace B.
Watkinson.) Reprinted by courtesy of Professor James McKeen Cattell.

36 The Mechanistic Conception of Life

the mechanism for memory is developed but sHghtly or not at
all. It seemed to me that some day it must become possible
to discover the physico-chemical laws underlying the apparently
random movements of such animals; and that the word ''animal
will" was only the expression of our ignorance of the forces
which prescribe to animals the direction of their apparently
spontaneous movements just as unequivocally as gravity
prescribes the movements of the planets. For if a savage
could directly observe the movements of the planets and should
begin to think about them, he would probably come to the con-
clusion that a ''will action" guides the movements of the planets
just as a chance observer is today inclined to assume that "will"
causes animals to move in a given direction.

The scientific solution of the problem of will seemed then
to consist in finding the forces which determine the movements
of animals, and in discovering the laws according to which
these forces act. Experimentally, the solution of the problem
of will must take the form of forcing, by external agencies, any
number of individuals of a given kind of animals to move in
a definite direction by means of their locomotor apparatus.
Only if we succeed in this have we the right to assume that we
know the force which under certain conditions seems to a lay-
man to be the ^\ill of the animal. But if one part only of the
animals moves in this definite direction and the other does not,
we have not succeeded in finding the force which unequivocally
determines the direction of their movement.

One other point should be observed. If a sparrow flies
down to a seed lying on the ground, we speak of an act of will,
but if a dead sparrow falls upon the seed this does not appear
to us as such. In the latter case purely phj^sical forces are
concerned, while in the former chemical reactions are also tak-
ing place in the sense-organs, nerves, and muscles of the animal.
We speak of an act of will, only when this latter complex, that
is, the natural movement of locomotion, plays its part also, and

Significance of Tropisms for Psychology 37

it is only with this sort of reactions that we have to deal in the
psychology of the will.

Some experiments on winged plant lice may serve as an
introduction to the methods of prescribing to animals the
direction of their progressive movements.

In order to obtain the material, potted rose bushes or Cine-
rarias infected with plant lice are brought into a room and
placed in front of a closed window. If the plants are allowed
to dry out, the aphids (plant lice), previously wingless, change
into winged insects. After this metamorphosis the animals
leave the plants, fly to the window, and there creep upward
on the glass. They can then easily be collected by holding a
test-tube underneath and touching one animal at a time from
above with a pen or scalpel, which causes the animals to drop
into the test-tube. In this manner a sufficiently large number,
perhaps twenty-five or fifty suitable subjects for the experi-
ment, may be obtained. With these animals it may be demon-
strated that the direction of their movement toward the light
is definitely determined — provided that the animals are healthy
and that the light is not too weak. The experiment is so arranged
that only a single source of light, e.g., artificial light, is used.

The animals place themselves with their heads toward the
source of light and move toward it in as straight a line as the
imperfection of their locomotor apparatus allows, approaching as
near to the source of light as their prison (the test-tube) permits.
When they reach that end of the test-tube which is directed
toward the source of light, they remain there, stationary, in a
closely crowded mass. If the test-tube is turned 180"^ the
animals again go straight toward the source of light until the
interference of the glass stops their further progressive move-
ments.^ It can be demonstrated in these animals that the

1 Loeb, Der Heliotropismus der Tiere und seine Uebereinstimmung mit dem
Heliotropismus der Pfiamen, Wurzburg, 1889. Translated in Studies in General
Physiology, 1906.

38 The Mechanistic Conception of Life

direction of their progressive movement is just as unequivocally
governed by the source of light as the direction of the move-
ment of the planets is determined by the force of gravity.

The theory of the compulsory movements of aphids under
the influence of light is as follows : Two factors govern the pro-
gressive movements of the animals under these conditions;
one is the symmetrical structure of the animal, and the second
is the photochemical action of light. We will consider the
two separately. In regard to the photochemical action of light,
we know today that a great many chemical reactions of organic
bodies are accelerated by light. Especially is this true of oxi-
dations.^ The mass of facts is already so great that we are
justified in assuming that the determining action of light upon
animals and plants is in its last analysis due to the fact that the
rate of certain chemical reactions in the cells of the retina or
of other photosensitive regions of the organisms is modified
by light; with increasing intensity of light the rate of certain
chemical reactions, e.g., oxidation, increases.

The second factor is the symmetrical structure of the ani-
mal. As expressed in the gross anatomy of the animal, the
right and left halves of the body are symmetrical. But it is
my belief that such a symmetry exists in a chemical sense, as
well as in an anatomical, by which I mean that s^inmetrical
regions of the body are chemically identical and have the same
metabolism, while non-symmetrical regions of the body are
chemically different, and in general have a quantitatively or
qualitatively different metabolism. In order to illustrate this
difference it is only necessary to point out that the two retinae,
which are certainly symmetrical, have an identical metabolism,
while a region of the skin which is not symmetrical with the
retina has a different metabolism. The individual points on

1 Luther, Die Aufgaben der Photochemie, Leipzig, 1905; C. Neuberg, Biochem.
Zeitschr., Xlll, 305, 1908; Loeb, The Dynamics of Living Matter, New York, 1906.
In addition, see the work of Ciamician, as also of Wolfgang Ostwald (Biochem.
Zeitschr., 1907).

Significance of Tropisms for Psychology 39

the retina are also chemically unlike. The observations upon
visual purple, the differences in the color sensitiveness of the
fovea centralis, and the peripheral parts of the retina indicate
that the points of sjnmmetry of the two retinae are chemically
alike, the non-symmetrical points chemically unlike.

Now if an unequal amoimt of light falls upon the two
retinae the photochemical reactions in the one which receives
more light will also be more accelerated than those in the other.
The same naturally holds true for every other pair of sym-
metrical photosensitive surface elements. For it should be
mentioned that photochemical substances are not found in
the eyes only, but also in other places on the surface of many
animals. In planarians, as my experiments and those of
Parker have shown, not only the eyes, but also parts of the
skin, are photosensitive. But if more light falls upon one
retina than upon the other, the chemical reactions will also be
more accelerated in the one retina than in the other, and
accordingly more intense chemical changes will take place in
one optic nerve than in the other. S. S. Maxwell and CD.
Snyder have demonstrated, independently of each other, that
the rate of the nerve impulse has a temperature coefficient of
the order of magnitude which is characteristic for chemical
reactions. From this we must conclude that when two retinae
(or other points of symmetry) are illuminated with unequal
intensity, chemical processes, also of unequal intensity, take
place in the two optic nerves (or in the sensory nerves of the
two illuminated points) . This inequality of chemical processes
passes from the sensory to the motor nerves and eventually to
the muscles connected with them. We conclude from this
that with equal illumination of both retinae the symmetrical
groups of muscles of both halves of the body will receive equal
chemical stimuli and thus reach equal states of contraction,
while, when the rate of reaction is unequal, the symmetrical
muscles on one side of the body come into stronger action than

40 The Mechanistic Conception of Life

those on the other side. The result of such an inequality of the
action of symmetrical muscles of the two sides of the body is
a change in the direction of movement on the part of the animal.

The change in the direction of movement can result either
in a turning of the head and, in consequence, of the whole animal
toward the source of light, or in a turning of the head and the
animal in the opposite direction. The structure of the central
nervous system is segmental and the head segments generally
determine^ the behavior of the other segments with their acces-
sory parts.

In the winged aphids the relations are as follows: Suppose
that a single source of light is present and that the light strikes
the animal from one side. As a consequence the activity of
those muscles which turn the head or body of the animal toward
the source of light will be increased.- As a result the head, and
with it the whole body of the animal, is turned toward the
source of light. As soon as this happens, the two retinae
become illuminated equally. There is therefore no longer
any cause for the animal to turn in one direction or the other.
It is thus automatically guided toward the source of light.
In this instance the light is the ''will" of the animal which
determines the direction of its movement, just as it is gravity
in the case of a falling stone or the movement of a planet. The
action of gravity upon the movement of the falling stone is
direct, while the action of light upon the^direction of movement
of the aphids is indirect, inasmuch as the animal is caused
only by means of an acceleration of photochemical reactions
to move in a definite direction.

1 Loeb, Comparative Physiology of the Brain and Comparative Psychology, New
York and London, 1900.

' If two sources of light of equal intensity are at an equal distance from the
animal, it will move in a direction at right angles to a line connecting the two
sources of light, because in this base both eyes are similarly influenced by the light.
Herein, as Bohn has rightly said, the machine-like heliotropic reaction of animals
differs from the movement of a human being toward one of two sources of light,
the movement in the latter case not being determined by heliotropism.

Significance of Tropisms for Psychology 41

We will now designate as positively heliotropic those animals
which are forced to turn their head or move toward the source
of light, and as negatively heliotropic those animals which are
oriented or compelled to move in the opposite direction.^

The aphids serve here only as an example. The same phe-
nomena of positive heliotropism may be demonstrated with
equal precision in a great many animals, vertebrates as well as
invertebrates. We cannot, of course, give here an account of
all these cases. The reader who is interested in them must look
into the voluminous literature upon this subject. Heliotropism
is unusually common among the larvae of marine animals and
insects, but also not lacking in sexually mature individuals.

Heliotropic animals are therefore in reality photometric
machines. According to photometric laws the intensity of
light varies with the sine of the angle at which the light strikes
a surface element of the animal (or with the cosine of the angle
of incidence). The animal is oriented by the light in such a
way that symmetrical elements of its photosensitive surface
are struck at about the same angle. In the presence of only
one source of light this condition is fulfilled if the axis of sym-
metry of the animal moves in the direction of the rays of light.
In this case the velocity of photochemical reactions on both sides
of the animal is the same and there is no reason why it should
deviate from this direction in its progressive motions.

Experiments on the heliotropism of plants as well as on the
perception of light by our retina have shown that the effect of
light equals the product of the intensity into the duration
of illumination. This law is identical with the general law of
Bunsen and Roscoe which states that the chemical effect
of light is within wide limits equal to this product. We do
not yet know whether or not Bunsen's law holds good for the
heliotropic animals. If it does, we shall have to substitute 5\(^^

1 Whether an animal is positively or negatively heliotropic depends upon the
fact whether the light causes an increase or a decrease in the tension of the muscles .
Why light should have these opposite effects is as yet imknown.

42 The Mechanistic Conception of Life

this law for what the metaphysician calls the will of these
animals.

Ill

The winged aphids serve as an example, because they fulfil
the above-mentioned requirement, namely, that all individuals,
without exception, move toward the light. For mechanistic
science it is a methodological postulate that the same law acts
without exception, or that the exception must be satisfactorily
explained. It was soon found, as was to be expected, that not
all organisms in their natural condition are equally suitable
for these experiments. Many animals show no heliotropism
at all ; many show only a slight reaction, while others show it in
a degree as pronounced as do the winged aphids. The problem
therefore presented itself of producing heliotropism artificially
in animals which, imder natural conditions, show no positive
heliotropism. If small crustaceans of a fresh-water pond or
lake are taken with a plankton net at noontime or in the after-
noon and placed in an aquarium which is illuminated from one
side only, it is often found that these animals move about in
the vessel pretty much at random and distribute themselves
irregularly. Some seem to go more toward the source of light,
others in the opposite direction, and the majority perhaps pay
no attention to the light.

This condition changes instantly if we add to the water
some acid, preferably carbonic acid, which easily penetrates the
cells of the animal. To every 50 c.c. of the fresh water a few
cubic centimeters of water charged with carbon dioxide are
slowly added. If the correct amount is added all the individuals
become actively positively heliotropic and move in as straight
a line as the imperfection of their swimming movements per-
mits, toward the source of light, and remain there closely
crowded together on the illuminated side of the vessel. If the
vessel is turned 180°, they go directly back again to the lighted
side of the vessel. Every other acid acts like carbonic acid and

Significance of Tropisms for Psychology 43

alcohol acts in the same manner, only more weakly and much
more slowly. Animals which were previously indifferent to
light become, under carbonic acid treatment, complete slaves
of the light.i

How does the acid produce this result? We will assume
that it acts as a sensitizer. The light produces chemical
changes, for instance, oxidation, on the surface of the animal,
especially in the eye, as was suggested in the case of the aphids.
The mass of photochemical substance which is acted upon by
the light is often relatively small, so that even when the light
strikes the crustacean (copepod) on one side only, the difference
in the chemical changes on the two sides of the body remains
still too small to call forth a difference in tension or action in the
muscles of the two sides of the body, sufficient to turn the ani-
mal toward the source of light. But if we add an acid this could
act as a catalyzer, as, for instance, in the catalysis of esters.
In the catalysis of esters, the acid acts, according to Stieglitz,
only to the extent of increasing the active mass of the substance
which undergoes a chemical change. In order to fix our ideas
provisionally we will assume that the acid makes the animal
more strongly positively heliotropic by increasing the active
mass of the photosensitive substance. In this way the same
intensity of light which before produced no heliotropic reaction
now may cause a very pronounced positively heliotropic reac-
tion; because if now the animal is struck on one side only by
the light, the difference in the reaction products in both retinae
becomes rapidly large enough to cause automatically a differ-
ence in the action of the muscles of both sides of the body and a
turning of the head toward the source of light.

In certain forms, for instance, in Daphnia and in certain
marine copepods, a decrease in temperature also increases the
tendency to positive heliotropism. If the mere addition of
acid is not sufficient to make Daphniae positively heliotropic,

1 Loeb, Pflugers Archiv, CXV, 564, 1906.

44 The Mechanistic Conception of Life

this may often be accomplished by simultaneously reducing the
temperature.

The animals which are strongly positively helio tropic and
those animals which do not react at all to light offer no diffi-
culties to the observer. Nevertheless, some zoologists seem to
have found difficulty in explaining the behavior of those animals
which come between the two extremes. For instance, one writer
has asserted that ^^ith greater intensity of light the laws of
heliotropic orientation hold good, while with a lessened light
intensity the animals react to light by the method of "trial
and error." From a chemical standpoint the behavior of
animals at low intensity is easily to be understood. If a posi-
tively heliotropic animal is illuminated from one side, a com-
pulsory turning of the head toward the source of light occurs
only when the difference in the rate of certain photochemical
reactions in the two eyes reaches a certain value. If the inten-
sity of the light is sufficient and the active mass of photochemical
substance in the animal great enough, it requires only a short
time, for instance, the fraction of a second, before the difference
in the mass of the reaction products formed on the two sides
of the animal reaches the value necessary for the compulsory
turning of the head toward the source of light. In this case the
animal is a slave of the light; in other words, it has hardly time
to deviate from the direction of the light rays; for if it turns
the head even for the fraction of a second from the direction
of the light raj's, the difference in the photochemical reaction
products in the two retinae becomes so great that the head is
at once turned back automatically toward the source of light.
But if the intensit}^ of the light or the photosensitiveness of
the animal is lessened the animal may deviate for a longer
period from the direction of the light rays. Such animals
do eventually reach the lighted side of the vessel, but they no
longer go straight toward it, moving instead in zig-zag lines

Significance of Tropisms for Psychology 45

or very irregularly. It is therefore not a case of a qualitative,
but of a quantitative, difference in the behavior of heliotropic
animals under greater or lesser illumination, and it is there-
fore erroneous to assert that heliotropism determines the
movement of animals toward the source of light only under
strong illumination, but that under weaker illumination an
essentially different condition exists.

Still another point is to be considered. We have seen
that acid increases the sensitiveness of certain animals to light,
possibly by increasing the active mass of the photochemical
substance. Every animal is continually producing acids in its
cells, especially carbonic acid and lactic acid; and such acids
increase the tendency in certain animals to react hehotropically.
It probably produces also substances which could have the
opposite effect and which decrease the heliotropic sensitiveness
of the animals. Fluctuations in the rate of the production of
these substances will also produce fluctuations in the helio-
tropic sensitiveness of the animal. If, for instance, the active
mass of the photosensitive substance in a copepod is relatively
small, a temporary increase in the production of carbonic acid
can increase the photosensitiveness of the animal sufficiently
to cause it to move for the period of a few seconds directly
toward the source of light. Later the production of carbonic
acid decreases and the animal again becomes indifferent to light
and can move in any direction. Then the production of car-
bonic acid increases again and the animal goes again, for a
short time, toward the light. Such animals finally gather at
the lighted side of the vessel because the algebraic sum of the
movements in the other directions becomes zero according to
the law of chance. But it is plain that such animals do not
reach the source of light by a straight path. A \^Titer who is
not trained to interpret the variations in the behavior of such
an animal chemically and physiologically, can naturally give
no explanation of their significance. If he is forced to find an

46 The Mechanistic Conception of Life

explanation he will wind up with the suggestion of ''trial and
error" which is no more chemical or scientific than the explana-
tions of metaphysicians in general.

Some authors have, it seems, worked only with animals
which were not pronouncedly heliotropic and whose photo-
sensitiveness wavered about the threshold of stimulation in the
manner described above. Such animals are not suitable for
experiments in heliotropism and it is necessary to first increase
their photosensitiveness if the laws of the action of light upon
them are to be investigated.

I also believe that observations upon animals which are
not sufl&ciently photosensitive have caused many writers to
assert that heliotropic animals do not place themselves directly
in the line of the rays of light,^ but that they first have to learn
the right orientation. A very striking experiment contradicts
this assertion. The larvae of Balanus perforatus develop
entirely in the dark. If the ovary filled with mature larvae
is placed in a watch crystal filled with sea-water in the dark,
the larvae emerge at once and, if they are brought into the light,
they move at once to the side of the watch crystal nearest to
the window. They were, therefore, pronouncedly positively
heliotropic before they came under the influence of the light.

In experiments with winged aphids I often found that
after having gone through the heliotropic reactions a few times
they react much more quickly to light than at the beginning.
This might be interpreted as a case of ''learning." In so far
as it is not a case of a lessening of the stickiness of the feet or
the removal of some other purely mechanical factor which
retards the rate of movement, it may be brought about by the
carbonic or lactic acids produced through the muscular activity .^

1 Provided that only a single source of light is present.

2 The so-called " staircase " phenomenon of stimulation of a muscle is ascribed,
probably rightly, also to the formation of acid. This phenomenon, that is, the
increase of the amount of contraction with every new stimulus, is, however, com-
parable to or identical with the increase in the rate of reactions in the experiments
described here.

Significance of Tropisms for Psychology 47

As far back as 1889 I pointed out that the photosensitive-
ness of an animal is different in different physiological conditions
and that, therefore, under natural conditions, heliotropism is
found often only in certain developmental stages, or in certain
physiological states of an animal. I have already mentioned
that in the aphids distinct heliotropic reactions may only be
expected when the animals have developed wings and have left
the plant. The influence of the chemical changes which take
place in animals upon heliotropism is much more distinct in
the larvae of Porthesia chrysorrhoea. The larvae hatch from
the eggs in the fall and, as young larvae, hibernate in a nest.
The rising temperature in the spring drives them out of the
nest, and they can also be driven out of the nest in winter by
an increase in temperature. When driven out of the nest in
this condition they are strongly positively hehotropic and I
have never found in natural surroundings any animals whose
heliotropic sensitiveness was more pronounced than it is in the
young larvae of Chrysorrhoea. But as soon as the animals
have once eaten, the positive heliotropism disappears and does
not return even if they are again allowed to become himgry.^
In this case it is clear that the chemical changes directly or
indirectly connected with nutrition lead to a permanent dimi-
nution or disappearance of the photochemical reaction. In
ants and bees the influence of substances from the sexual
organs seems to be the determining factor in the production of
positive heliotropism. The ant workers show no heliotropic
reactions, while in the males and females, at the time of sexual
maturity, a distinct positive heliotropism develops, the intensity
of which continues to increase.

It is a well-known fact that during sexual maturity special
substances are formed which influence various organs. For

1 Loeb, op. cit., p, 24. (This latter fact has been overlooked by several
writers.)

48 The Mechanistic Conception of Life

instance, Leo Loeb has found that the substances which are
set free by the bursting of an egg foUicle cause a special sensi-
tiveness in the non-pregnant uterus, so that every mechanical
stimulus causes the latter to form a decidua. In this way he
could cause the formation of any number of deciduae in non-
pregnant uteri, while without the circulation of follicle sub-
stance in the blood the uterus did not react in this manner.

It is a common phenomenon that animals in certain larval
stages are positively heliotropic, while in others they are not
sensitive to light or are even negatively heliotropic. I will
not discuss these facts further in this place, but refer my readers
to my earlier papers.

This change in the heliotropic sensitiveness, produced by
certain metabolic products in the animal body, is of great biologi-
cal significance. I pointed out in former papers that it serves
to save the lives of the above-mentioned yoimg larvae of Chrysor-
rhoea. When the young larvae are awakened from their winter
sleep by the sunshine of the spring they are positively helio-
tropic. Their positive heliotropism leaves them no freedom
of movement, but forces them to creep straight upward to the
top of a tree or branch. Here they find the first buds. In this
way their heliotropism guides them to their food. Should
they now remain positively heliotropic they would be held fast
on the ends of the twigs and would starve to death. But we
have already mentioned that after having eaten they once more
lose their positive heliotropism. They can now creep down-
ward, and the restlessness which is characteristic of so many
animals^ forces them to creep downward until they reach a
new leaf, the odor or tactile stimulus of which stops the pro-
gressive movement of the machine and sets their eating
activity again in motion.

The fact that ants and bees become positively heliotropic
at the time of sexual maturity plays an important role in the

1 The physico-chemical cause of this "restlessness" which is noticeable in
many insects and crustaceans is at present unknown.

Significance of Tropisms for Psychology 49

vital economy of these creatures. As is well kno^\Ti, the mating
of these insects takes place during the so-called nuptial flight.
I found that among the male and female ants of a nest the
heliotropic sensitiveness increases steadily up to the time of
the nuptial flight and that the direction of their flight follows
the direction of the rays of the sun. I gained the impression
that this nuptial flight is merely the consequence of a very
highly developed heliotropic sensitiveness. The case seems to
be similar among the bees according to the following experi-
ment described by Kellogg. The bees were ready to swarm
out of the opening of the box used for the experiment when
he suddenly removed the dark covering of the box so that the
light now entered it from above. The heliotropic sensitiveness
of the animals was so great that they crept upward within
the box, following the direction of the light rays, and were not
able to make the nuptial flight. Thus, according to these obser-
vations the bees at the time of the nuptial flight are positively
heliotropic machines.

These observations may serve as examples of the way in
which the analysis of the vital phenomena of certain animals
shows tropisms to be elements of these phenomena. Many
observations of a similar nature are found in the papers of
Georges Bohn, Parker, Radl,^ and myself.

Under the influence of the theory of natural selection the
view has been accepted by many zoologists and psychologists
that everything which an animal does is for its best interest.
The exact doctrine of heredity, founded by Mendel and
advanced to the position of a systematic science in 1900,
reduces this idea to its proper value. It is only true that
species possessing tropisms which would make reproduction
and preservation of the species impossible must die out.

I Radl, Der Phototropismus der Tiere, Leipzig, 1903.

50 The Mechanistic Conception of Life

Galvanotropism illustrates this fact in a striking manner.
If a galvanic current is passed through a trough filled Tvith
water, and animals are placed in this trough, it can be observed
that an orientation in relation to the direction of the current
takes place in many of the animals, and that they move in the
direction either of the positive or of the negative current.
This phenomenon we call galvanotropism. In galvanotropism
the current lines or the current curves play the same role as the
light rays in heliotropism. At those points where the current
curves enter the cells^ a collection of ions takes place which
influences the chemical reactions. The number of species
which show typical galvanotropic reactions is not so great as the
number of those showing typical heliotropism. In my opinion
this difference is the result of the physical difference in the action
of light and of the electric current. Light acts essentially
upon the free surface of the animal, while the electric current
affects all the cells and nerves. Thus the action of the current
upon the skin becomes complicated and modified by its simul-
taneous effect upon the nerve branches and upon the central
nervous system. The result is thus much more complicated
than that of the action of light where essentially only the effect
upon the skin and retina is involved. For this reason, a dis-
tinct galvanotropism is found more often in organisms with
a simple structure, as, for instance, in unicellular organisms,
than in vertebrates, although it is also demonstrable in the
latter.

Galvanotropism is, however, purely a laboratory product.
With the exception of a few individuals, which have in recent
years fallen into the hands of physiologists who happened to be
working on galvanotropism, no animal has ever had the chance
to come under the influence of an electric current. And yet
galvanotropism is a remarkably common reaction among
animals. A more direct contradiction of the view that the

1 Or where the movement of the ions withm the cell is retarded.

Significance of Tropisms for Psychology 51

reactions of animals are determined by their needs or by natural
selection could hardly be found.

One might be led to suppose that galvanotropism and heli-
otropism are not comparable. They are, however, as a matter
of fact, phenomena of the same category with the exception of
the aforementioned fact, that light acts generally only upon the
surface of the skin, while the electric current influences all the
cells of the body. As already mentioned, the disturbing com-
plications arising from this latter circumstance disappear for
the most part when we work with unicellular organisms, and we
should expect that galvanic and heliotropic reactions would
more nearly resemble one another in this case, provided that
we work with organisms possessing both forms of sensitiveness.
And this expectation is fulfilled. The algae of the species
Volvox show heliotropism and galvanotropism. The investi-
gations made by Holmes and myself upon heliotropism, as well
as those of Bancroft upon the galvanotropism of these organisms
indicate that the mechanism of these reactions in Volvox is
the same and the degree of determinism of the heliotropic
and galvanotropic reactions in Volvox is equally great.

Claparede raises the objection that the galvanotropic
reactions are purely compulsory, while the heliotropic reactions
are governed by the ''interest of the animal. "^ Such a view,
however, is not supported by the facts. The reason whj- heli-
otropism may occasionally, as we have seen, be of use, while
galvanotropism has no biological significance, is because the
electric current does not exist in nature. It can, however, be
shown also that heliotropism is just as useless to many animals
as galvanotropism. For instance, I pointed out twenty years
ago that some varieties of animals which do not live in the light
at all, for instance, the larvae of the goat moth, which live mider
the bark of trees, may show positive heliotropism. I found,
moreover, that the crab, Cuma Rathkii, which lives in the mud of

1 Claparede, "Les tropismes devant la psychologic," Journ. /. Psychologie und
Neurologie, XIII, 150, 1908.

The Mechanistic Conception of Life

the harbor at Kiel, when brought into the Ught and removed
from the mud shows positive heUotropism. It is, therefore,
just as incorrect to assert that the hehotropic reactions are

governed by the bio-
^ ' logical interests of the
animal as that this is
true for galvanotrop-
ism. We must, there-
fore, free ourselves at
once from the over-
valuation of natural
selection and accept
the consequences of
Mendel's theory of
heredity, according
to which the animal
is to be looked upon
as an aggregate of in-
dependent hereditary
qualities.

VII

The attempt has
been made to prove
that organisms are
attuned to a certain
intensity of light and
so regulate their
heliotropism that they
invariably reach that intensity of light which is best suited
to their well-being. I believe that this is also a suggestion
forced upon the investigators by the extreme application
of the theor}' of natural selection. I have made experi-
ments upon a large number of animals, but, with a clear

Fig. 21. — Arrangement to prove that posi-
tively hello tropic animals move toward the source
of light even if by so doing they go from the sim-
light into the shade. W W is & window through
which smihght S falls into the room. By a piece
of board d e the sunlight 5 is prevented from
striking the region 6 c of a table near the window
and this part of the table is in the shade. Only the
dayUght D can reach this part of the table.

A test-tube a c is put on this table at right
angles to the plane of the window. At the be-
ginning of the experiment the animals (e.g., the
winged aphides) are all at a. The animals move
at once toward the window, but instead of remain-
ing at b they keep on moving from the direct
svmhght into the shade toward the sovu"ce of light
until they all reach the end of the tube c near the
window (in the shade) where they remain perma-
nently.

Significance of Tropisms for Psychology 53

arrangement of the physical conditions of the experiment, I
have never found a single indication of such an adaptation.
In every case it has been sho\\Ti that positively heliotropic
animals are positive to any intensity of light above the threshold.
Thus winged plant lice or wingless larvae of Chrysorrhoea or
copepods, which have been made heliotropic by acids, go toward
the light whether the source of light is the direct sunlight or
reflected light from the sky or weak lamp light, provided that
the (threshold) value of the intensity of light required for the
reaction is exceeded. Indeed, I have been able to show that
positively heliotropic animals also move toward the source of
light even if the arrangement is such that by so doing they go
from the light into the shadow.^ I have never observed a
*' selection" of a suitable intensity of light.

What probably lies behind these interpretations of the
"selection of a suitable intensity of light" is the fact that under
certain conditions reaction products formed by the photo-
chemical action of light may inhibit the positive heliotropism.
I found a very clear instance of this sort in the newly hatched
larvae of Balanus perforatus, which are positively heliotropic.
If they are placed in the light of a quartz mercury lamp (of
Heraus), which is very rich in ultra-violet rays, the positively
heliotropic larvae soon become negatively heliotropic. For
these experiments the larvae should be placed only in a very
shallow depth of sea-water.

Even in a strong light which is not so rich in ultra-violet
rays as the light of the mercury lamp, it is sometimes possible
to cause positively heliotropic animals to become negatively
heliotropic. This is the case, for instance, with the larvae of
Polygordius. But it would be wrong in this case to speak of
an adaptation of the animal to a certain intensity of light.

1 Quite often without even stopping for a moment. In animals sensitive to
differences (see next chapter) a stopping occurs in this experiment in the passing
from the light into the shadow, but they go, nevertheless, immediately on in the
direction of the source of light. The reader will find a further account of this
experiment in my book on The Dynamics of Living Matter.

54 The Mechanistic Conception of Life

In my opinion it is merely a case where a metabolic product either
alters the photochemical action or so influences the central
nervous system that the excitation of the retina by the light
weakens the tonus of the muscles, instead of strengthening it.

Some of the other mistakes have perhaps also arisen because
the writers worked with complicated experimental conditions
instead of with simple ones, for instance, because they used a
hollow prism filled with ink in order to produce a gradual
decrease in the light intensity. In the semidarkness thus pro-
duced, the intensity of light often remains beneath or near
the threshold of stimulation, and the writers fall victims to
that class of errors which we have already pointed out in
speaking of the influence of lesser intensities of light.

VIII

Heliotropic phenomena are determined by the relative
rates of chemical reactions occurring simultaneously in sym-
metrical surface elements of an animal. There is a second class
of phenomena which is determined by a sudden change in the
rate of chemical reactions in the same surface elements. Reac-
tions to a sudden change in the intensity of light are sho^vn most
clearly in marine tube-worms, whose gills are exposed to light.
If the intensity of the light in the aquarium is suddenly dimin-
ished the worms withdraw quickly into their tubes. A sudden
increase in the intensity of light has no such effect. With other
forms, for instance, with planarians, a sudden decrease in the
intensity of the light causes a decrease in movement. Such ani-
mals gather chiefly in parts of the space where the intensity of
light is relatively small. I have designated such reactions as
the expression of sensitiveness to changes in the intensity of a
•stimulus (''Unterschiedsempfindlichkeit") differential sensi-
bility, in order to distinguish them from tropisms.^

1 Loeb, " Ueber die Umwandlung positiv heliotropischer Tiere, u.s.w.," Pflugers
Archiv, 1893. See also the recent investigations of Georges Bohn, La naissance de
V intelligence, Paris, 1909; "Les essais et les erreurs ctiez les etioles de mer," Bull.

Significance of Tropisms for Psychology 55

It is hardly necessary to point out here that the effects of
rapid changes in intensity, when they are very marked, can
easily complicate and entirely obscure the heliotropic phe-
nomena. In Hypotricha and other infusoria this differential
sensibility is very pronounced in response to sudden touch or
sudden alteration of the chemical medium, and like the tube-
worms they thereupon draw back very quickly. Since their
locomotor organs are not symmetrical, but are arranged in a
peculiar unsymmetrical manner, they do not, after the next
progressive movement, return to the former direction of move-
ment, but deviate sideways from it, and it is therefore easy
to understand that such animals do not furnish the best ma-
terial for demonstrating the laws of heliotropism, especially
since they possess only a slight photochemical sensitiveness.
But Jennings^ has with special preference used observations on
such organisms to argue against the theory of tropisms. Just
as the action of a constant current in muscles and nerves is
different from that of an intermittent current, so we find an
analogous case in the action of light. If we wish to trace all
animal reactions back to physico-chemical laws we must take
into consideration besides the tropisms not only the facts of the
differential sensibility but also all other facts which exert an
influence upon the reactions. The influence of that mechanism
which we call "associative memory" also belongs in this cate-
gory, but we cannot discuss this further at this place. The
reader is referred to my book^ as well as to the more recent
works of Bohn, La naissance de V intelligence^ and La nouvelle
psychologie animale.^ Let us bear in mind that "ideas" also

Inst. gen. psychol., 1907; "Intervention des reactions oscillatoires dans les tro-
pismes," Ass. franc, d. Sciences, 1907.

1 Jennings, The Behavior of Lower Organisms, 1906.

2 Comparative Physiology of the Brain and Comparative Psychology, New York
and London, 1900.

3 Paris, "Bibliothequedephilosophiescientifique," 1909.

* Paris, " Bibliotheque de philosophic contemporaine," 1911.

56 The Mechanistic Conception of Life

can act, much as acids do for the hehotropism of certain
animals, namely, to increase the sensitiveness to certain stimuli,
and thus can lead to tropism-like movements or actions directed
toward a goal.

Besides light and the electric current, the force of gravity
also has an orienting influence upon a number of animals.
The majority of such animals are forced to turn their heads
away from the center of the earth and to creep upward. It was
uncertain for a long time how the orientation of cells in relation
to the center of gravity of the earth could influence the rate of
the chemical reactions within, but it has been suggested that
an enlargement or shifting of the reacting surfaces formed the
essential connecting link. If it is assumed that in such geo-
tropically sensitive cells two phases (for instance, two fluid
substances which are not at all, or not easily, miscible, or one
solid and one fluid substance) of different specific gravities are
present, which react upon one another, a reaction takes place
at the surfaces of contact. Every enlargement of the latter in-
creases the mass of reacting molecules. A shifting of the surfaces
would act in the same manner. Finally, a third possibility re-
mains which could perhaps be realized in plant roots and stems.
If in the geotropically sensitive elements two masses of differ-
ent specific gravity are present, only one of which reacts to the
flowing sap in the center or the periphery of the stem, the cells
of the upper side of a stem which is laid horizontally will ac-
quire a different rate of reaction from those of the lower side,
because in the former the specifically heavier substances are
directed toward the center of the stem, while in the latter the
specifically lighter ones are directed toward the center. Con-
sequently, one side will grow faster than the other, hence the
geo tropic bending.^ In the frog's egg, we can actually demon-
strate directly the existence of two substances of different

1 Chapter on "Tropisms" in Dynamics of Living Matter.

Significance of Tropisms for Psychology 57

specific gravity and can study their behavior, since in this case
they are of different color.

In animals it has been observed that orientation toward the
center of gravity of the earth often becomes less compulsory
when the inner ear has been removed. Mach first pointed
out the possibility that the otoliths are responsible for this.
He believed that they might press upon the end-organs of the
sensory nerves and every change of pressure might cause a cor-
rection of the position of the animal. It is generally assumed
that this view has been verified by experiment but I cannot
entirely agree with it although I once described experiments
which seemed to support Mach's otolith theory. I had found
that when the otoliths of the inner ear of the shark are scraped
out with a sharp spoon the normal orientation of the animal
suffers; but if the otoliths are simply washed out from the
inner ear by a weak current of sea-water the orientation does
not so easily suffer.

In the latter case, it is doubtful whether all the otolith
powder has been removed from the ear. The problem was
solved by experiments on flounders, which have only a single
large otolith that can easily be removed from the ear. E. P.
Lyon carried out these experiments, which showed that no
disturbance of the orientation resulted from this operation. We
may conclude, therefore, that in my experiments of scraping
out the otoliths a disturbance of the orientation occurred,
because in so doing the nerve endings in the ears were injured.
We have, therefore, no right to maintain that the orientation of
animals in relation to the center of gravity of the earth is regu-
lated by the pressure of the otoliths upon the nerve endings,
but that this regulation takes place in the nerve endings them-
selves, and probably, indeed, as a result of the existence there
of two different phases of different specific gravity which react
upon one another. Through the change of orientation of the
cells in relation to the center of gravity of the earth, the two

58 The Mechanistic Conception of Life

phases undergo a shifting by means of which a change in the
rate of reaction is brought about according to one of the ways
described above. Since then I have looked through the htera-
ture on the function of the otoliths or statoliths, and have
reached the conclusion that all writers who assert that the
removal of the otoliths disturbs the geotropic orientation of
animals have been victims of the same fallacy as myself. They
have injured or removed the nerve endings. In the only case
in which a removal of the otoliths without tearing or other
injury of the nerve endings can be justifiably assumed, no dis-
turbance of the orientation occurred.

While in my own work I have aimed to trace the complex
reactions of animals back to simpler reactions like those of
plants and finally to physico-chemical laws, the opposite
tendency has lately been gaining influence. Some botanists,
namely, Haberlandt, Nemec, and F. Darwin, endeavor to show
that the relatively simpler reactions of plants may be traced
back to the more complex relations found in animals. Instead
of deriving the tropic reactions of plants as directly as possible
from the law of mass action or the law of Bunsen and Roscoe,
they try to show that ''sense-organs" exist in the cells of
plants and France even attributes to the latter a ''soul" and
"intelligence." I believe that in order to be consistent, these
writers ought to base the law of mass action upon the assump-
tion of the existence of sense-organs, souls, and intelligence in
the molecules and ions. It is probably unnecessary to empha-
size the fact that it is better for the progress of science to derive
the more complex phenomena from simpler components than to
do the contrary. For all "explanation" consists solely in the
presentation of a phenomenon as an unequivocal function of
the variables by which it is determined, and if in nature we find
a function of two variables, it does not, in my opinion, tend
toward progress to assert that this is a case of functions of
more than two variables, without furnishing sufficient proof for
this assertion.

Significance of Tropisms for Psychology 59

These writers explain the geotropic reactions of plants
by saying that in certain cells starch grains are present which
serve the purpose of the otoliths in animals. These starch
grains are believed to press upon the sense-organs or nerve
endings in the plant cells concerned and the ''pressure-sense"
of the plant is then supposed to give rise to the geotropic curva-
ture. I have no opposition to offer to the assumption that the
starch grains change their position with a change in the position
of the cells, and I am also willing to pass over for the present
the view that the starch grains form one of the two phases in
the cell. But I see no necessity for assuming besides this
the existence of intracellular sense-organs which perceive the
pressure of the starch grains. This is, in my opinion, an
unnecessary complication of simple relations.

The progress of natural science depends upon the dis-
covery of rationalistic elements or simple natural laws. We
find that there are two classes of investigators in biology,
grouped according to their attitude toward such simple laws
or rationalistic elements. One seems to aim at the denial of
the existence of such simple laws and every new case which
does not fall at once imder such a law offers an opportunity
for them to point out the inadequacy of the latter. The other
group of investigators aims to discover and not to disprove
laws. When such investigators have discovered a simple law
which is generally applicable, they know that an apparent
exception does not necessarily overthrow the law, but that
possibly an opportunity is offered for a new discovery and an
extension of the old law. Mendel's laws have been brilliantly
confirmed in a number of cases. In some cases of apparent
deviations (from these laws), however, it has not always been
possible at once to recognize the cause. One group of investi-
gators has recognized that these deviations do not indicate the
incorrectness of Mendel's laws, but that they are merely the

60 The Mechanistic Conception of Life

result of secondary and often minor complications; the latter
investigators have from this standpoint made further fruitful
discoveries. The role of the other group of investigators in
this case has consisted, primarily, in an attempt to minimize
the importance of Mendel's laws and thus to retard the progress
of science.

The case is similar in the realm of tropisms. Tropisms
and tropism-like reactions are elements which pave the way
for a rationalistic conception of the psychological reactions of
animals and I believe, therefore, that it is in the interest of the
progress of science to develop further the theory of animal
tropisms. The fact that in an electric current the same animal
often moves differently from what it does under the influence
of light finds its explanation for the observer conversant with
physical chemistry in the fact that the electric current causes
changes in the concentration of ions within, as well as upon the
surface, while the chemical action of light is essentially limited
to the surface. Certain A\Titers, however, leave this difference
in the action of the two agents out of consideration and make
use of the difference in the behavior of certain organisms in
response to light and to the electric current, to assert that it
is not permissible to speak of tropisms as being governed
by general laws; in other words, they say that tropisms are
without significance. Animals in general are symmetrically
built and the motor elements of the right and left sides of the
body usually act symmetrically. Consequently the heliotropic
orientation, for instance, comes about as we have already
described. There are animals, however, which move sideways,
for instance, certain crabs, such as the fiddler crab. Holmes
has found that these crustaceans also go sideways toward the
light. Jemiings draws from this fact the following conclusions:
''The symmetrical position is an incident of the reaction, not
its essence."

In other words, he uses these observations of Holmes to

Significance of Tropisms for Psychology 61

indicate that the role ascribed to symmetry has no importance
for the theory of tropisms. I am, however, inclined to draw
another conclusion, namely, that in the fiddler crabs in the
first place there is an entirely different connection between the
retina and the locomotor muscles from that in other crustaceans,
and that, secondly, there is a special peculiarity in regard to
the function of the two retinae whereby they do not act like
symmetrical surface elements. I believe that a new discovery
may be made here.^

These data may suffice to explain my point of view. To me
it is a question of making the facts of psychology accessible
to analysis by means of physical chemistry. In this way it is
already possible to reduce a set of reactions, namely, the tro-
pisms to simple rationalistic relations. Many animals, because
their body structure is not only morphologically, but also
chemically, symmetrical, are obliged to orient their bodies in
a certain way in relation to certain centers of force, as, for in-
stance, the course of light, an electric current, the center of
gravity of the earth, or chemical substances. This orientation
is automatically regulated according to the law of mass action.
The application of the law of mass action to this set of reactions
is thus made possible. I consider it unnecessary to give up
the term *' comparative psychology," but I am of the opinion
that the contents of comparative psychology will under the
influence of the above-mentioned endeavors be different from
the contents of speculative psychology. But I believe also
that the further development of this subject will fall more to
the lot of biologists trained in phj^sical chemistry than to the
specialists in psychology or zoology, for it is in general hardly

1 From which. I expect, furthermore, that they will only confirm still more
the laws of heliotropism. This expectation is based upon analogous relations in
the pleiironectids, which I cannot, however, discuss further here. However,
probably no one will maintain that the existence of the pleiironectids invalidates
all laws in regard to the symmetrical body structure.

62 The Mechanistic Conception of Life

to be expected that zoologists and psychologists who lack a
physico-chemical training will feel attracted to the subject of
tropisms.

In closing let me add a few remarks concerning the possible
application of the investigations of tropisms.

I believe that the investigation of the conditions which
produce tropisms may be of importance for psychiatry. If we
can call forth in an animal otherwise indifferent to light by
means of an acid a heliotropism which drives it irresistibly into
a flame; if the same thing can be brought about by means of a
secretion of the reproductive glands, we have given, I believe,
a group of facts, within which the analogies necessary for
psychiatry can be called forth experimentally and can be
investigated.

These experiments may also attain a similar value for
ethics. The highest manifestation of ethics, namely, the con-
dition that human beings are willing to sacrifice their lives for
an idea is comprehensible neither from the utilitarian stand-
point nor from that of the categorical imperative. It might be
possible that under the influence of certain ideas chemical
changes, for instance, internal secretions within the body, are
produced which increase the sensitiveness to certain stimuli to
such an unusual degree that such people become slaves to cer-
tain stimuli just as the copepods become slaves to the light
when carbon dioxide is added to the water. Since Pawlow
and his pupils have succeeded in causing the secretion of saliva
in the dog by means of optic and acoustic signals, it no longer
seems strange to us that what the philosopher terms an ''idea'*
is a process which can cause chemical changes in the body.

III. SOME FUNDAMENTAL FACTS AND CONCEP-
TIONS CONCERNING THE COMPARATIVE PHYS-
IOLOGY OF THE CENTRAL NERVOUS SYSTEM

Ill

SOME FUNDAMENTAL FACTS AND CONCEPTIONS CON-
CERNING THE COMPARATIVE PHYSIOLOGY
OF THE CENTRAL NERVOUS SYSTEM i

1. The understanding of complicated phenomena depends
upon an analysis by which they are resolved into their simple
elementary components. If we ask what the elementary com-
ponents are in the physiology of the central nervous system,
our attention is directed to a class of processes which are called
reflexes. A reflex is a reaction which is caused by an external
stimulus, and which results in a coordinated movement, the
closing of the eyelid, for example, when the conjuctiva is
touched by a foreign body, or the narrowing of the pupil under
the influence of light. In each of these cases, changes in the
sensory nerve endings are produced which bring about a change
of condition in the nerves. This change travels to the central
nervous system, passes from there to the motor nerves, and
terminates in the muscle-fibers, producing there a contraction.
This passage from the stimulated part to the central nervous
system, and back again to the peripheral muscles, is called a
reflex. There has been a growing tendency in physiologj^ to
make reflexes the basis of the analysis of the functions of the
central nervous system, and consequently much importance
has been attached to the processes underlying them and the
mechanisms necessary for reflex.

The name reflex suggests a comparison between the spinal
cord and a mirror. Sensory stimuli were supposed to be
reflected from the spinal cord to the muscles; destruction of the
spinal cord would, according to this, make the reflex impossible,

1 Reprinted from Loeb, J., Comparative Physiology of the Brain and Compara-
tive Psychology (1899). By courtesy of G. P. Putnam's Sons of New York and
London.

66 The Mechanistic Conception of Life

just as the destruction of the mirror might prevent the reflection
of Hght. This comparison, however, of the reflex process in
the central nervous system with the reflection of light has,
long since, become meaningless, and at present few physiologists
in using the term reflex think of its original significance. In-
stead of this, another feature in the conception of the term
reflex has gained prominence, namely, the purposeful character
of many reflex movements. The closing of the eyelid and the
narrowing of the pupil are eminentlj' purposeful, for the cornea
is thereby protected from hurtful contact with foreign bodies,
and the retina from the injurious effects of strong light.
Another striking characteristic in such reflexes has also been
emphasized. The movements which are produced are so well
planned and coordinated that it seems as though some intelli-
gence were at work either in devising or in carrying them out.
The fact, however, that even a decapitated frog will brush
with its foot a drop of acetic acid from its skin, suggests that
some other explanation is necessary. A prominent psychologist
has maintained that reflexes are to be considered as the mechani-
cal effects of acts of volition of past generations.^ The ganglion-
cell seems the only place where such mechanical effects could
be stored up. It has therefore been considered the most
essential element of the reflex mechanism, the nerve-fibers
being regarded, and probably correctly, merely as conductors.

Both the authors who emphasize the purposefulness of the
reflex act and those who see in it only a physical process have
invariably looked upon the ganglion-cell as the principal bearer
of the structures for the complex coordinated movements in
reflex action.

I should have been as little inclined as any other physiolo-
gist to doubt the correctness of this conception had not the
establishment of the identity of the reactions of animals and
plants to light proved the untenability of this view and at the

1 A statement for which no trace of experimental proof exists.

Physiology of Central Nervous System 67

same time offered a different conception of reflexes. The flight
of the moth into the flame is a typical reflex process. The
light stimulates the peripheral sense-organs, the stimulus passes
to the central nervous system, and from there to the muscles of
the wings, and the moth is caused to fly into the flame. This
reflex process agrees in every point ^vith the heliotropic effects
of light on plant organs. Since plants possess no nerves and no
ganglia, this identity of animal with plant heliotropism can
force but one inference — these heliotropic effects must depend
upon conditions which are common to both animals and plants.
At the end of my book on heUotropism^ I expressed this view
in the following words: ''We have seen that, in the case of
animals which possess nerves, the movements of orientation
toward light are governed by exactly the same external condi-
tions, and depend in the same way upon the external form of the
body, as in the case of plants which possess no nerves. These
heliotropic phenomena, consequently, cannot depend upon
specific qualities of the central nervous system." On the other
hand, the objection has been raised that destruction of the
ganglion-cells interrupts the reflex process. This argument,
however, is not sound, for the nervous reflex arc in higher animals
forms the only protoplasmic bridge between the sensory organs
of the surface of the body and the muscles. If we destroy
the ganglion-cells or the central nervous system, we interrupt
the continuity of the protoplasmic conduction between the
surface of the body and the muscles, and a reflex is no longer
possible. Since the axis cylinders of the nerves and the
ganglion-cells are nothing more than protoplasmic formations,
we are justified in seeking in them only general protoplasmic
qualities, unless we find that the phenomena cannot be explained
by means of the latter alone.

2. A further objection has been raised, that although these

1 Loeb, J., Der Heliotropismus der Tiere und seine Uebereinstimmung mit dem
Heliotropismus der Pflanzen, Wurzburg, 1890. A preliminary note on these
experiments appeared January, 1888.

68 The Mechanistic Conception of Life

reflexes occur in plants possessing no nervous system, yet in
animals where ganglion-cells are present the very existence of
the ganglion-cells necessitates the presence in them of special
reflex mechanisms. It was therefore necessary to find out if
there w^ere not animals in which coordinated reflexes still con-
tinued to exist after the destruction of the central nervous
system. Such a phenomenon could be expected only in forms
in which a direct transmission of stimuli from the skin to the
muscle or direct stimulation of the muscle is possible, in addi-
tion to the transmission through the reflex arc. This is the
case, for instance, in worms and in ascidians. I succeeded^ in
demonstrating in Ciona intestinalis that the complicated reflexes
still continue after removal of the central nervous system.^

A study, then, of comparative physiology brings out the
fact that irritability and conductibility are the only qualities
essential to reflexes, and these are both common qualities of
all protoplasm. The irritable structures at the surface of the
body, and the arrangement of the muscles determine the
character of the reflex act. The assumption that the central
nervous system or the ganglion-cells are the specific bearers of
reflex mechanisms cannot hold. But have we now to conclude
that the nerves are superfluous and a waste ? Certainly not.
Their value lies in the fact that they are quicker and more
sensitive conductors than undifferentiated protoplasm. Because
of these qualities of the nerves, an animal is better able to adapt
itself to changing conditions than it possibly could if it had no
nerves. Such power of adaptation is absolutely necessary for
free animals.

3. While some authors explain ail reflexes on a psychical
basis, the majority of investigators explain in this way only a

1 Loeb, J., Untersuchungen zur physiologischen Morphologic der Tiere, II,
Wurzburg, 1892.

2 This animal closes the oral opening when we touch it. This is a reflex com-
parable to the closing of the eyelid if we touch the cornea. The central nervous
sytem of the animal consists of one ganglion. When the latter is removed the
oral opening still closes upon mechanical stimulation.

Physiology of Central Nervous System 69

certain group of reflexes — the so-called instincts. Instincts
are defined in various ways, but no matter how the definition
is phrased the meaning seems to be that they are inherited
reflexes so purposeful and so complicated in character that
nothing short of intelligence and experience could have produced
them. To this class of reflexes belongs the habit possessed by
certain insects of laying their eggs on the material which the
larvae will afterward require for food. When we consider that
the female fly pays no attention to her eggs after laying them,
we cannot cease to wonder at the seeming care which nature
takes for the preservation of the species. How can the action
of such an insect be determined if not by mysterious structures
which can only be contained in the ganglion-cells ? How can we
explain the inheritance of such instincts if we believe it to be a
fact that the ganglion-cells are only the conductors of stimuli ?
It was impossible either to develop a mechanics of instincts or
to explain their inheritance in a simple way from the old stand-
point, but our conception makes an explanation possible.
Among the elements which compose these complicated instincts,
the tropisms (heliotropism, chemotropism, geotropism, stere-
otropism) play an important part. These tropisms are identical
for animals and plants. The explanation of them depends first
upon the specific irritability of certain elements of the body-
surface, and, second, upon the relations of symmetry of the
body. Symmetrical elements at the surface of the body have
the same irritability; unsymmetrical elements have a different
irritabiUty. Those nearer the oral pole possess an irritability
greater than that of those near the aboral pole. These circum-
stances force an animal to orient itself toward a source of stimu-
lation in such a way that symmetrical points on the surface
of the body are stimulated equally. In this way the animals
are led without will of their own either toward the source of
the stimulus or away from it. Thus there remains nothing
for the ganglion-cell to do but to conduct the stimulus, and

70 The Mechanistic Conception of Life

this may be accomplished by protoplasm in any form. For
the inheritance of instincts it is only necessary that the egg
contain certain substances — which will determine the different
tropisms — and the conditions for producing bilateral symmetry
of the embryo. The mystery with which the ganglion-cell has
been surroimded led not only to no definite insight into these
processes, but has proved rather a hindrance in the attempt to
find the explanation of them.

It is evident that there is no sharp line of demarkation
between reflexes and instincts. We find that authors prefer
to speak of reflexes in cases where the reaction of single parts
or organs of an animal to external stimuli is concerned; while
they speak of instincts where the reaction of the animal as a
whole is involved (as is the case in tropisms).

4. If the mechanics of a number of instincts is explained
by means of the tropisms common to animals and plants, and
if the significance of the ganglion-cells is confined, as in all
reflex processes, to their power of conducting stimuli, we are
forced to ask what circumstances determine the coordinated
movements in reflexes, especially in the more complicated ones.
The assumption of complicated but unkno\\Ti and perhaps
unknowable structures in the ganglion-cells served formerly as
a convenient terminus for all thought in this direction. In
giving up this assumption, we are called upon to show what
conditions are able to determine the coordinated character of
reflex movements. Experiments on galvanotropism of animals
suggest that a simple relation may exist between the orientation
of certain motor elements in the central nervous system and
the direction of the movements of the body which is called
forth by the activity of these elements. This perhaps creates
a rational basis for the further investigation of coordinated
movements.^

1 Since this was written von UexkueU found a law which will go far in explaining
the mechanism of coordination, namely, that a stretched muscle shows an increased
irritability while the contracted muscle shows a decreased irritability. Since

Physiology of Central Nervous System 71

5. We must also deprive the ganglion-cells of all specific
significance in spontaneous movements, just as we have done
in the case of simple reflexes and instincts. By spontaneous
movements we mean movements which are apparently deter-
mined by internal conditions of the living system. Strictly
speaking, no movements of animals are exclusively determined
by internal conditions, for atmospheric oxygen and a certain
range of temperature are always necessary in order to preserve
the activity beyond a short period of time.

We must discriminate between simple and conscious spon-
taneity. In simple spontaneity we must consider two kinds
of processes, namely, aperiodic spontaneous processes and
rhythmically spontaneous or automatic processes. The rhyth-
mical processes are of importance for our consideration.
Respiration and the heart beat belong in this category. The
respiratory movements seem to indicate that automatic
activity can arise in the ganglion-cells, and from this the con-
clusion has been drawn that all automatic movements are due
to specific structures of the ganglion-cells. Recent investiga-
tions, however, have transferred the problem of rhythmical
spontaneous contractions from the field of morphology into that
of physical chemistry. The pecuHar qualities of each tissue are
partly due to the fact that it contains certain ions (Na, K, Ca,
and others) in definite proportions. By changing these pro-
portions, we can impart to a tissue properties which it does not
ordinarily possess. If in the muscles of the skeleton the Na
ions be increased and the Ca ions be reduced, the muscles are
able to contract rhythmically Hke the heart. It is only the
presence of Ca ions in the blood which prevents the muscles
of our skeleton from beating rhythmically in our body. As the
muscles contain no ganglion-cells, it is certain that the power of
rhythmical spontaneous contractions is not due to the specific

the contraction of one group of muscles necessitates the stretching of their antago-
nists the coordinated character of locomotive action seems to become intelligible
(1912).

72 The Mechanistic Conception of Life

morphological character of the ganglion-cells, but to definite
chemical conditions which are not necessarily confined to
ganglion-cells.^

The coordinated character of automatic movements has
often been explained by the assumption of a '^ center of coordina-
tion," which is supposed to keep a kind of police watch on the
different elements and see that they move in the right order.
Observations in lower animals, however, show that the coordina-
tion of automatic movements is caused by the fact that that
element which beats most quickly forces the others to beat in
its own rh^'thm. Aperiodic spontaneity is still less a specific
function of the ganglion-cell than rhythmical spontaneity. The
swarm spores of algae, which possess no ganglion-cells, show
spontaneity equal to that of animals having ganglion-cells.

6. Thus far we have not touched upon the most important
problem in physiology, namely, which mechanisms give rise
to that complex of phenomena which are called psychic or
conscious. Our method of procedure must be the same as in
the case of instincts and reflexes. We must find out the ele-
mentary physiological processes which underlie the complicated
phenomena of consciousness. Some physiologists and psy-
chologists consider the purposefulness of the psychic action
as the essential element. If an animal or an organ reacts as a
rational human being would do under the same circumstances,
these authors declare that we are dealing with a phenomenon of
consciousness. In this way many reflexes, the instincts
especially, are looked upon as psychic functions. Conscious-
ness has been ascribed even to the spinal cord, because many of
its functions are purposeful. We shall see in the follow^ing
chapters that many of these reactions are merely tropisms
which may occur in exactly the same form in plants. Plants
must therefore have a psychic life, and, following the argument,
we must ascribe it to machines also, for the tropisms depend

1 Loeb, J., American Journal of Physiology, III, 327 and 383, 1900.

Physiology of Central Nervous System 73

only on simple mechanical arrangements. In the last analysis,
then, we would arrive at molecules and atoms endowed with
mental qualities. We can dispose of this view by the mere fact
that the phenomena of embryological development and of
organization in general show a degree of purposefulness which
may even surpass that of any reflex or instinctive or conscious
act. And yet we do not consider the phenomena of develop-
ment to be dependent upon consciousness.

On the other hand, physiologists who have appreciated the
untenable character of such metaphysical speculations have
held that the only alternative is to drop the search for the
mechanisms underlying consciousness and study exclusively the
results of operations on the brain. This would be throwing
out the ^^heat with the chaff. The mistake made by meta-
physicians is not that they devote themselves to fundamental
problems, but that they employ the wrong methods of investi-
gation and substitute a play on words for an explanation by
means of facts. If brain physiology gives up its fundamental
problem, namely, the discovery of those elementary processes
which make consciousness possible, it abandons its best possi-
bilities. But to obtain results, the errors of the metaphysician
must be avoided and explanations must rest upon facts, not
words. The method should be the same for animal psj'chology
that it is for brain physiology. It should consist in the right
understanding of the fundamental process which recurs in
all psychic phenomena as the elemental component. This
process, according to my opinion, is the activity of the associative
rnemory, or of association. Consciousness is only a metaphysical
term for phenomena which are determined by associative
memory. By associative memory I mean that mechanism by
which a stimulus brings about not only the effects which its
nature and the specific structure of the irritable organ call for,
but by which it brings about also the effects of other stimuli
which formerly acted upon the organism almost or quite

74 The Mechanistic Conception of Life

simultaneously with the stimulus in question. ^ If an animal
can be trained, if it can learn, it possesses associative memory.
By means of this criterion it can be shown that Infusoria,
Coelenterates, and worms do not possess a trace of associative
memory. Among certain classes of insects (for instance, ants,
bees, and wasps), the existence of associative memory can be
proved. It is a comparatively easy task to find out which
representatives of the various classes of animals possess, and
which do not possess, associative memory. Our criterion
therefore might be of great assistance in the development of
comparative psychology.

7. Our criterion puts an end to the metaphysical ideas that
all matter, and hence the whole animal world, possesses con-
sciousness. We are brought to the theory that only certain
species of animals possess associative memory and have con-
sciousness, and that it appears in them only after they have
reached a certain stage in their ontogenetic development. This
is apparent from the fact that associative memory depends
upon mechanical arrangements which are present only in certain
animals, and present in these only after a certain development
has been reached. The fact that certain vertebrates lose all
power of associative memory after the destruction of the cere-
bral hemispheres, and the fact that vertebrates in which the
associative memory either is not developed at all or only slightly
developed (e.g., the shark or frog) do not differ, or differ but
slightly, in their reactions after losing the cerebral hemispheres,
support this view. The fact that only certain animals possess
the necessary mechanical arrangements for associative memory,
and therefore for consciousness, is not stranger than the fact
that only certain animals possess the mechanical arrangements
for uniting the rays from a luminous point in one point on the
retina. The liquefaction of gases is an example of a sudden

1 Loeb, J., "Beitrage zur Gehirnphysiologie der Wiirmer," PflUgers Archiv,
LVI, 247, 1894.

Physiology of Central Nervous System 75

change of condition which may be produced when one variable
is changed; it is not surprising that there should be sudden
changes in the ontogenetic and phylogenetic development
of organisms when there are so many variables subject to
change, and when we consider that colloids easily change their
state of matter.

It becomes evident that the unraveling of the mechanism
of associative memory is the great discovery to be made in the
field of brain physiology and psychology. But at the same
time it is evident that this mechanism cannot be unraveled by
histological methods, or by operations on the brain, or by
measuring reaction times. We have to remember that all life
phenomena are ultimately due to motions or changes occurring
in colloidal substances. The question is, Which peculiarities
of the colloidal substances can make the phenomenon of asso-
ciative memory possible? For the solution of this problem
the experience of physical chemistry and of the physiology of
the protoplasm must be combined. From the same sources
we must expect the solution of the other fundamental problems
of brain physiology, namely, the process of conduction of
stimuli.

IV. .PATTERN ADAPTATION OF FISHES AND THE

MECHANISM OF VISION

PATTERN ADAPTATION OF FISHES AND THE MECHAN-
ISM OF VISION^

The mechanism of the action of the brain is entirely
unknown to us. We are unable to look into the active brain
and the objective results of brain action are in general so differ-
ent in their nature from the external stimulus which leads to the
action that we are prevented in most cases from drawing any
conclusions concerning the nature of the processes occurring
in the brain.

From results obtained in experiments on dogs Munk stated
years ago that there existed a projection of the retina on a part
of the cortex which he had designated as the visual sphere and
that the extirpation of definite parts of this sphere caused
blindness in definite parts of the retina. I repeated these
experiments but was not able to confirm his statements.
Henschen has recently, however, furnished the proof, on the
basis of excellent pathological observations on man, that such
a projection after all exists, but that it is situated in another
part of the cortex from where Munk had believed it to be,
namely, in the area striata. Minkowski was able to confirm
Henschen's conclusions through experi lents on dogs. These
observations and experiments suggest the possibility that in
vision an image is formed not only on the retina but also on the
cortex.

The possibility that vision is based on the formation of an
image in the brain is supported by a group of facts which to
my knowledge have never received any consideration in this
connection.

1 Reprinted from Physiologisches Centralblatt, XXV, No. 22, 1912. This
note is given merely as a suggestion concerning the mechanism underlying certain
brain processes.

80 The Mechanistic Conception of Life

It has been known for some time that many animals,
especially certain fishes, adapt their color and pattern to the
ground upon which they happen to be. This fact has been
extensively utilized for the theory of natural selection. It
seems to me that the same facts furnish also the proof that an
image of the objects is formed in the brain. Pouchet many
years ago showed that the adaptation of fishes to the ground
ceases as soon as their eyes are removed or as soon as the forma-
tion of retinal images is prevented through the turbidity of the
refractive media of the eye. This fact (confirmed by many
observers) proves that the harmony between color and pattern
of the skin of fishes with their surroundings is transmitted
through the retinal image; in other words, that the so-called
adaptation of fishes to their surroundings is only the trans-
mission of the retinal image to the skin.

It has, moreover, been shown that the destruction of the
optic fibers and the optic ganglia in the brain acts like the extir-
pation of the eyes; and finally it has been proved that the
cutting of the sympathetic fibers which go to the pigment cells
of the skin also prevents the formation of a picture of the
ground on the skin. Hence we know the path by which the
retinal image is transferred to the skin of fishes. One station
is the ending of the optic fibers in the brain. Since we are able
to prove the existence of an image of the object on the retina
of fishes; since it is proved that the image on the skin of the
fish is a picture of the retinal image but not of the object (in
this case the ground) itself; since, moreover, the transmission
of the retinal image upon the skin takes place through the
optic nerve, it follows that the image must pass the central
stations of the optic nerve during the transmission to the skin.

An image consists of a number of points of different intensity
of light, the mutual arrangement of which is definite and char-
acteristic for the object. Sumner has shown that certain
fishes are able to reproduce on their skin rather complicated

Pattern Adaptation of Fishes 81

patterns (e.g., a chess board), which form the bottom of the
aquarium. This reproduction of the pattern is somewhat
imperfect, but if we deduct the secondary disturbing factors the
fact remains that the pattern on the skin is a tolerably true
picture of the pattern of the ground. There exists, therefore, a
definite arrangement of the images of the different luminous
points of the ground on the retina and a similar arrangement of
the images of the luminous points on the skin of the fishes. We
may consider each point of the retinal image as a luminous or a
stimulating point which produces a corresponding image point
in the primary optic ganglia through the action of the nerve-
fiber through which it is connected with the ganglia. Every
image point in the primary optic ganglia may be considered
again as a luminous or stimulating point which through the
mediation of a special nerve-fiber influences an individual
chromatophore or a small group of chromatophores of the skin.
Considering the fact that the retina is a mosaic, we cannot well
imagine the transmission of the retinal image upon the skin
in any other way than by assuming that the relative arrange-
ment of the individual points of the retinal image is preserved
in the optic fibers and the end ganglia of the optic nerve.
Under this assumption a relative distribution of the stimulating
intensities must occur in the primary optic ganglion which cor-
responds to the distribution of the image points on the retina
and which again can be called an image.

These observations in fish and the conclusions drawn in this
note suggest the idea that vision is a kind of telephotography.

V. ON SOME FACTS AND PRINCIPLES OF
PHYSIOLOGICAL MORPHOLOGY

ON SOME FACTS AND PRINCIPLES OF PHYSIOLOGICAL

MORPHOLOGY ^

I. HETEROMORPHOSIS-

The various organs of the higher animals have a definite
arrangement; from the shoulders arms originate, from the hips
legs, but we never see legs growing out from the shoulders
or arms from the hips. In the lower animals the same definite
arrangement of organs exists.

Fig. 22 gives a diagram of a hydroid, Antennularia anten-
nina, which is quite common in the Bay of Naples. From a
bundle of roots or stolons a straight stem arises to a height of
six inches or more. From this main stem originate, in regular
succession, short and slender branches, which carry polyps on
their upper sides.

In this animal we never find a root originating at the apex,
or in place of a branch, or polyps originating on the under side
of a branch.

In observing these phenomena the question arose : What are
the circumstances which determine that only one kind of organ
shall originate at certain places in the body? It occurred to
me that the answer to this question might be obtained by finding
out first of all whether or not it were possible to make any
desired organ of an animal grow at any desired place. In case
this could be done, the question to be decided was whether the
same circumstances by which the arrangement of organs can
be changed experimentally also determine the arrangement of

1 Reprinted from Biological Lectvires delivered at the Marine Biological
Laboratory of Woods Hole, 1893. by courtesy of Ginn & Co.

2 Untersuchungen zur physiologischen Morphologie der Tiere. I, Hetero-
morphosis, Wtirzburg, 1891. II. Organbildung und AVachsthum. Wurzburg, 1892.
Translated in Studies in General Physiology.

The Mechanistic Conception of Life

Fig. 23. — Diagram
of normal regenera-
tion if a piece a 6 of
Antennularia is liung
up vertically in the
water. The piece
forms roots W at the
lower end b and a new
stem »S at the upper
end a. The old nor-
mal arrangement of
organs is thus restor-
ed through the pro-
cess of regeneration.

Fig. 24. — Diagram
of heteromorphic re-
generation in Anten-
nularia. A piece a b
cut out of the stem
is hung up in an in-
verted position, i.e.,
the root end b up-
ward and the stem
end a downward. In
this case the apical
end a forms roots W,
and the basal end b
forms a new stem S
which grows upward.

organs in the natural development. The
hydroid, Antennularia antennina, above
mentioned, seemed to afford a suitable sub-
ject for experimentation in an attempt to
solve this problem and the following simple
experiments were performed.

A piece ab (Fig. 23) of an Antennu-
laria was cut out and hung up vertically in
the water of the aquarium, the apical end
a above and the root end b below. It was found that after a
few days the root end b had formed little roots, W, which

Fig. 22. — A piece
of the normal stem
of Antennularia an-
tennina,& hydroid of
the Bay of Naples.
Approximately natu-
ral size. S S, stem.
W, stolons or roots.

Physiological Morphology

grew downward, and the apical end, a, had formed a new
stem, S.

A similar piece was cut out from another specimen and was
hung upside dowTi in the aquarium (Fig. 24). The root end b,
which was now above, formed a new stem, *S, and the apical
end a, which was below, formed roots, W. In the newly
formed stem the arrangement of the organs was the same as
in the normal animal, namely, the branches which were growing

. ^\,.,:n>...^ ::^.,b.„..:

Fig. 25. — From nature. Regeneration of a piece a b cut out from the stem
of Antennularia and put horizontally into the water. The branches on the lower
side which had ceased to grow, grow downward as stolons and attach themselves
to solid bodies. On the upper side a new stem c d grows vertically upward.

obliquely upward bore polyps on their upper side. From this
we see that it was possible to substitute a root for a stem and
an apex for a root. This phenomenon of the substitution of
one organ for another I termed heteromorphosis. If the excised
piece of an Antennularia was placed horizontally instead of
vertically in the aquarium, something still more remarkable
occured, namely, the branches on the lower side suddenly began
to grow vertically downward, and these downward growing
elements were no longer branches but roots (Fig. 25). This
could be proved by their physiological reactions, for the roots
attach themselves to the surface of solid bodies, e.g., the glass

The Mechanistic Conception of Life

of the aquarium, while the stems never show such a reaction.
These new parts growing out from the branches of the under
side of the stem attached themselves to the solid bodies with
which they came in contact. Moreover, they were positively
geotropic (that is, they grew toward the center of the earth),

while the branches never
showed any positive geo-
tropism. The branches
on the upper side were
not transformed into
roots. They either per-
ished or gave rise to long,
slender, perfectly straight
stems, which grew ver-
tically upward. These
stems, as a rule, were too
slender to bear branches,
but at parts of the
upper surface of the
main stem there origi-
nated new stems (c d,
Fig. 25), which grew
vertically upward and
produced the typical
little branches bearing
polyps.

If we brought the stem into an obhque position (Fig. 26),
with the apex a upward, from every element of the main stem
new stems and roots originated, but with this difference, that
stems always originated from the upper side of an element and
roots from its lower side. If the stem were placed in an oblique
position, with the root end above, the branches on the mider
side grew out as roots, and at the upper end a stem arose as
usual.

Fig. 26. — Diagrammatic regeneration in a
piece o b of a stem of Antennularia put ob-
liquely into the water. On the upper side of
the stem a b new stems S, S„ S,„ grow verti-
cally upward, while at the lower end of the
piece a b opposite the new stems roots W,
W„ W,„ grow out.

This influence of gravitation is foimd only
in Antennularia antennina, not in other forms
of Antennularia.

Physiological Morphology 89

What circumstances had all these experiments in common ?
The stems always originated from the upper end or side of an
element, and roots always from the lower side or end of the same
element. These facts can be explained only on the assump-
tion that in this case gravitation determines the place of origin
of organs.

Now we may ask whether the action of this force, gravita-
tion, is also responsible for the natural arrangement of the organs
in this form, namely, that roots appear only at the base of the
stem and never at the apex or in the place of a branch. I believe
that this is the case. By reason of its negative geotropism, the
stem grows vertically upward. Gravitation does not permit
roots to arise at any place except at the under side of the
organs, and that is, under normal conditions, at the base of the
stem. The same force determines that polyps can originate
only on the upper side of branches, and thus the general
arrangement of organs is brought about by gravitation. But
how does gravitation determine that stems grow on the upper
and roots at the under side? This is a question to which we
shall return later.

Fig. 27 gives a drawing of an example of heteromorphosis
in Margelis, a hydroid common at Woods Hole, upon which
another set of experiments was carried on. If we cut off a stem,
or a small piece of a stem of this hydroid, and place it in a dish
containing sea-water, protecting it carefully from every motion,
a curious change takes place in the organism. Almost all, and
in some cases all, of the stems which touch the glass give rise
to roots that spread out and very soon cover a large area of the
glass. In this way the apical end of a stem may continue to
grow as a totally different organ, namely, as a root. Every
organ not in contact with some solid body gives rise to polyps.
Even the main root, if not in contact with a solid body, no longer
grows as a root, but gives rise to a great number of small polyps
which appear at the end of long stems. Fig. 27, which Mr.

The Mechanistic Conception of Life

Tower was kind enough to draw for me, shows a branch which
formed roots at its apex and polyps at its roots in this manner.

Fig. 27. — Heteromorphosis in Margelis, a hydroid. At a and 6, where the
points of stems touch the ground of the aquarium, new roots or stolons grow out.

The stem touched the bottom of the dish with the apical ends,
a and h. All these ends gave rise to roots. From the upper
side of the original root, which was not in contact with the

Fig. 28. — Heteromorphosis in Pennaria. A piece a b of this hydroid was cut
out and put into a jar with sea-water. The ends a and b touched the bottom of the
jar. At both points new roots grew out.

glass, later on small polyps grew out. Every place which

was in contact with solid bodies gave rise to roots, and every

place which was in contact with sea- water gave rise to polyps.

This is not the only species of hydroid found at Woods Hole

Physiological Morphology 91

in which such forms of heteromorphosis can be produced.
Another form, Pennaria, is just as favorable. In Pennaria I
succeeded repeatedly in producing roots at both ends of a
small stem that bore no polyps (Fig. 28).^

In these experiments on Margelis and Pennaria organs
brought into contact wdth solid bodies continue to grow as
roots, if they grow at all. Organs surrounded on all sides by
water continue to grow in the form of polyps, if they grow at
all. In Margelis, contact with a solid body plays the same role
as did gravitation in the case of Antennularia. In what way
the contact may have an influence shall be mentioned later on,
but here one more point may be mentioned. In Antennularia,
gravitation not only determines the place of origin of the various
organs, but also the direction of their growth; the stem,
growing upward, is negativelj' geotropic, the root, growing
downward, is positively geotropic. In Pennaria, the nature
of the contact not only determines the place of origin of the
various organs, but also the direction of their gro\\'th. If we
bring an outgrowing pol^-p of Pennaria into contact with a
solid body, the polyp begins to grow away from the body, and
the new stem is very soon nearly' perpendicular to the part of
the surface with which it came into contact.

I have called this form of irritability stereotropism. We

1 In a Tubularian I was able to produce the opposite result, namely, to get
an animal that ended at both ends in a polyp and had no root. Weismann seems
to assume, in his Germ Plasm, that the latter result is to be explained by the
principle of natural selection, inasmuch as an animal without polj-ps could not
continue to live, and hence it would be impossible to produce roots at both ends.
In Pennaria this supposed impossibility was realized; one may say that these
roots in Pennaria may give rise later on to polyps. In the special case that I
observed they did not, although as a rule they do; but the same is the case in
Tubularia, in which polyps also arise from the roots. It might be said, perhaps,
that the formation of roots in Pennaria is, for some reason, absolutely necessary;
but it is just as easy to produce polyps at both ends. Even if it were possible to
reconcile these facts with the principles of natural selection, causal or physiological
morphology would not gain thereby, as the circumstances that determine the
forms of animals and plants are only the different forms of energy in the sense in
which this word is used by the physicist, and have nothing to do with natural
selection.

92 The Mechanistic Conception of Life

may speak of positive stereotropism in the case of the root, and
of negative stereotropism in the case of the polyp.

Here, too, we may ask whether the contact with foreign
bodies, which in these experiments determines the arrangement
of the various organs, may not have the same effect in the
natural development of the organism. I believe that such is
the case. Negative stereotropism forces the polyps to grow
away from the ground into the water, and hence parts sur-
rounded by water form polyps only. Positive stereotropism
forces roots in contact with the ground to hold to it, hence
parts in contact with the ground give rise to roots only. Thus
it happens that, under ordinary circumstances, in this animal
we find roots only at the base where it touches the ground. In
other hydroids the place of origin of the different organs is
determined by light, and in others we find more complicated
relations.

It may appear from the foregoing that such cases of hetero-
morphosis are confined to hydroids, but such is not the case.
We find similar cases in Tunicates. Ciona intestinalis, a solitary
ascidian, has eye-spots around the two openings into the
pharyngeal cavity. If we make an incision eye-spots are
formed on both sides of the incision.^

II. POLARIZATION

While the foregoing experiments were in progress, I observed
that in many animals I was unable to produce any kind of
heteromorphism. These animals showed, in regard to the
formation of organs, a phenomenon with which we are familiar
in a magnet. If a magnet is broken into pieces, every piece
has its north pole on that side which in the unbroken magnet
was directed toward the north. Likewise, there are animals
every piece of which produces, at either end, that organ toward

1 Since this was written phenomena of heteromorphosis have been produced
In many animals. Herbst found that in crustaceans an antenna could be caused
to be formed in the place of an excised eye, Van Duyne, Bardeen, and Morgan
observed phenomena of heteromorphosis in Planarians and so on (1912).

Physiological Morphology

which it was directed in the normal condition. We may speak

in such cases of polarization. The clearest example of this I

found in an actinian, Cerianthus membranaceus.
If we cut a rectangular piece, c d ef, out

of the body-wall of Cerianthus new tentacles

soon begin to grow out of this piece, but

only from the side ef (Fig. 29), which was

directed toward the oral end of the animal.

Nothing of the sort oc-
curs in the side c e, or
d c , or f d. The produc-
tion of tentacles takes
place before any other
regeneration begins.
The same polarization is
shown in the following
variation of the preceding
experiment. If we make
an incision, acb (Fig. 30),
into the body-wall of the actinian, only the
lower lip, h c, produces tentacles, while the
upper lip, a c, produces none. The two
ends heal together in such a way that one-
half of a mouth, with its surrounding ten-
tacles, b (Fig. 31), is formed. It is curious
to see how these tentacles behave if we offer
them bits of meat. They endeavor to force
them into the new oral disc, where the
mouth should be, and only after a struggle

of some minutes do they give up the futile attempt. I tried

in every possible way to produce tentacles in the aboral end

of a piece which had been cut out, but without success.

Hydra behaves, as regards polarization, a little differently

from Cerianthus. If we make an incision in the stem, a

C^J

Fig. 29. — Diagram-
matic. If a piece c d
ef is cut out from the
wall of Cerianthus, a
sea anemone, new
tentacles are formed
only at the upper cut
ef.

Fig. 30. — Diagram-
matic. If an incision
a cb is made into the
body of Cerianthus
new tentacles grow
out only from the
lower edge c h.

The Mechanistic Conception of Life

whole new oral pole grows out, but otherwise it too shows
polarization.

A good many animals, so far as we know, reproduce only
the lost organ, but never show any heteromorphism. We see,

Fig. 31. — From natvire. Formation of a second head in Cerianthus after a
lateral incision at b. Only a fraction of the normal number of tentacles are formed
corresponding to the fraction of the periphery laid bare by the incision. No new
mouth is formed, but if a piece of meat is offered to the group of tentacles at b
they seize it and press it to the place where a mouth ought to be, sho\ving the
purely machine-like character of all these reactions.

therefore, that while in some animals we are able to produce
heteromorphosis, in others the most definite polarization exists,
and we are able to produce regeneration of lost parts only in the
arrangement which exists in the normal animal. In this case
we must assume that unkno^Mi internal conditions determine
the arrangement of limbs.

In addition to examples of heteromorphosis or polarization
occurring separately, we find cases in which both phenomena

Physiological Morphology

are exhibited by the same animal. If we cut out a sufficiently
large piece of the stem of Tuhularia mesemhryanthemum, and
place it in the bottom of a dish of water,
carefully protected from jarring, the ante-
rior end of the piece gives rise to a new
polyp, the posterior end to a root; but if
we hang up the stem in such a way that the
posterior end does not touch the surface of
the glass, and is sufficiently provided with
oxygen, this end, too, produces a polyp, and
we have a true case of heteromorphosis
(Fig. 32). In all cases the polyp at the oral
end is formed first, and a relatively long
time (one or more weeks) elapses before
the aboral polyp is formed. Under one con-
dition, however, I could cause the stem to
form a polyp at the aboral as quickly as at
the oral end, namely, by inhibiting or re-
tarding the formation of the oral pol3rp.
This could be done readily by diminishing
the supply of oxygen at the oral end. In
such cases the aboral polyps were produced
almost as quickly as the oral polyps.^

III. THE MECHANICS OF GROWTH IN
ANIMALS

In order to arrive at an explanation
of the phenomena of organization we must
ask what the physical forces are that
determine the formation of a new organ.
We know that the ultimate sources of
energy for all the functions of living bodies

Fig. 32. — Hetero-
morphosis in Tuhula-
ria. From nature.
Thenormal Tubularia
ends at one end in a
stolon, at tlie other
in a head or polyp.
If a piece o 6 is cut
out and suspended in
water a new liead or
polyp c and d is formed
at both ends. We
can thus produce an
animal wliich termi-
nates in a head at
both ends of its body;
while in Fig. 28 an
animal was represent-
ed which ended at
both ends in a stolon
or foot.

1 It was found later independently by both Godlewski and myself that if we
ligature the stem of a Tubularian the polyps at both ends are formed simultane-
ously (1912).

96 The Mechanistic Conception of Life

are chemical processes. The question is, How can these
chemical forces be brought into relation with the visible changes
which take place in the formation of a new organ ? The answer
to this question is to be obtained by a knowledge of the
mechanics of growth. It is very remarkable that the mechanics
of growth forms almost an empty page in the history of animal
morphology and physiology. I can refer here only to the few
experiments I have made on this subject; but fortunately the
subject has been worked out very carefully in plants, and as my
experiments show that the conditions for growth in animals
are, to a certain extent at least, the same as the conditions for
growth in plants, we have the beginning of a basis for work.

A brief outline of the manner of growth in plants is as
follows : Before the cell grows it forms substances which attract
water from the surroundings, or, as the physicist expresses it,
it forms substances which determine a higher osmotic pressure
within the cell than did the substances from which the}' originate.
The walls of the cell, or rather the protoplasmic layer that lines
the cell-wall, possesses peculiar osmotic properties, in conse-
quence of which it allows molecules of water to pass through
freely while remaining resistant to the passing through of the
molecules of many salts dissolved in the water. The result is
that when substances of higher osmotic pressure are formed
inside the cell, water from the outside passes in until the pres-
sure within again equals the pressure without. The cell-wall
becomes stretched and, according to Traube, new material is
precipitated in the enlarged interstices, thus rendering growth
permanent. This method of growth is most conspicuous,
perhaps, in the germinating seed. The rising temperature in
spring produces in the seed substances of higher osmotic pres-
sure (with greater attraction for water) than the substances
from which they originate. The result is that water enters the
seed; by the pressure of the water within the cells their walls
are stretched out and the seed grows. The chemical and

Physiological Morphology 97

osmotic changes are the sources for the energy which is needed
to overcome the resistance to growth.^

In order to ascertain whether I could determine what are
the mechanical causes of growth in animals, I began at Naples
some experiments on Tubularia mesemhryanthemum. I chose
long stems belonging to the same colony and distributed them
in a series of dishes containing sea-water of different concentra-
tions. In some of the dishes the concentration had been raised
by adding sodium chloride, and in others it had been lowered by
adding distilled water. According to the laws of osmosis the
amount of water absorbed by the cells of these Tubularians
differed with the concentration of the sea-water, the amount
being greatest in the most diluted solution and least in the most
concentrated solution. If now in reality the mechanics of
growth is the same for animals as for plants, we should expect
that the more diluted the sea-water the more rapid would be the
growth in the Tubularian stem. Of course, finally, a limit is
reached where the water begins to have a poisonous effect. It
was found, indeed, that within certain limits of concentration
the increase in the length of the stems during the same period
was greatest in the most diluted and least in the most concen-
trated sea-water. It is remarkable that the maximum of growth
took place not in sea-water of normal concentration, but in
more diluted sea-water, though this of course may not be the
case in all animals. The following curve (Fig. 33) will give an
idea of the dependence of growth upon the concentration of
the sea-water in Tuhidaria. The values for the amount of
sodium chloride, in 100 cubic centimeters of sea-water, are
represented on the axis of the abscissa, the values for the
increase in growth on the axis of ordinates.

These and similar experiments, which for lack of space
cannot be mentioned here, show that growth in animals is

1 The substance which is formed and which causes the swelling may be an
acid. I found that acids cause a swelling of muscles and it has since been shown
that this is a general phenomenon.

The Mechanistic Conception of Life

determined by the same mechanical forces which determine
growth in plants. An obstacle to such a conclusion seems to lie
in the fact that many plant-cells have solid walls, while this is
not the case in most animal cells. The solid cell-wall, however,
does not determine the peculiar character of growth. This
character is determined first, by chemical processes within the
cell, which result in a higher osmotic pressure, and, secondly, by
the osmotic qualities of the outer layer of protoplasm, which

Pig. 33. — Curve representing the influence of diluted sea-water. The
abscissae represent the concentrations, the ordinates the corresponding growth
in the unit of time. The maximum growth is at a concentration between 2 and 3
per cent of salt, while the normal concentration is indicated by the vertical line
between 3 and 4.

allows water to pass through freely, but does not allow all salts
dissolved in it to do the same. Both these qualities are inde-
pendent of the solid cell-wall, and I see no reason why the animal
cell should not agree in these two salient features with the
plant-cell.

In order that the foregoing explanation of the mec hanism of
growth in the animal cell might be based only upon knoTVTi pro-
cesses, it was necessary to find out whether, in case of growth,
chemical processes of such a character take place that substances
of higher osmotic pressure are formed than those from which
they originate. Everyone knows that by exercise our muscles
increase in size. No satisfactory explanation of this fact has

Physiological Morphology 99

been given. If my interpretation of the method of g^o^^i:h
were correct, I must expect that during activity substances are
formed in the muscle, which determine a higher osmotic pressure
than those from which they originate. This is exactly the case.
Ranke had already sho\\Ti that the blood of a tetanized frog
loses water and that this water is taken up by the muscles.
In experiments which were carried on by Miss E. Cooke in my
laboratory, we were able to show directly that during activity
the osmotic pressure inside the cell-wall is raised. We deter-
mined the concentration of a solution of NaCl, or rather of
a so-called Ringer's mixture, in which the gastroconemius of a
frog neither lost nor took up water. We fomid that while
this concentration for the resting gastroconemius was about
0.75 per cent to 0.85 per cent, for the gastroconemius that
had been tetanized from twenty to forty minutes it varied from
1.2 per cent to 1.5 per cent.^

This increase of osmotic pressure inside the muscle-cell leads,
during normal activity, to a taking up of water from the blood
and lymph, and the consequence is an increase in volume. The
same muscle, as soon as it ceases to be active, begins to decrease
in size. Activity, therefore, plays the same role in the growth of
a muscle that the temperature plays in the gro^\i:h of the seed.

I tried to ascertain whether segmentation, like gro^iih in
general, is influenced by the amount of water contained in the
cell. If we decrease the amount of water in the egg of the
sea-urchin segmentation is retarded, and if we use a sufficiently
high concentration of sea-water it may be stopped entirely.
Therefore the amount of water contained in the cell plaj^s still
another role in the process of organization and influences the
process of cell-division.

1 This increase in osmotic pressure is probably caused by the formation of
acid. Two years after the publication of this lecture I showed that the muscle
swells in an isomotic solution if this solution is acid. The recent work of Pauli
and Handovski indicates that the swelling is caused through a formation of salt
between the acid and a weak base, e.g., a protein. The protein salt is more
strongly dissociated than the protein base (1912).

100 The Mechanistic Conception of Life

IV. THE artificial PRODUCTION OF DOUBLE AND MULTIPLE

MONSTROSITIES IN SEA-URCHINS^

The idea that the formation of the vertebrate embryo is a
function of growth has been made the basis of the embryological
investigations of His. In a masterly way, His has sho^\^l how
inequahty of growth determines the differentiation of organs.
In the blastoderm of a chick, for example, the first step in the
formation of the embryo is a process of folding. There origi-
nates a head fold, a tail fold, a medullary groove, and the system
of amniotic folds. According to His, all these processes of
folding are due simpl}' to inequalities of growth, the center of
the blastoderm growing more rapidly than the periphery. It
can be shown, very simply, that such a process of unequal
growth must, indeed, lead to the formation of exactly such a
system of folds as we find in the blastoderm of a chick. If we
take a thin, flat plate of elastic rubber, and lay it on a drawing-
board, we can imitate the stronger growi:h in the center by
sticking two tacks into the middle of the rubber, a short
distance apart, and then pulling them in opposite directions.
In this way we may imitate unequal growth, the center growing
faster than the periphery. If we then fix the tacks in the
drawing-board, so that the rubber in the middle remains
stretched, we get the same system of folds as that shown by the
embryo of a chick. I mention this way of demonstrating the
effects of unequal growth as the ideas of His are still doubted
by some morphologists.

His raised the question. Why is growth different in different
parts of the blastoderm? But instead of trying to answer it
from the physiological standpoint he answered it from the
anatomical standpomt. According to him, the different regions
of the unsegmented egg correspond already to the different
regions of the differentiated embryo. But this so-called theory

1 Another method of producing twins from one egg is discussed in the last
chapter of this book.

Physiological Morphology 101

of preformed germ-regions gives no answer to the question, why
some parts of the embryo grow faster than others. Neverthe-
less, it is not necessarily in opposition to the theory of growth
offered in the preceding chapter. Starting with the idea of His,
we may well imagine that the different regions of the ovum
are somewhat different chemically, and that these chemical
differences of the different germ-regions determine the differ-
ences of growth in the blastoderm. Thus the phenomena of
heteromorphosis would show that, in some animals at least, the
arrangement of preformed germ-regions may be changed by
gravitation, light, adhesion, etc.

It must be asked, however, what, from the standpoint of
causal morphology, determines the arrangement of the different
germ-regions in the egg. If we answer ''heredity," causal
morphology can make no use of such an explanation. Our
blood has the temperature of about 37°, but although our
parents had the same temperature, the heat of our blood is not
inherited, but is the result of certain chemical processes in our
tissues. Still it may be possible that the molecular forces of
the chemically different substances of the egg determine a
separation of these substances and thereby give rise to the
chief directions of the future embryo.

Driesch has shown^ that by shaking a sea-urchin's egg in the
four-cell stage the four cells may be separated, and each one be
capable of giving rise to a complete embryo, which differs only
in size from the normal embryo. If the theory of preformed
germ-regions with its later modifications were true, we should
expect that every one of the isolated cells would give rise to
one-fourth of an embryo. But it has been said that the arti-
ficial isolation of one cleavage-cell causes a process of post-
generation or regeneration. Driesch, moreover, changed the
mode of the first cleavage by submitting the ovum to one-sided
pressure. In this way the nuclei were brought into somewhat

1 Zeitschrift f. wissensch. Zoologie, LIII, LV.

102 The Mechanistic Conception of Life

different places from those they would have held m the case of
normal segmentation. Still, normal embryos resulted. One
might object again that the preformation of the germ-regions
existed in the protoplasm, and not in the nucleus, I have made
a series of experiments to the results of which these objections
cannot be made. I shall describe these experiments somewhat
fully, as they have not yet been published, though I cannot
enter into details at this place.

I brought eggs of a sea-urchin, within ten to twenty minutes
after impregnation, into sea-water that had been diluted by the

Fig. 34 Fig. 35

Fig. 34. — Fertilized egg of a sea-urcliin (Arbacia) put into dilute sea-water.
The protoplasm swells until the membrane m bursts and part of the protoplasm
6 flows out; each of the two droplets may develop into a blastula, so that from
such an egg two larvae may arise, as Indicated m Fig. 35.

addition of about 100 per cent distilled water. In this solution
the eggs took up so much water that the membrane (m. Fig. 34)
burst and part of the protoplasm escaped in the form of a drop
(6, Fig. 34), which often, however, remained in connection
with the protoplasm inside the membrane after the eggs were
brought back into normal sea-water. These eggs gave rise
to adherent twins, the ejected part h of the protoplasm, as well
as the part remaining inside the membrane, developing into a
normal and perfectly complete embryo. The part of the proto-
plasm, which at first had connected the two drops, formed the
part where the twins remained gro\Mi together. Of course, it
often happened that, by accident or rapid movement, the twins
were separated, and they then developed into perfectly normal
single embryos. Since we caimot assume that in every case the

Physiological Morphology 103

same part of the protoplasm escapes, we must conclude that
every part of the protoplasm may give rise to fully developed
embryos without regard to preformed germ-regions.^ In many
eggs a repeated outflow of the protoplasm takes place. In such
cases each of the drops of the protoplasm may give rise to an
embryo, and I obtained not only double embryos, but triplets
and quadruplets all grown together.

It is remarkable that the development of these monstrosities
goes on nearly at the same rate as that of the normal embryo,
provided they are equally well supplied with oxygen and equally
protected from microbes and infusoria. The development in
most eggs takes place in so regular and typical a manner that it
seems as if there were a prearrangement of some kind. It is,
however, perfectly well possible that this prearrangement
consists in a separation of different liquid substances in the
ovum by the molecular qualities of these liquids. Such a
separation, of course, might be called a preformation of germ-
regions, but it would be something totally different from what
is now understood by that term.

V. THEORETICAL REMARKS

1. All life phenomena are determined by chemical processes.
This is equally the case whether we have to do with the contrac-
tion of a muscle, with the process of secretion, or with the forma-
tion of an embryo or a single organ. One of the steps that
physiological morphology has to take is to show in every case
the connecting link between the chemical processes and the
formation of organs. I have tried to show that in a few cases
at least this connecting link was to be sought in the changes of
osmotic pressure determined by the chemical changes which
take place in the growing organ.

But this fact alone does not explain why it is that we get

I In the light of more recent experiments it is possible, that after all only such
pieces can develop into a normal embryo which contain the different germ-regions
(1912).

104 The Mechanistic Conception of Life

differences in the forms of organs. In order to understand
this we must bear in mind that the processes of growth must
necessarily be different for different organs, as for example in
the formation of a root, and the formation of a stem. As
growth is a process in which energy is used up in overcoming
the resistance to growth, differences of growth can only be
determined either by differences in the amount of energy set free
in the growing organ or by differences in resistance. Differ-
ences in the energy must be the outcome of differences in the
chemical processes which determine growth. Therefore we are
led to the idea that differences in the forms of different organs
must be determined by differences in their chemical constitu-
tion, or, if the chemical constitutions be similar, by differences
in resistance to growth. That organs which differ in shape
are very often chemically different is a well-known fact. The
formation of urea in the liver and the synthesis of hippuric
acid by the kidneys are the consequences of chemical differences.

In this way we are led through the mechanics of growth to a
conclusion which forms the nucleus of Sachs's theory of organi-
zation, namely, 'Hhat differences in the form of organs are
accompanied by differences in their chemical constitution, and
that according to the principles of science we have to derive the
former from the latter." According to Sachs there are at least
as many ''spezifische Bildungsstoffe " in a plant as there are
different organs.^

2. In adopting the theory of Sachs and applying it to animal
morphology, we must avoid a mistake very often made even
in the case of good theories, namely, the endeavor to explain
special cases which are complicated by unknown conditions.
Huyghens explained by his theory of light the phenomena of
refraction, but he could not and did not attempt to explain the
sensations of color. For these phenomena the wave theory of

1 J. Sachs, Stoff und Form der Pflanzenorgane, Gesammelte Abhandlungen,
II, 1893.

Physiological Morphology 105

light remains true, but color sensations depend not only on the
wave motion of the ether, but also on the chemical and physical
structure of the retina. I think it perfectly safe to say that
every animal has specific germ substances, and that the germ
substances of different animals differ chemically. Its chemical
qualities determine that from a chick's egg only a chick can
arise. But it would be a mistake to attempt at present an
explanation of how the unknown chemical nature of the germ
determines all the different organs and characters that belong
to the species. For instance, the yolk sac of the Fundulus
embryo has a tiger-like coloration. We might say that these
markings may be due to a certain arrangement of molecules or
complexes of molecules (determinants), which later on give rise
to the colored places of the yolk sac, but I found that this
coloration originates in a manner much more simple. The
pigment cells are formed irregularly on the surface of the yolk.
The pigment is chemically closely related to hemoglobin, and
so its formation may from the first be connected with the
formation of the blood corpuscles. But the arrangement of
the pigment cells during the first days of development is not
such as to produce any definite markings. They lie upon the
walls of the blood-vessels as well as in the spaces between the
capillaries (Fig. 36). Later on, however, all of the pigment
cells have crept upon the surface of the neighboring blood-
vessels (Fig. 38). I succeeded experimentally in showing it to
be probable that some of the substances contained in the blood
determine this reaction. These substances, if they diffuse from
the blood-vessel and touch the chromatophore, make, according
to the laws of surface tension, the protoplasm of the chroma-
tophore flow toward and at last over the blood-vessel and form
a sheath around it, while the gaps between the blood-vessels
become empty of chromatophores. In this way the chroma-
tophores are arranged in stripes, and possibly changes in the
surface tension, and not a preformed arrangement of the germ.

106 The Mechanistic Conception of Life

Fig. 36

Fig. 37

Fig. 38

Figs. 36, 37, and 38. — From nature. The origin of the pattern on the yolk
sac of a fish embryo {Fundulus heteroclitus). Fig. 36 is a drawing of the blood-
vessels at the surface of the yolk sac at an early stage of development. The black
pigment cells show no definite orientation in regard to the blood-vessels. Fig,
37, the same egg a few days later. Here we notice that some pigment ceUs show
a tendency to creep on the blood-vessels. Fig. 38, the same egg still a few days
later. The black pigment cells have completely crept on the blood-vessels and
formed a sheath around them. The red chromatophores are omitted in this
drawing.

This was the first observation pro\'ing that tropisms play a role in the arrange-
ment of the organs in the body.

Physiological Morphology 107

determine the marking. We do not know what processes
determine the coloration of animals which owe their markings
to interference colors, but the task of deriving such a coloration
in the adult from a similar arrangement of molecules in the
germ plasm would prove too much even for a genius like
Huyghens, and without the possibility of such a derivation the
theory is of no use.

3. The reasons why roots grow on the under side of the
stem of Antennularia and stems on the upper side can only be
given when the special physical and chemical conditions inside
the stem of Antennularia have been worked out. At present we
can only think of possibiHties. It is possible that the hj'potheti-
cal root substances of Sachs may have a greater specific gravity
than the substances which form stems, and therefore take the
lowest position in the cell. Since outgro\\i:h can take place
only at the free surface of a stem or branch, roots can on this
assumption grow only at the under side and stems only at the
upper side of an element. But there are still other possibilities
which we must omit here. In the case of Margelis and other
hydroids, it might happen that contact with solid bodies pro-
duced an increase of surface in the touched elements in case they
contained specific root substances, while the opposite took place
in the case of elements containing polyp substances. The con-
sequence would be an increase in the surface of the roots if they
came into contact with solid bodies, while polyps only would
grow out in the opposite direction. I found, indeed, in some
forms at Naples that roots of hydroids which grew free in the
water began to grow much faster and to branch off more
abundantly when brought into contact with solid bodies. But
in these cases we must wait with our attempts at explanation
until the physical and chemical conditions for the form are
worked out. For the same reasons I will not go into a discus-
sion of the question of what determines the polarization of ani-
mals like Cerianthus. It may suffice to suggest the possibility

108 The Mechanistic Conception of Life

that in polarized animals the tissues or cells may have such a
peculiar structure as to allow the specific formative substances
to migrate or arrange themselves only in one direction, while in
cases of heteromorphosis migration or arrangement in every
direction or in several directions is possible.

4. The egg of a sea-urchin under normal conditions gives
rise to but one embryo. This circumstance is due simply to
the geometrical shape of the protoplasm, which, under normal
conditions, is that of a sphere. When we make the eggs burst,
the protoplasm outside the egg membrane and that which
remains within it assume spherical forms, by reason of the
surface tension of the protoplasm. When this happens, as a
rule, we get twins, if two separate segmentation cavities are
formed, and only one embryo, if both cavities communicate
with one another. Whether the first or the second case will
happen depends upon the molecular condition of the part of
the protoplasm connecting the two drops. Therefore, the
number of embryos which come from one egg is not determined
by the preformation of germ-regions in the protoplasm, or
nucleus, but by the geometrical shape of the egg and the
molecular condition of the protoplasm, in so far as these circum-
stances determine the number of blastulae. In my experiments,
I got double or triple embryos when the egg formed two or
three droplets or spheres, as every sphere gives rise to a blastula.
In Driesch's experiments, one single cell of the four-cell stage
necessarily formed a whole embryo after it had been isolated,
as it assumed the shape of a single sphere or ellipsoid. Of
course, there must be a limit to the number of embryos that can
arise from one egg; but the limit is not due to any preformation,
but to other circumstances, the chief one being that with too
small an amount of protoplasm the formation of a blastula —
from merely geometrical reasons, as there must be a minimum
size for the cleavage-cells — becomes impossible.^ Without the

1 1 stated that the minimal size is about one-eighth of the mass of the sea-
urchin's egg and I do not think that this is very far off the limit.

Physiological Morphology 109

formation of the blastula, of course it is not possible to get the
later stages which are determined by the blastula.

I have chosen the name Physiological Morphology for these
investigations, inasmuch as their object has been to derive the
laws of organization from the common source of all life phenom-
ena, i.e., the chemical activity of the cell. In what way this
is to be done is indicated in the chapter on the mechanics of
growth.

But the aim of Physiological Morphology is not solely
analytical. It has another and higher aim, which is synthetical
or constructive, that is, to form new combinations from the
elements of living nature, just as the phj'sicist and chemist
form new combinations from the elements of non-living nature.

VI. ON THE NATURE OF THE PROCESS OF

FERTILIZATION

ON THE NATURE OF THE PROCESS OF FERTILIZATION^

Experimental biology is a very recent science. Not until
recently have biologists begun to become conscious of the
uncertainty of conclusions which are not tested and verified by
adequate experiments.

Leeuwenhook demonstrated in 1677 the existence of motile
elements in the sperm, the so-called spermatozoa. He believed
that the spermatozoa represented the future embryo. The
majority of his contemporaries assumed that the spermatozoa
were parasitic organisms which had nothing to do with fertiliza-
tion. The idea that spermatozoa are not parasites did not
subside until it was proved about 160 years later that the
spermatozoa originate from the cells of the testes.

That sperm was needed to bring about fertilization of the
egg was too obvious a fact to escape even those biologists who
never made an experiment, but that the spermatozoa and not
the liquid constituents were the essential element in the sperm
was a fact which could not be established except experimentally.
It was generally assumed that no direct contact between sperm
and egg was necessary and that something volatile contained
in the sperm, the imaginary ''aura seminalis" was sufficient
for the act of fertilization. That contact between sperm and
egg was really necessary for fertilization was at last proved
experimentally by Jacobi (1764) who showed that fish eggs
can only be fertilized if the sperm is brought into direct con-
tact with the eggs; and by Spallanzani who put the males of
frogs during the act of cohabitation into trousers and convinced

1 Reprinted, from Biological Lectures delivered at Woods Hole, 1899, by
courtesy of Ginn & Co., Boston.

113

114 The Mechanistic Conception of Life

himself that under such conditions the eggs remained unferti-
Hzed although the "aura seminalis" was not prevented from
acting upon the eggs. This ended the reign of the ''aura
seminalis.''

It was reserved to two experimenters, Prevost and Dumas
(the latter the famous chemist) to prove that the spermatozoa
are the essential element in the sperm. They made the simple
experiment of filtering the sperm and demonstrated that the
sperm whose spermatozoa had been retained by the filter had
lost its power of fertilizing the eggs. But even this did not
convince many of the descriptive biologists and nine years later
K. E. von Baer still expressed the opinion that spermatozoa
had nothing to do with fertilization. In 1843 the entrance of
the spermatozoon into the egg was directly observed by Barry
and this fact has since been verified by an endless number of
investigators for the egg of all kinds of animals. It is probably
no exaggeration to say that with the general recognition of the
experimental method in biology it would probably have taken
about as many years as it took centuries to establish the simple
fact that the spermatozoon is the essential element in sperm.

The mere observation of the fact that the spermatozoon
must enter the egg in order to bring about fertilization did not
lead to any understanding of the mechanism of the activation of
the egg. Nevertheless four theories or rather suggestions were
offered.

The first theory of fertilization is a morphological one.
According to this theory, it is the morphological structure of the
spermatozoon which is responsible for the process of fertiliza-
tion.

The second theory is a chemical one. According to this
theory it is not a definite morphological or structural element
of the spermatozoon, but a chemical constituent, that causes
the development of the egg. Against this second view Miescher
raised the objection that his investigations showed the same

Nature of the Process of Fertilization 115

compounds in the egg and the spermatozoa. I do not beheve
that this objection is valid. We know that simple variations
in the configuration of a molecule have an enormous effect upon
life phenomena. This is shown among others by the work of
Emil Fischer on the relation between the molecular configura-
tion of sugars and their fermentability. When Miescher made
his experiments he was not familiar with such possibilities.
Moreover, Miescher was not able to state whether the
spermatozoa contain enzymes or not.

A third theory was a physical theory (Bischof). This
theory assumes that a peculiar condition of motion exists in
the spermatozoon which is transmitted to the egg and causes
its development. It should be said, however, that this idea
is not so very different from the chemical conception, because
it assumes exactly the same for the spermatozoon that Liebig
assumes for the enzymes. Liebig thought that the enzymes
owed their power of producing fermentation to the motions of
certain atoms or groups of atoms.

The fourth conception is the stimulus conception, which
was originated by His. According to this conception the egg is
considered as a definite machine which if once wound up will
do its work in a certain direction. The spermatozoon is the
stimulus which causes the egg to undergo its development. It
is to be said in connection with this stimulus conception that
the main point at issue is omitted as to whether the stimulus
carried by the spermatozoon is of a physical or a chemical
character, and in this way, of course, the stimulus conception is
nothing but a disguised repetition of the chemical or physical
theory of fertilization.

All these theories are so vague that we do not need to be
surprised that none of them has led to any further discovery.
If we want to make new discoveries in biology, we must start
from definite facts and observations, and not from vague
speculations. Among these observations the most important

116 The Mechanistic Conception of Life

are those on parthenogenesis. It had been observed for a
long time that the imfertihzed egg of the silkworm can develop
parthenogenetically. It was, moreover, kno^\^l that plant lice
can give rise to new generations without fertilization. The
most impressive fact concerning the parthenogenesis of animals
was contributed by Dzierzon, who discovered that the unferti-
lized eggs of bees develop and give rise to males, while the
fertilized eggs give rise to females. Similar conditions seem to
exist in wasps. It is, moreover, certain that a few crustaceans
show parthenogenesis.

A beginning of parthenogenetic development had been
observed in the case of a great many marine animals which
develop outside of the female in sea-water. It was found that
such eggs when left long enough in sea-water may divide into
two or three cells, but no farther. On the other hand, in
ovaries of mammals now and then eggs were found that were
segmented into a small number of cells. ^ These facts and the
occurrence of a certain class of tumors in the ovary, the so-called
teratomata, suggest the possibility of at least partial partheno-
genesis in the eggs of mammals. But all these phenomena
were considered to be of a pathological character. It must be,
however, admitted that we cannot utilize these facts with any
degree of certainty for the theory of fertilization, as in this
case certainty can only be obtained bj^ the experiment. It
was not until very recently that such experiments were made.

Eight years ago I observed that if the fertilized eggs of the
sea-urchin were put into sea-water whose concentration was
raised by the addition of some neutral salt they were not able
to segment, but that the same eggs, when put back after they
had been in such sea-water for about two hours, broke up into
a large number of cells at once instead of dividing successively

1 Hertwig, O., Die Zelle und die Gewebe, p. 239, Jena, 1893.

Nature of the Process of Fertilization 117

into two, four, eight, sixteen cells, etc. Of course it is neces-
sary for this experiment that the right increase in the concen-
tration of the sea-water be selected. The explanation of this
fact is as follows: The concentrated sea-water brings about
a change in the condition of the nucleus which permits a division
and a scattering of the chromosomes in the egg.^ As soon as
the egg is put back into normal sea-water it at once breaks up
into as many cleavage-cells as nuclei or distinct chromatin
masses had been preformed in the egg. Morgan tried the same
experiment on the unfertilized eggs of the sea-urchin, and
found that the unfertilized egg, if treated for several hours with
concentrated sea-water, was able to show the beginning of a
segmentation when put back into normal sea-water. A small
number of eggs divided into two or four cells, and, in a few
cases, went as far as about sixty cells, but no larvae ever devel-
oped from these eggs. Morgan^ had used the same concen-
tration of sea-water as Norman^ and I had used in our previous
experiments. I had added about 2 grams of sodium chloride
to 100 c.c. of sea-water. Norman used instead of this 3§ grams
of MgCl2 to 100 c.c. of sea-water, and Morgan used the same
concentration. Mead'' made an observation somewhat similar
to Morgan's upon Chaetopterus. He found that by adding a
very small amount of KCl to sea-water he could force the unfer-
tilized eggs of Chaetopterus to throw out their polar bodies.
The substitution of a little NaCl for KCl did not have the
same effect. While continuing my studies on the effects of
salts upon life phenomena, I was led to the fact that the peculiar
actions of protoplasm are influenced to a great extent by the
ions contained in the solutions which surround the cells. As is

1 Loeb, J., "Experiments on Cleavage," Journ. of Morph., VII, 1892.

2 Morgan, T. H., "The Action of Salt Solutions, etc.," Arch. f. Entwickelungs-
mechanik, VIII, 1899.

3 Norman, W. W., " Segmentation of the Nucleus without Segmentation of the
Protoplasm," Arch. f. E ntwickelungsmechanik. III, 1896.

* Mead, A., " The Rate of Cell-Division and the Fimction of the Centrosome,"
Woods Hole Biol. Led., 1898.

118 The Mechanistic Conception of Life

well kno^Mi, if we have a salt in solution, e.g., sodium chloride,
we have not only NaCl molecules in solution, but a certain
number of NaCl molecules are split up into Na ions (Na atoms
charged with a certain quantity of positive electricity) and CI
ions (CI atoms charged with the same amount of negative
electricity). When an egg is in sea-water, the various ions enter
it in proportions determined by their osmotic pressure and the
permeability of the protoplasm. It is probable that some of
these ions are able to combine with, the proteins of the proto-
plasm. At any rate, the physical qualities of the proteins of
the protoplasm (their state of matter and power of binding
water) are determined by the relative proportions of the
various ions present in the protoplasm or in combination with
the proteins.^ By changing the relative proportions of these
ions we change the physiological properties of the protoplasm,
and thus are able to impart properties to a tissue which it does
not possess ordinarily. I have found, for instance, that by
changing the amount of sodium and calcium ions contained
in the muscles of the skeleton we can make them contract
rhythmically like the heart. It is only necessary to increase
the number of sodium ions in the muscle or to reduce the num-
ber of calcium ions or do both simultaneously.- On the basis
of this and similar observations I thought that by changing the
constitution of the sea-water it might be possible to cause the
eggs not only to show a beginning of development but to
develop into living larvae, which were in every way similar
to those produced by the fertilized egg.

There seemed to be three ways in which this might be
accomplished. The first way was a simple change in the con-
stitution of the sea-water without increasing its osmotic pres-
sure. The second way was to increase the osmotic pressure

1 Loeb, J., "On lon-Proteid Compounds and Their Role in the Mechanics of
Life-Phenomena," Amer. Joum. of Phys., Ill, 1900.

- It is due to the Ca ions of our blood that the muscles of our skeleton do not
beat rhythmically, like our heart .

Nature of the Process of Fertilization 119

of the sea-water by adding a certain amount of a certain salt.
The third way was by combining both of these methods. The
first way did not lead to the result I desired.^ All the various
artificial solutions I prepared had only the one effect of causing
the unfertilized egg to divide into a few cells, but I was not
able to produce a blastula. I next tried the effects of an
increase in the sea-water by adding a certain amount of mag-
nesium chloride. In this case I had no better results than
Morgan. Very few eggs began to divide, but these did not
develop beyond the first stages of segmentation. I then tried
the combination of both methods. The osmotic pressure of
ordinary sea-water is roughly estimated to be the same as that
of a f n NaCl solution or a ^-n MgCl.^ solution. I found, after
a number of experiments, that by putting the unfertiHzed eggs
of the sea-urchin into a solution of 60 c.c. of -^^-n MgClg solu-
tion and 40 c.c. of sea-water for two hours the eggs began
to develop when put back into normal sea-water. Such eggs
reached the blastula stage. I do not think that anybody has
ever seen before such blastulae as resulted from these unferti-
lized eggs. As these eggs had no membrane, the amoeboid
motions of the cleavage-cells led very frequently to a discon-
nection of the various parts of one and the same egg, and the
outlines of the egg became extremely irregular. The blastulae
showed, as a rule, the same outline as the egg had in the morula
stage. It was, moreover, a rare thing that the whole mass of
the egg developed into one blastula. The disconnection of the
various cleavage-cells led, as a rule, to the formation of more
than one embryo from one egg. The results were in a certain
way similar to those I had obtained when I caused the fertilized
eggs of sea-urchins to burst. In such cases a part of the proto-
plasm flowed out from the egg but was able to develop. These
extraovates had no membrane, and of course showed some
irregularity in their outlines, but the irregularity in this case

1 Later experiments gave, however, positive results. See next chapter.

120 The Mechanistic Conception of Life

was far less than that observed in the unfertilized eggs of my
recent experiments. But although I had thus far satisfied my
desire to see the unfertilized eggs of the sea-urchin reach the
blastula stage, I was not able to keep these eggs alive long
enough to see them grow into the pluteus stage. They developed
more slowly than the normal eggs, and died, as a rule, on the
second day.

It was my next task to find a solution which would allow
the eggs to reach the pluteus stage. I found that this can be
done by reducing the amount of magnesium chloride and
increasing the amount of sea-water. By putting the unferti-
lized eggs for about two hours into a mixture of equal parts of
-2^-n MgCl^ and sea-water, the eggs, after they were put back
into normal sea-water, not only reached the blastula stage, but
went into the gastrula and pluteus stages. The blastulae that
originated from these eggs looked much healthier and more
normal than those of the former solution with more MgCla-
Of course as these unfertilized eggs had no membrane it
happened but rarely that the whole mass of an egg developed
into one single embryo. Quadruplets, triplets, and twins were
much more frequently produced than a single embryo. The
outlines of each blastula were much more spherical than in the
previous experiment. These eggs reached the pluteus stage on
the second day (considerably later than the fertilized eggs do).
Thus I had succeeded in raising the unfertilized eggs of sea-
urchins to the same stage to which the fertifized eggs can be
raised in the aquarium. I have not yet succeeded in raising the
fertilized eggs in my laboratory dishes beyond the pluteus stage.

Though I do not wish to go into the technicalities of these
experiments, I must mention a few of the precautions that I
took in order to guard against the possible presence of sperma-
tozoa in the sea-water.^ The reader who is interested in this

1 Today it may seem strange that I had to meet such objections, but when my
first papers on artificial parthenogenesis appeared, very few biologists were willing
to accept the correctness of my statements. The most absurd sources of error
were suggested.

Nature of the Process of Fertilization 121

technical side of the experiments will find all the necessary data
in my publication in the American Journal of Physiology}
Here I wish only to mention the following points:

1. These experiments were made after the spa^\Tiing season
was practically over.

2. Bacteriological precautions were taken against the possi-
bility of contamination of the hands, dishes, or instruments
with spermatozoa.

3. The spermatozoa contained in the sea-water lose, accord-
ing to the investigation of Gemmill,^ their fertilizing power
within five hours if distributed in large quantities of sea- water.

4. We have a criterion by which we can tell whether the
egg is fertilized or not in the production of a membrane. The
fertilized egg forms a membrane and the unfertilized egg has no
distinct membrane. None of the unfertilized eggs that devel-
oped artificially had a membrane.^

5. With each experiment a number of control experiments
were made. Part of the unfertilized eggs were put into the
same normal sea-water that was used for the eggs that did
develop. None of these eggs that remained in normal sea-water
formed a membrane or showed any development, except that a
few of them were divided into two cells after about twenty-
four hours.

6. I made another set of control experiments by putting a
lot of eggs of the same female into a solution w^hich differed less
from the normal sea-water than the one which caused the
formation of blastulae or plutei from the unfertilized eggs.

1 Loeb, J., "On the Artificial Production of Normal Larvae from the Unferti-
lized Egg of the Sea-Urchin," Amer. Joum. of Phys., III.

2 Gemmill, "The Vitality of the Ova and Spermatozoa of Certain Animals,"

Joum. of Anat. and Phys., 1900.

3 The method used in these experiments was primitive inasmuch as no ferti-
lization membrane was formed. A few years later I found a method for the
artificial production of a fertilization membrane, which is described in the next
paper. In the earlier experiments in which no fertilization membrane was
developed, nevertheless a change in the cortical layer of the egg was brought
about by the combined action of the hydroxyl-ions of the solution, and the
increased osmotic pressure.

122 The Mechanistic Conception of Life

In this case it was shown, that although these eggs received the
same sea-water as the ones which developed, and although
they were injured less than the ones which developed, yet not
one single egg formed a membrane or reached the blastula
stage. If the sea-water had contained any spermatozoa these
eggs should have reached the blastula stage.^ Hence, as in
nine different series of experiments these results were confirmed,
we may assume that by treating the eggs for two hours with a
solution of equal parts of a ^ n MgCl, solution and sea-water
we can cause them to develop parthenogenetically into plutei.

Ill

What conclusions may we draw from these results ? If we
wish to avoid wild and sterile speculations, I think we should
confine ourselves to the following question: What alterations
can be produced in an egg by treating it for two hours
with a solution of equal parts of ^^n MgClg and of sea-water ?
Even in this regard we can only give a very indefinite answer
which, however, will have to be in the following direction: The
bulk of our protoplasm consists of colloidal substances. This
material easily changes its state of matter and its power of
binding water. It seems probable that changes of these two
qualities are mainly responsible for muscular contraction and
perhaps amoeboid motions. Among the agencies that cause
changes of these physical qualities we know of three that are
especially powerful. The one is specific enzymes (trypsine,
plasmase, etc.). The second is ions in definite concentration.
The concentration varies for various ions. The third agency
is temperature. In our experiments it is obvious that only the
second possibility can have been active. I do not consider
it advisable to enter into theoretical discussions beyond these

1 Through other control experiments I convinced myself that a treatment of
eggs or spermatozoa with equal parts of a -g'^n MgClg solution and sea-water
diminishes the impregnability of the eggs and annihilates the fertilizing power of
spermatozoa in a very short time.

Nature of the Process of Fertilization 123

statements. The next question that should be raised would be
whether the spermatozoa act in the same way. It is true that
the spermatozoon contains a considerable proportion of salts,
especially KgPO^, but it may contain enzj-mes or it may con-
tain substances which have similar effects upon the physical
qualities of the colloids, like the three agencies mentioned above.
In the last volume of these lectures I pointed out that it is
impossible to derive all the various elements that constitute
heredity from one and the same condition of the egg.^ Our
recent experiments suggest the possibility that different con-
stituents of the egg are responsible for the process of fertiliza-
tion and for the transmission of the hereditary qualities of the
male. While we are able to produce the process of fertilization
by a treatment of the unfertilized egg with certain salts in
certain concentrations, we cannot hope to bring about the
transmission of the hereditary qualities of the male by any such
treatment. Hence, the inference must be that the transmis-
sion of the hereditary qualities of the male and the agency that
causes the process of fertilization are not necessarily one and
the same thing. I consider the chief value of the experiments
on artificial parthenogenesis to be the fact that they transfer
the problem of fertilization from the realm of morphology
into the realm of physical chemistry .^

1 Loeb, J., " The Heredity of the Markmg in Fish Embryos," Woods Hole Biol.
Led., Boston, 1899.

2 This paper was written immediately after I had succeeded in producing
larvae from the unfertilized egg. In the following years the methods of artificial
parthenogenesis were improved and this led to the unraveling of the mechanism
by which the spermatozoon causes the egg to develop. An account of this work
is given in the two following papers.

VII. ON THE NATURE OF FORMATIVE STIMU-
LATION (ARTIFICIAL PARTHENOGENESIS)

VII

ON THE NATURE OF FORMATIVE STIMULATION
(ARTIFICIAL PARTHENOGENESIS)!

PREFACE

The title of this paper was chosen in reference to Virchow's
paper on ''Stimulation and Irritability" {Virchow's Archiv,
XIV, 1, 1858) in which he discriminates between three forms of
stimulation : functional, nutritive, and formative. By formative
stimuli he means those which give rise to nuclear and cellular
division. He considers as the classic example for formative
stimulation the fertilization of the egg and the parallel drawn
by him between this process and the causation of a pathological
process of growth is so characteristic that we may quote it in full :

If we admit the identity between the pathological and the embry-
onic neoformation, the egg will have to be considered as the analogue
of the pathological mother cell and the act of impregnation as the
analogue of pathological stimulation. This view is not essentially
altered through the discovery of the entrance of the spermatozoon
into the egg, since there is no reason to consider the spermatozoon as
the direct morphological starting-point for the development of definite
parts of the egg. If, as seems to be the case, the spermatozoa are
dissolved in the egg, they carry into it only certain chemical substances,
which serve as specific stimuli, by calling forth new chemical and
morphological arrangements of the atoms. Each specific contagium
offers the same possibilities.

The supposition prevalent at Virchow's time, that the
spermatozoon is entirely dissolved in the egg was not correct;
but his view, that the spermatozoon carries chemical substances
into the egg, which form the stimulus for its development, is
perfectly correct; and like\\ise the analogy between the causa-
tion of the development of the egg by a spermatozoon and the
causation of a pathological growth seems correct. I therefore

1 Address delivered at the International jSIedical Congress at Budapest, 1909.

127

128 The Mechanistic Conception of Life

believe that it may be of interest to the medical profession
to follow me in a brief survey of my experiments on artificial
parthenogenesis and the causation of the development of the
egg by a spermatozoon.

Cellular physiology has shown that tissues and organs
develop only from cells through nuclear and cellular division.
The conditions which cause cells to divide and to develop into
new normal or pathological tissues have, since Virchow, been
called formative stimuli. It is the task of modern biology to
ascertain first what is the nature of these stimuli, and second,
which change occurs in the cell in the process of formative
stimulation. Virchow already emphasized the fact that the
fertilization of the egg is the model of all phenomena of forma-
tive stimulation and that the spermatozoon may be considered
as the formative stimulus in this case.

Pathologists have not yet succeeded in determining what
the physico-chemical nature of the formative stimulus in the
case of a tumor is, or what changes a cell undergoes in such a
process. This task has, however, been accomplished to a high
degree in the animal egg, and it may therefore interest the
pathologist and the physician in general to become familiar
with the essential features of the data thus obtained.

It is known that aside from a few exceptions the animal egg
can only develop if a spermatozoon enters into it. If no sperma-
tozoon enters, as a rule no segmentation of the egg takes place
and it perishes after a comparatively short period of time. The
questions which I tried to solve were the following: B}^ which
physico-chemical agencies does the spermatozoon cause the
egg to divide and to develop into an embryo; and second, which
changes does the egg undergo in this formative stimulation by
a spermatozoon ? Or in other words, what is the mechanism by
which the unfertilized egg is caused to segment and to develop ?

Nature of Formative Stimulation 129

Two ways were open to find an answer to this question:
first to try to cause the development of the unfertiUzed egg
with extracts from sperm. I have spent a good deal of time in
trying to succeed in this task, but met at first with only negative
results for the reason that I used at first only extracts from the
sperm of the same species of animals from which the eggs were
taken. Only recently have I found that the extract of sperm is
effective only if it is taken from a foreign species. We shall
return to this curious fact later on and show that it has a
bearing upon the problem of the immunity of our cells to the
lysins of our body.

The second way which could lead to a decision of the ques-
tion concerning the nature of formative stimulation lay in the
direction of artificial parthenogenesis, i.e., of the causation of
the development of the animal egg, not by extracts of sperm
but directly by physico-chemical agencies. This method of
procedure has a special advantage. Since in this case we know
the nature of the agencies we employ, it is easier to get an
insight into the mechanism by which they cause the development
of the egg; while if we work with extracts of sperm we are in
the dark as to the chemical character of the active substances.

We will begin with a description of the method of artificial
parthenogenesis in the egg of the Cahfornian sea-urchin,
since here this method has been worked out most completely.
It may be mentioned that in the eggs of many animals the effect
of the entrance of the spermatozoon manifests itself almost
instantly by a characteristic change, namely, the formation
of the so-called membrane of fertilization. Briefly stated this
process may possibly consist in the entrance of sea-water
between the surface film and the protoplasm of the egg, where-
by the former is lifted up from the protoplasm of the egg and
separated from it by a more or less wide, clear space. Figs. 1

130 The Mechanistic Conception of Life

and 2 (page 7) show these changes m the sea-urchin egg. Fig.
1 represents the unfertilized egg, Fig. 2 shows the same egg
after the entrance of the spermatozoon.

In 1905 I succeeded in finding a method by which it is
possible to call forth the formation of a membrane of fertiliza-
tion without apparent injury to the egg. This method consists
in putting the eggs for about two minutes (at a temperature
of 15°) into a mixture of 50 c.c. of sea-water-f3 c.c. of an n/10
lower monobasic fatty acid, e.g., acetic, propionic, butyric, or
valerianic acid. In this mixture no membrane formation
takes place; if, however, the eggs are transferred into normal
sea-water all the eggs form a perfect fertilization membrane.
The experiments showed that this process of membrane forma-
tion is the essential condition which causes the egg to develop.
In all these eggs in the course of the next hours after the mem-
brane formation those changes begin which lead to a cell-
division. If the temperature is very low not only cell-divisions
begin but the egg may develop into a swimming larva; it
reaches the so-called blastula stage. At room temperature,
however, the artificial production of a membrane in the egg
by fatty acid only calls forth a nuclear and possibly a cell-
division; after this the egg slowly begins to disintegrate.

We therefore see that the artificial membrane formation
by a fatty acid induces the developmental process, but that
at ordinary temperature the latter does not go far. In order
to cause a complete development, a second influence is needed,
as we shall see later.

Before we describe this second influence, another question
has to be settled, namely, how we know that the membrane
formation and not any other action of the acid, e.g., a catalytic,
is the formative stimulus in this case. The answer is, that if
we apply the acid but prevent the changes leading to a mem-
brane formation, divisions of the nucleus and of the cell do not
occur. On the other hand, we shall see later on that we can

Nature of Formative Stimulation 131

call forth the membrane formation not alone by fatty acids
but by a number of different agencies and that all these means
act as formative stimuli.

The causation of the membrane formation by a fatty acid
starts, therefore, the development in the sea-urchin egg, but
this development is abnormal and the egg is sickly and perishes
the more rapidly the higher the temperature. The question
arises, how can we inhibit this sickliness and grant a normal
development to the egg?

I found that two different means are at our disposal for
this purpose. The one which never fails consists in putting
the eggs about twenty minutes after the artificial membrane
formation into hypertonic sea-water (or any other hj-pertonic
solution, e.g., sugar solution), i.e., into sea-water or any other
solution the osmotic pressure of which has been rendered
50 per cent higher than that of the sea-water. In this solution
the eggs remain from twenty to sixty minutes — according to
the temperature and the concentration of hydroxj'1-ions in the
solution. If after this time the eggs are transferred into normal
sea-water they develop at room temperature in a wa}' similar to
the eggs which are fertilized by sperm. ^

The second method of causing the eggs to develop normally
at room temperature after the artificial causation of the mem-
brane formation consists in putting these eggs for three hours
in sea-water free from oxygen or into sea-water to which a
trace of KCN has been added. After the eggs are transferred
into normal sea- water they develop often but not always. This
method is, therefore, not quite as reliable as the other method
mentioned previously.

We see, therefore, that the formative stimulus in the artificial
activation of the egg of the sea-urchin consists of two phases,

1 The larvae originating from eggs fertilized by sperm live no longer than those
originating from eggs which develop parthenogenetically, if the larvae are not fed.
The feeding of these larvae is a tedious process and for this reason I have not
undertaken the task. Delage has, however, raised two such larvae until they were
sexually mature.

132 The Mechanistic Conception of Life

namely, first the artificial causation of the membrane formation
and second the subsequent short treatment of the egg with a
hypertonic solution; (or a longer treatment with an isotonic
solution free from oxygen or containing KCN).

We may add that these observations do not hold good for
the sea-urchin egg only. Similar observations were made on
the eggs of annelids (Polynoe) and of star-fish (Asterina).
In Polynoe and star-fish the artificial membrane production is
often sufficient to allow the eggs to develop into larvae. But
the number of eggs which reach the larval stage and the type
of segmentation is improved if the eggs are treated subsequently
with one of the above-mentioned methods, as R. Lillie found for
Asterias and I for Polynoe. The experiments on annelids and
star-fish, therefore, confirm the fact, that the calling forth of
the membrane is the essential feature in formative stimulation
and that the subsequent treatment of the eggs with a hyper-
tonic solution (or an isotonic solution free from oxygen) has
merely a corrective effect; it probably counteracts a secondary
detrimental effect connected with the membrane formation.

Ill

We will now try to gain some insight into the mechanism of
these two agencies. How can the fatty acid cause the formation
of a membrane? In order to get an answer to this question
we must find out whether there are other agencies which
act like fatty acids. It was noticed that all the agencies
which cause cytolysis also cause membrane formation, namely,
first the specifically cytolytic agencies like saponin, solanin,
digitalin, bile salts, and soaps. Experiments with these agencies,
especially with saponin, solanin, and digitalin, yielded a curious
result. If the unfertilized eggs are put into a weak solution of
saponin in sea-water we notice as the first effect on the eggs
the formation of a fertilization membrane. Then ensues a pause
of sometimes several minutes and after this pause a sudden

Nature of Formative Stimulation

133

cytolysis of the whole egg follows. If we take the eggs during
this pause (i.e., after the membrane is formed, but before the

d^--V-\--

FiG. 39

Fig. 40

Fig. 41

Fig. 42

Fig. 43

Figs. 39-43. — Membrane formation and subsequent cytolysis of the sea-
urchin egg in a weak solution of saponin in sea-water. Camera drawings from
nature. Fig. 39, unfertilized egg at the beginning of the experiment. In this
condition the egg was put into sea-water containing a small amount of saponin.
The following figvires show the changes it underwent in this solution. Fig. 40,
membrane formation under the influence of saponin, eight minutes later. If the
egg is taken out of the saponin-sea-water in this stage, washed and put into a
hypertonic solution for about one half-hour, it will develop into a larva, after it is
put back into normal sea-water. If, however, it is left in the saponin solution it
undergoes the rapid cytolysis represented in Figs. 41, 42, and 43. In the above
drawing of the egg, cytolysis began at G, Fig. 41, five minutes after the membrane
formation. The stages represented in Figs. 42 and 43 were reached a few minutes
later.

cytolysis of the egg occurs) out of the sea-water containing
saponin and free them from all traces of saponin by wash-
ing them repeatedly with sea-water, they behave as if the

134 The Mechanistic Conception of Life

membrane formation had been called forth by a fatty acid.
Such eggs begin to develop, but do not go beyond the first
nuclear division at room temperature. If, however, the eggs
are treated for half an hour Tvdth hypertonic sea-water they
can develop to normal plutei, i.e., larvae with skeletons.

The second group of cytolytic agencies is formed by the
specific fat solving hydrocarbons like amylen, benzol, toluol,
and in a much lesser degree chloroform, etc. Hert^vig had
alreadv observed that chloroform calls forth the membrane
formation and Herbst had seen the same effect brought about
by benzol and toluol. But these substances act so violently
that the membrane formation is followed almost immediately
by a cytolysis of the egg, and for this reason these authors could
not notice that the membrane formation was followed by the
development of the egg. I have, however, been able to con-
vince myself that if amylen or benzol are allowed to act only
for one moment and if the eggs are then quickly transferred
into normal sea-water a membrane formation can be produced in
some of them ^vithout subsequent c}i:olysis. If such eggs were
afterward treated ^^'ith hypertonic sea-water they developed
into larvae.

A further group of cytolytic agencies is ether or alcohols.
Cytolysis of the eggs by these agencies is also preceded by a
membrane formation. If the eggs are taken out from such
solutions immediately after membrane formation they can be
saved from cji^ol^-tic destruction (Figs. 44-47).

Bases can also call forth membrane formation, but their
action is rather slow and depends on the presence of free ox>'gen.
One gains the impression as if the alkali acted in this case only
as an accelerator of oxidations and as if a product of oxidation
was the proper cause for the membrane formation. The
membrane formation usually becomes manifest only if one
treats the eggs afterward for a short time with a hypertonic
solution ; such a treatment causing them to develop into larvae.

Nature of Formative Stimulation

135

An increase in temperature can also produce a cytolytic
effect. I have observed that at 34° or 35° the eggs of Strongylo-
centrotus purpuratus form often but not always a membrane

Fig. 44

Fig. 45

Fig. 46

Fig. 47

Figs. 44-47. — Membrane formation and subsequent cytolysis of the egg
under the influence of the addition of a minute quantity of salicylaldehyde to
sea-water. Camera drawings. Fig. 44, unfertilized egg at the beginning of the
experiment. Fig. 45, membrane formation in the salicylaldehyde-sea-water.
Fig. 46, beginning of the cytolysis. Fig. 47, cytolysis completed. The cytolyzed
egg has in this case an entirely different appearance from that of an egg cytolyzed
in saponin.

of fertilization. Such a temperature kills these eggs almost
instantly and consequently they are no longer able to develop
after this treatment. The eggs of the star-fish Asterias for-
hesii are, however, not killed so rapidly after the membrane

136 The Mechanistic Conception of Life

formation, and R. Lillie was able to show that such eggs can
develop into larvae if the membrane formation is called forth
by raising the temperature. Von Knaffl has shown that if a
high temperature acts for some time on these eggs they perish
by c}i:olysis and are transformed into ^'ghosts."

We have been able to convince ourselves, therefore, that all
the agencies which cause c>i:olysis also call forth the membrane
formation; while the agencies which do not call forth cytolysis
do not cause a membrane formation. We find in addition that
the cytolytic power of these agencies runs parallel with their
power of causing membrane formation.

From this we draw the inference that the membrane forma-
tion depends upon the c\i:ol3"sis of the surface layer of the egg.
We shall see later on that we must discriminate between a
cortical layer and the core of the unfertilized egg. This
superficial cortical layer of the egg is very thin. The essential
feature of the developmental stimulus consists in the cj^tolysis
of this cortical layer of the egg and this cytolysis is caused by
the spermatozoon.

We have already mentioned that the c5i:olysis which under-
lies the membrane formation causes the development of the
egg, but that the egg is as a rule sickly after this membrane
formation. To fix our ideas provisionally we assume that
through the membrane formation a substance is formed which
must be abolished or destroyed before the egg is able to develop
normally. If we permit the egg to begin its development while
it still contains this hypothetical detrimental substance in a
sufficient quantity it is sickly and dies prematurely. The
destruction of this hj^^othetical substance can be brought about
in two ways: first, by treating the egg for a short time wdth a
hypertonic solution. When I discovered this fact there was no
analogue known which allowed us to draw an inference concern-
ing the mode of action of a hypertonic solution. I succeeded

Nature of Formative Stimulation 137

in showing that such a solution is only effective in artificial
parthenogenesis if it contains free oxygen. If the hypertonic
solution is deprived of oxygen it remains without any effect. It
remains also inefficient if a trace of KCN is added to it. Since
KCN inhibits the oxidations in the cell it is obvious that the
hypertonic solution only acts by a modification of the process
of oxidation.

The second method of saving the life of the egg consists in
putting it after the membrane formation for about three hours
into sea-water which is practically free from oxygen, or contains
a trace of KCN whereby the oxidations in the egg are suppressed.
If these eggs are transferred after this time into normal sea-
water containing free oxygen they are often able to develop
normally.^

Thus far we have dealt only with artificial parthenogenesis.
We are now about to take up the causation of development by
a spermatozoon. Is the formative stimulation of the egg by
spermatozoon of the same character as that in artificial partheno-
genesis? This question can be answered in the affirmative.
It is possible to show that the spermatozoon also calls forth the
normal development of the egg by at least two substances and
that one of these substances acts like butyric acid or saponin
in artificial parthenogenesis, inasmuch as it causes the cytolysis
of the thin cortical layer of the egg; while the second substance
has an effect similar to that of the hypertonic solution. The
correctness for this view is proved by the fact that I succeeded
in separating these two effects of the spermatozoon.

If we wish to bring about a separation of these two agencies
in the spermatozoon we cannot use the spermatozoa of the
same species of sea-urchins from which the egg is taken; for
in this case the spermatozoon penetrates at once into the

1 A further discussion of the facts in this chapter is contained in the next paper
on " The Prevention of the Death of the Egg through the Act of Fertilization. "

138 The Mechanistic Conception of Life

protoplasm of the egg as soon as it comes in contact with it.
In this way almost simultaneously both substances of the
spermatozoon become effective, the cytolytic substance, which
causes the membrane formation, and the ''corrective" substance.
The experiments, however, result differently when we add the
spermatozoa of a foreign species, e.g., star-fish, to the egg of
the sea-urchin. Under ordinary conditions sperm of the star-fish
cannot cause the egg of the sea-urchin to develop; it becomes
effective, however, in sea- water which has been rendered a little
more alkaline through the addition of some NaHO. If 0.6 c.c.
n/10 NaHO is added to 50 c.c. of sea- water all the eggs of the
sea-urchin form fertilization membranes in such a mixture if
only a trace of living sperm of a star-fish {Asterias ochracea) is
added. It takes, however, some time, mostly from ten to fifty
minutes, until the living star-fish sperm brings about this effect;
while after the addition of sea-urchin sperm this result is
obtained in one minute.

If the sea-urchin eggs, all of which have formed membranes
upon the addition of living star-fish sperm, are put back into
normal sea-water and if we watch their further fate, we soon
notice that we are dealing with two groups of eggs. The one
group acts as if only one of the two agencies, namely, the
cytolytic one, had taken effect. These eggs show at room
temperature only the beginning of nuclear division and then
disintegrate, while at a lower temperature they may develop a
little farther. If we treat them, however, after the membrane
formation by star-fish sperm for from thirty to fifty minutes
with a hypertonic solution they all develop at room temperature
mostly into normal larvae. The other eggs develop without
any subsequent treatment with a hypertonic solution into
normal larvae.

What causes this difference in the behavior of both groups
of eggs ? A histological examination of these eggs decides this
point. My assistant, Mr. Elder, found that a spermatozoon

Nature of Formative Stimulation 139

had entered into those eggs which develop after the addition
of star-fish sperm without subsequent treatment \\'ith a hyper-
tonic solution into normal larvae; while the eggs which behave
as if only an artificial membrane formation had taken place
do not contain any spermatozoon.

This behavior of the eggs under the influence of foreign
sperm is comprehensible under the assumption that the
spermatozoon also causes the development of the egg through
two agencies; one of these agencies is a cytolytic substance,
a so-called lysin. This substance is probably situated at the
surface of the spermatozoon. This lysin only calls forth
the membrane formation and it acts like the but>Tic acid in
the method of artificial parthenogenesis. The second agency
seems to be more in the interior of the spermatozoon and it
exercises an influence similar to the short treatment of the egg
with a hypertonic solution. A normal development will result
only if the spermatozoon enters the egg since in this case only
both agencies, the cytolytic and the corrective, get into the
egg. We have already mentioned that foreign spermatozoa
penetrate only slowly into the egg. If a spermatozoon pene-
trates partially through the surface of the egg without entirely
penetrating into the protoplasm, enough of the lysin sticking
to the surface of the spermatozoon can be dissolved to cause
the cytolysis of the surface film of the egg which gives rise to the
membrane formation. Such eggs receive from the spermato-
zoon only the lysin, and they act therefore as if only the mem-
brane formation had been called forth in them by the treatment
with but>Tic acid, since in the formation of the membrane
the spermatozoon is thro\\'n out.

In the eggs of Strongylocentrotus purpuratus the mem-
brane formation can in general only be called forth by living
star-fish sperm while the extract of dead star-fish sperm in the
same concentration remains without effect. This fact is of im-
portance to disprove the possibilit^'that the membrane formation

140 The Mechanistic Conception of Life

in these experiments was caused by star-fish blood which was
added with the sperm.

That it is possible to separate a lysin from the sperm can
be proved for the eggs of another species of sea-urchins, namely,
Strongylocentrotus franciscanus, the eggs of which are very
sensitive to lysins. In these eggs it is possible to call forth the
membrane formation with a very dilute watery extract of the
sperm of star-fish which was killed by heating it to 60° C, or
more. Such eggs can be caused to develop into plutei by treat-
ing them after the membrane formation for a short time with
hypertonic sea-water. In the place of star-fish sperm the
sperm of other foreign species can be used. I have called forth
the membrane formation in the sea-urchin egg with the living
sperm of sharks or even roosters. Such eggs act as if only the
membrane formation with the aid of butyric acid had been
caused. At room temperature they begin to develop but they
are sickly and soon perish. If, however, they are treated
afterward with a hypertonic solution they develop into normal
plutei. In this case only the lysin entered the egg but not the
spermatozoon. It was, therefore, necessary to treat such eggs
subsequently with hypertonic sea-water in order to cause them
to undergo normal development at room temperature.

The idea that a lysin contained in the spermatozoon is the
formative stimulus which causes the egg to develop can be
tested experimentally. We know that blood contains lysins
which destroy the blood corpuscles of foreign species, while
it does not destroy the cells of the same species. If the idea is
correct that the spermatozoon acts upon the egg through a
lysin which calls forth the membrane formation it should be
possible to call forth the membrane formation in the unferti-
lized egg of the sea-urchin by foreign blood and such is the case.
I was able to show three years ago that the blood of certain

Nature of Formative Stimulation 141

worms, namely SipuncuUdes , can call forth the membrane
formation in the sea-urchin egg even if it is diluted a hundred
times or more with sea-water. This effect is not produced
in the eggs of every female sea-urchin but of only about 20
per cent of the females. I think the difference is caused by
differences in the permeability of the eggs for lysins; and the
degree of permeability seems to vary slightly for the eggs of
different females.

Instead of wasting time on an examination of the effects of
the blood of invertebrates^ I examined the effects of the blood
serum of warm-blooded animals. I succeeded in causing
membrane formation in the sea-urchin egg (purpuratus) with the
blood serum of cattle, sheep, pigs, and rabbits; and such eggs
behaved like the eggs which had been treated with the living
sperm of roosters or with butyric acid. They began to develop,
but they became sickly at room temperature and soon disinte-
grated. If, however, they were treated after the membrane
formation for a short time with a hypertonic solution they
developed at room temperature. The blood, therefore, contains
the lysin, but not the second substance necessary for the
full development. It is, therefore, necessary to substitute for
the action of the latter the treatment with a hypertonic solu-
tion if we wish to call forth a normal development of the egg
treated with serum.

The lysin of the blood is like that of the spermatozoon
relatively resistant to heat. The blood does not lose its power
to call forth membrane formation by heating it for some time to
60° or 65° C.^ It is curious that SrCl, and Babl2 increase the
membrane-forming power of the blood.

Not only blood but also the extracts of the organs of foreign

1 Since this was written the blood and the extracts of organs of a number of
invertebrates were used successfully to produce the membrane formation and
development of the egg of the sea-urchin.

2 The substance which causes membrane formation can be precipitated with
acetone (Loeb, P fingers Archit, CXXIV, 37, 1908).

142 The Mechanistic Conception of Life

species call forth membrane formation in the sea-urchin egg.
An extract of the coecum of the star-fish was very effective.

We have already mentioned the fact that the extract of
dead sperm of foreign species, e.g., of star-fish, certain mollusks,
certain worms, sharks, fowl, causes membrane formation in the
eggs of franciscanus. Experiments with the extract of dead
sperm of their owti species on the egg oi franciscanus or purpura-
tus fail; and the same is true for extracts from the tissues of
these species.^ What causes this difference in the action of
the lysins from their own and a foreign species? We know
that the lysins of our own blood do not hurt our cells while
they hurt the cells of foreign species. There exists, therefore,
an immunity of the eggs as well as of the rest of the cells against
the lysins of their own blood or tissues.

Our experiments throw a light upon the nature of this
immunity. If the lysins contained in our blood do not injure
our cells it can only be due to one of two facts: The lysins of
our own blood can either not diffuse into our cells, while they
can diffuse into the cells of foreign forms, or the cells contain
antibodies against the lysins of their own body, but not against
those of foreign species. As far as the lysins of the blood are
concerned we cannot decide between the two possibilities.
We can, however, reach a decision for the lysins of the sperma-
tozoa. The extract from the dead sperm of the sea-urchin is
ineffective for the eggs of the sea-urchin solely for the reason
that it cannot diffuse into the sea-urchin egg. For if the sea-
urchin lysin is carried by the living sea-urchin spermatozoon
(which acts as a motor for the lysin) into the sea-urchin egg,
the lysin is very active and probably more active than the
lysin of foreign species. If the sea-urchin egg contained an
antibody against the lysin of the sea-urchin sperm, the sea-
urchin sperm should not be able to call forth membrane
formation when it enters the sea-urchin egg.

1 If eggs were sensitized with SrCl., they could be caused to develop by
extracts from the coecvun of the sea-urchin, though this was trueonlj- exceptionally.

Nature of Formative Stimulation 143

We now understand the paradoxical fact, that by foreign
sperm we can cause membrane formation and development of
the sea-urchin egg in two different ways: namely, first by the
living sperm and second by the extract from the dead sperm;
while the sperm of the same species can only cause the eggs to
develop when it is alive. We now understand the fact alluded
to at the beginning of this chapter that my first experiments
to cause the development of the egg with extract of sperm did
not succeed, since I took it for granted that it was necessary
to use the extract of the sperm of the same species from which
the eggs were taken. The lysins in this case were not able to
diffuse into the egg.

The further unraveling of the nature of the immunity of the
egg-cell against the dissolved lysins of the blood and the tissues
of the same species depends upon the explanation of the fact
that the lysins of a species cannot diffuse into the egg of the
same species. It would be of interest if the same principle
formed for the immunity of the egg-cell would hold also for the
immunity of the body-cells against the lysins in the blood of
their o%vn species.

We may, therefore, say that the substance to which the
sperm owes its fertilizing power is a lysin and we may express
the suspicion that the lysins which we have thus far kno\\TL
only as protective agencies against bacteria play a great physio-
logical role in the mechanism of life phenomena. We may call
our theory of the developmental action of the spermatozoon
the lysin theory; thereby designating that the impulse for the
development of the egg is given by a lysin contained in the
spermatozoon. In artificial parthenogenesis we substitute for
the natural lysin a cytolytic substance. Aside from the lysin
action the normal development demands, as a rule (but not
always), a second corrective influence which in artificial par-
thenogenesis may be given by a hypertonic solution.

144 The Mechanistic Conception of Life

VII

The experiments on the artificial parthenogenesis of other
forms of animals show that the eggs of different animals possess
a varying tendency for parthenogenetic development. There
are eggs which can easily be induced to develop, so easily in
fact, that the experimenter cannot always be sure whether he
has caused the development by a substance applied by him or
whether some accidental condition of the experiment was
responsible. The eggs of the silkworm, of the star-fish, and
of certain worms belong to this class. In working with star-
fish eggs we can observe that occasionally a few of them develop
in normal sea-water, apparently without any demonstrable
cause, into swimming larvae. The eggs of the Californian
sea-urchin Strongylocentrotus purpuratus, on the other hand,
show not the slightest tendency to segment parthenogenetically ;
only the above-mentioned very specific and quantitative method
causes them to develop. For this reason I selected these eggs
for the investigation of the nature of the experimental stimulus,
since I could always be sure that the same stimulus gave the
same results; while, e.g., in the star-fish eggs we can never be
perfectly certain that some internal condition in the egg or some
overlooked unimportant secondary condition in the experiment
may not have caused the development. Although eggs with
such a strong tendency for spontaneous development as the
star-fish eggs are not the best material for the study of the
nature of the developmental stimulus yet we have to answer
the question how it happens that some eggs have a greater
tendency for parthenogenetic development than others.

Mathews has observed that by gently shaking the star-fish
eggs the number of eggs which develop "spontaneously" can
be increased. I made a similar observation in the eggs of
Amphitrite, an annelid. In the eggs of the sea-urchin nobody
has ever been able to obtain such a result. I am inclined to
believe that if a sea-urchin should be found, the eggs of which

Nature of Formative Stimulation 145

possess a greater tendency to develop spontaneously, it might
also be found that the number of eggs developing spontaneously
might be increased by agitation.

I tried whether it is possible to cause the eggs to cytolyze
also mechanically. If we exercise only a slight pressure with
a finger upon the ovary of a star-fish we find that many of
the eggs which afterward leave the ovary are cytolyzed. This
cytolysis is not caused by a bursting of the egg membrane; on
the contrary, in this case the cytolysis of the egg is, as usual,
preceded by the formation of a membrane of fertilization and
this membrane remains intact in the star-fish egg which is
caused to cytolyze by mechanical pressure. In the sea-urchin
egg, however, it is impossible to produce cytolysis by a slight
pressure.

The eggs of the star-fish which develop spontaneously first
form a membrane. Shaking causes a development of the star-
fish eggs only if the-shaking first leads to a membrane formation.
The greater tendency of the star-fish to develop spontaneously
is, therefore, due to the greater ease with which cytolysis can
be produced in this egg.

How can mere agitation or pressure call forth membrane
formation or cytolysis ? It seems to me that this fact is most
easily understood under the assumption, first suggested by
Biitschli, that the cytoplasm is an emulsion. It would then
follow that the membrane formation as well as the cytolysis
depends upon the destruction of this emulsion. We know that
different emulsions have a different degree of durability. The
eggs which upon gentle pressure undergo cytolysis have an
emulsion with a lesser degree of durability than the eggs in which
pressure has no such effect. Let us assume that membrane
formation as well as cytolysis depends upon the destruction
of an emulsion; in this case the membrane formation depends
upon the destruction of the emulsion in the cortical layer of the
egg only. The lysin of the egg destroys only the emulsion in

146 The Mechanistic Conception of Life

the cortical layer of the egg and thereby causes development.
The greater tendency of the eggs of certain animals for spon-
taneous parthenogenetic development thus depends upon the
relatively small degree of durability of the emulsion which
constitutes the cortical layer of the egg. But it should be
stated that this hypothesis is not essential for the lysin theory
of the activation of the egg.

VIII

The assumption that the membrane formation is only a
superficial cytolysis of the egg presupposes that the cortical
layer of the egg is different from the rest of the cytoplasm.
Biitschli had already reached such a conclusion on the basis
of histological observations. I am inclined to accept this view
on the basis of my observations on the action of cytolytic
agencies on the unfertilized egg. The action of these agencies
on the unfertilized egg always occurs in two stages which are
often separated from each other by a considerable interval of
time. The first stage is the cytolysis of the superficial layer;
the second stage is the cytolysis of the rest of the egg. This
is most obvious in experiments with weak solutions of saponin
or solanin in sea-water. In this case first a membrane forma-
tion occurs, then a pause ensues, often of several minutes, and
then cytolysis of the whole egg follows. If instead of saponin
benzol is used a pause can also be observed between membrane
formation and cytolysis of the whole egg but this pause is short,
often only a fraction of a second, or at the best a few seconds.

It can also be shown directly that there is a qualitative
difference between the cortical layer of the protoplasm and the
rest. If for the artificial membrane formation the lower fatty
acids, from the formic to the capronic acid, are used, cytolysis
of the cortical layer only is observed, i.e., membrane formation
follows but no cytolysis of the whole egg. If, however, the
higher fatty acids of the same series from the heptylic acid on

Nature of Formative Stimulation 147

and upward are applied the membrane formation is always
followed after a short pause by a cytolysis of the whole egg.

The lysins contained in the blood and the spermatozoon
act according to my present experience only upon the cortical
layer of the cytoplasm but not on the rest of the egg. We get a
membrane formation and development but not a cytolj'sis of
the whole egg.

If we go back to the idea of Butschli that protoplasm has
the structure of an emulsion we are led to the view that the
emulsion of the cortical layer of the egg differs from that of the
rest of the egg. There are certain cytolytic agencies which
destroy only the cortical layer; while all general cytolytic
agencies destroy the cortical layer as well as the rest of the egg.

How can the cytolysis of the cortical layer of the egg lead
to a membrane formation? Von Knaffl has expressed the
following view on this point: ''Protoplasm is rich in lipoids,
it is probably mainly an emulsion of these and of proteins.
Every physical and chemical agency which is able to liquefy
lipoids calls forth a cytolysis of the egg. The protein of the
egg can only swell or be dissolved if the state of the lipoids is
altered by chemical or physical means. The mechanism of
cytolysis consists in the liquefaction of the lipoids and the
subsequent swelling or liquefaction of proteins by absorption

of water This confirms Loeb's view that membrane

formation is caused by the liquefaction of lipoids."

We can accept this with a slight modification which refers
to the nature of the emulsion. An emulsion requires not only
two substances or phases as von Knaffl assumes but in addition
a third substance. The third substance serves the purpose of
making the emulsion more durable (Lord Rayleigh's theory).
The droplets of the emulsion are surrounded by a thin layer
of a substance which lessens the surface tension between the

148 The Mechanistic Conception of Life

droplet and the second phase of the emulsion. I assume that
only this stabilizing substance consists of lipoids, especially
cholesterin. The two other phases which constitute the
emulsion need not be lipoids. To fix our ideas provisionally
we may assume that these phases are first protein with little
water and second water with little protein. The existence of
these two phases has been established by Hardy. The emulsion
at the surface of the egg consists, according to this view, of a
system of protein droplets poor in water surrounded by a
stabilizing film of a lipoid (cholesterin or lecithin). If the sea-
urchin egg is treated with a lipoid solvent like benzol the
stabilizing film of cholesterin is dissolved and the protein drop-
let can absorb water. If we use saponin the film is destroyed
by the precipitation of cholesterin by saponin. The absorp-
tion of water leads to the lifting up of the surface film which
surrounds the egg.^

We wish to add a few remarks concerning the nature of this
surface film, although this does not belong to our problem.
According to Overton and Koeppe the surface film of cells
consists of lipoids, and according to Koeppe cytolysis is deter-
mined by the solution or tearing of this film. This view is not
tenable, since the surface film which is lifted off in the form of
the fertilization membrane does not consist of a lipoid but of
protein. This is suggested by the fact that this membrane is
absolutely insoluble in any lipoid solvent. Moreover, this
membrane remains perfectly intact when the egg is transformed

into a ''ghost."

Since we can cause the formation of a membrane of fertili-
zation in the star-fish egg by gentle agitation or mere pressure,

1 We have assumed here that the fertilization membrane is preformed in the
imfertilized egg and lifted up in consequence of the cytolysis of the layer beneath
it. As I stated in my book on Die chemische Entwicklungserregung des tierischen
Eies, it is also possible that the fertilization membrane is a membrane of precipita-
tion formed through the reaction of a constituent of the liquefied cortical layer
with a constituent of the sea-water (Ca?). It is immaterial for the problem
discussed in this paper which view we adopt temporarily.

Nature of Formative Stimulation 149

this membrane is apparently preformed in the unfertilized egg;
and if this be true the process of membrane formation must
consist in the lifting up of a preformed film from the underlying
cytoplasm through the entrance of sea-water between this film
and the cytoplasm. In this process the surface film undergoes
a change, since the spermatozoon can enter into the egg before
but not after the membrane formation. That merely a change
in the nature of the surface film prevents the entrance of a
spermatozoon into the egg after the membrane formation can
be proved by the fact that if we tear the membrane mechanically
a spermatozoon can penetrate into the egg. This proves that
the surface film, even if it is already preformed in the unferti-
lized egg, has different qualities or a different structure when it is
in close contact with the cytoplasm than when it is lifted off
from the cytoplasm by a layer of sea-water.

We have assumed that the membrane formation is deter-
mined by the action of a lysin or cytolytic agency upon the
cortical layer of the egg, whereby a protein in this layer absorbs
sea-water, and is thereby dissolved. This assumption leads to
two consequences: first it must be possible to show that the
fertilization membrane is permeable for sea-water and crystal-
loid substances but impermeable for colloids. The correctness
of this view can be proved. If we add to the sea-water, con-
taining eggs with a fertilization membrane, a certain quantity
of dissolved white of egg, tannin, or blood serum, the membrane
collapses and closes tight around the cytoplasm. The reason
is that almost all the liquid which existed between the mem-
brane and the cytoplasm diffused into the surrounding sea-
water. If the eggs are brought back into normal sea-water
(free from protein) it diffuses again into the space between
the membrane and the cytoplasm, and the fertilization mem-
brane resumes its former distance from the cji^oplasm and
its round shape. The membrane is, therefore, impermeable
for the colloids dissolved in sea-water.

150 The Mechanistic Conception of Life

If salts are added to the sea-water or if it is diluted by
the addition of distilled water the tension and the diameter
of the membrane do not change. This proves that the
membrane is permeable for salts, but not for colloids, and that
the lifting up of the fertilization membrane is determined by
the swelling and subsequent liquefaction of a colloid. This
dissolved colloid exercises an osmotic or colloidal pressure and
sea-water must diffuse from the outside imder the fertilization
membrane of the egg until the tension of this membrane equals
the osmotic or colloidal pressure of the dissolved colloid. This
explains also why it is that the fertilization membrane as a rule
assumes a spherical shape.

We now can understand why not in all cases of fertilization
a distinct fertilization membrane is formed. This may be due
to the fact that the degree of swelling of the colloid of the cortical
layer varies under different conditions.

We now possess a pretty complete picture of what happens
to the egg in the case of ''formative stimulation," i.e., when it
is caused to develop. Through a lysin or some other cytolytic
agency a certain substance of the cortical layer, presumably a
lipoid, is dissolved or precipitated, whereby a protein substance
of that layer is able to absorb water and swell. Formerly it
was thought that the spermatozoon caused the development
of the egg by carrying a ferment or enzyme into it and that this
ferment set the mechanism of development into action. Others
expressed the opinion that the entrance of the sperm nucleus or
of a centrosome was responsible for the development. We see,
however, that it suffices to call forth the artificial membrane
formation in the unfertilized sea-urchin egg, in order to observe
after two or three hours the formation of normal astrospheres or
spindles. This disproves the suggestion that the fusion of egg
and sperm nucleus is essential for the development of the egg.^

1 The fusion of the nuclei is of course of importance for the transmission of
paternal qualities.

Nature of Formative Stimulation 151

The ferment theory of the activation of the egg by the sperma-
tozoon is also wrong. If it were correct the velocity of develop-
ment should be accelerated if not doubled if two spermatozoa
enter the egg instead of one; or if fertilization by sperm and
artificial parthenogenesis are superposed in the same egg. But
this is not the case. In neither case is a shortening of the
time which elapses between two successive periods of segmen-
tation observed.^

The further development will be connected with the ques-
tion how can the cytolysis of the cortical layer of the egg lead
to its development? I may mention the possibility that the
cytolysis of the cortical layer facilitates the diffusion of oxygen
or of HO ions (bases) or other substances, necessary for the
development, into the egg.

XII

Let us summarize our results concerning the activation or
formative stimulation of the egg. For the normal development
at least two agencies are required: the one is the cytolysis of
the thin cortical layer of the egg. Any agency which causes
this cytolysis (without causing the cytolysis of the rest of the
egg) induces development. The spermatozoon as well as the
blood and the tissues contain a substance (lysin) which causes
only cytolysis of the cortical layer. The lower fatty acids,
from formic to capronic, cause only the cytolysis of the cortical
layer. Since most cytolytic agencies cause a cytolysis of the
whole egg they can be used only if the eggs are withdrawn
from their influence after the cortical layer is destroyed but
before the rest of the egg has undergone destruction.

Cytolysis of the cortical layer leads often but not always to
the formation of the membrane of fertilization.

Since all cytolytic substances are lipoid soluble (or destroy
lipoids) it is probable, but not proved, that the formative

1 Another reason is that the velocity of segmentation is purely determined
by the egg, no matter what is the nature of the spermatozoon.

152 The Mechanistic Conception of Life

stimulus in the activation of the egg consists in a liquefaction
or precipitation or some other modification of the lipoids of the
cortical layer of the egg which results in an imbibition or solu-
tion of a colloidal substance of the cortical layer. If the cyto-
plasm has the structure of an emulsion it is possible that lipoids
form the stabilizing envelope for the droplets which, according
to Lord Rayleigh, is necessary for the durability of the emulsion.
The cytolysis of the cortical layer of the egg causes its
development, but this development is often abnormal and comes
prematurely to a standstill. In order to induce a more normal
type of development a second agency is often required, the
mode of action of which is not yet so clearly understood as that
of the cytolytic agency, namely, a short treatment of the egg
with a hypertonic solution containing oxygen or a longer inhi-
bition of the development of the egg in normal sea-water which
is free from oxygen. The spermatozoon carries in addition
to the lysin a second substance into the egg, which acts
similarly to the hypertonic solution in our method of artificial
parthenogenesis .

VIII. THE PREVENTION OF THE DEATH OF THE
EGG THROUGH THE ACT OF FERTILIZATION

VIII

THE PREVENTION OF THE DEATH OF THE EGG
THROUGH THE ACT OF FERTILIZATION^

The unfertilized egg dies in a comparatively short time,
while the act of fertilization gives rise to a series of generations
which, theoretically at least, is of infinite duration. The act
of fertilization is, therefore, a life-saving act for the egg. The
question arises, in which way can the spermatozoon save the
life of the egg ?

If the ovaries of a star-fish are put into sea-water the eggs
are shed. They are generally immature, and in this condition
they cannot be fertilized, either by spermatozoa or by chemical
means. If they remain, however, for some time in sea-water,
all or a number of them gradually become mature; that is to say,
their nuclear mass is diminished by the extrusion of the two
so-called polar bodies. If immediately after the extrusion of
the polar bodies sperm is added, the eggs develop. They can
at that period likewise be caused to develop by certain chemical
and physical agencies.

Ten years ago I made the following observations. If the
eggs are not caused to develop by sperm or by physico-chemical
agencies, they perish very rapidly. At summer temperature
they may die in from four to six hours. The death of the egg
manifests itself morphologically in a darkening and blackening
of the otherwise clear egg. I found that the death of the egg
can be prevented by withdrawing the oxygen, or by diminish-
ing the rate of oxidations in the egg through the addition of a
trace of potassium cyanide. The life-saving action of lack of

1 Reprinted from the Harvey Lectures, 1911, by courtesy of Messrs. J. B.
Lippincott & Co.

155

156 The Mechanistic Conception of Life

oxygen can be shown in various ways. The maturation of the
egg itself depends upon oxidations. If the ox;y^gen is withheld
from the immature eggs, or if the oxidations in the immature
eggs are inhibited by potassium cyanide, the process of matura-
tion does not take place. Maturation is, therefore, also a
function of oxidations. The eggs of a female, which were
unripe, were divided into two groups: the one group remained
in sea-water in contact with oxygen; the other was put into
sea-water whose oxygen had been removed by a current of
hydrogen. The eggs of the second group remained alive; the
eggs of the first group perished in a few hours.

It is not even necessary to drive out the air by hydrogen;
the life of the unfertilized eggs can also be preserved by putting
large masses of them into a narrow glass tube which is sealed
at the bottom. The eggs sink to the bottom of the tube, and
those which are lying near the bottom receive no oxygen, since
the oxygen which diffuses from the air through the sea-water
is consumed by the uppermost layer of the eggs. On account
of this lack of ox^^gen the eggs at the bottom of the tube do not
mature and do not perish; hence by withholding ox^^gen from
the immature eggs their maturation and death are prevented.

If the oxygen is withheld from the eggs immediately after
they become mature their life is also saved. A. P. Mathews
has repeated this experiment and obtained the same results.
This proves that the death of the mature but unfertilized egg is
determined by oxidations. If these oxidations are inhibited
death does not occur. When these experiments were first
published they caused opposition. This opposition was based
on the fact that potassium cyanide was used in part of the
experiments. The objection was raised that the potassium
cyanide in these experiments acted only by preventing the
development of bacteria. The authors, however, who raised
this objection, overlooked the fact that lack of oxygen acts in
exactly the same w^ay as the addition of potassium cyanide,

Prevention of Death by Fertilization 157

and that it is entirely immaterial how lack of oxygen is pro-
duced, whether the oxygen is driven out by carefully purified
hydrogen or whether the eggs are put together in a large heap,
whereby only those lying on the surface of the heap receive
sufficient oxygen.

It is, however, easy to show directly that the above-
mentioned objection is incorrect. The eggs of the star-fish can
easily be put into sterilized sea-water without bacterial infec-
tion. The following experiment was tried. The eggs of a
star-fish were separated into three parts: one part was put
aseptically into a series of flasks with sterilized sea- water; the
second part was put into ordinary sea- water without asepsis;
the third part was put into sea-water to which a large quantity
of a putrid culture of bacteria had been added that had
developed on the dead eggs of the star-fish. It was found that
in all three cases the mature eggs died within the same period of
time. The sterilization of the eggs of the first group was
complete, as was shown by the fact that the eggs although dead
preserved their form for two months, while the dead eggs in the
normal sea-water were completely destroyed in a few days by
the action of the bacteria.

It is, therefore, certain that the death of the star-fish eggs
which are not fertilized is not caused by bacteria, but by the
process of oxidation in the egg. If no spermatozoon enters the
egg, or if the egg is not caused to develop by chemical treatment
it perishes very rapidly. If, however, a spermatozoon enters
the egg, the latter remains alive in spite of the fact that the
entrance of the spermatozoon causes an acceleration of the
oxidations in the egg. Warburg found for the eggs of the sea-
urchin at Naples that fertilization raises the velocity of the
process of oxidations to six times their original value, while
Wasteneys and I found that fertilization caused an increase
in the velocity of oxidations of Arbacia in Woods Hole to three
or four times the rate found in the unfertilized eggs.

158 The Mechanistic Conception of Life

How can we explain the fact that fertiHzation saves the life
of the egg ? Let us make the following preliminary assumption :
The unfertilized egg contains a poison, or some faulty combina-
tion of conditions which, if oxidations take place, causes the
death of the egg. In the unfertilized but mature egg oxidations
take place. The spermatozoon carries into the egg among
other substances something which protects the egg against the
fatal effects of the oxidations, and allows them even to carry on
oxidations at an increased rate without suffering. We might
say that the mature but unfertilized egg is comparable to an
anaerobic being for which oxidations are fatal, and that the
spermatozoon transforms the egg into an aerobic organism.

If we compare the eggs of different animals, w^e j&nd great
differences in regard to the above-mentioned conditions. The
eggs of certain annelids (Polynoe) also perish rapidly if they
become mature without being caused to develop, while the
eggs of the sea-urchin remain alive for a longer period of time
after they have become mature. The problem as to what
determines this difference has not yet been investigated.

The analysis of the process of fertilization by the spermato-
zoon shows that we must discriminate between two kinds of
effects, the hereditary effect and the activating or develop-
mental effect. The experiments on artificial parthenogenesis
make it very probable that the two groups of substances, the
substances which determine the heredity of paternal characters
and the substances which cause the egg to develop, are entirely
different. In this paper we are concerned only with the
second group of substances, namely, those which cause the
development of the egg.

The analysis of the causation of development of the egg
by a spermatozoon has shown that the latter acts by carrying
at least two substances or groups of substances into the egg.

Prevention of Death by Fertilization 159

The first of these substances causes the formation of a mem-
brane; the second serves the purpose of rendermg the egg
immune against the fatal action of oxidations.

I have shown in a number of papers that the essential
feature in the causation of the development of the egg is a

Fig. 48

Fig, 49

Fig. 50

Figs. 48-50. — Disintegration of the sea-urchin egg after the membrane for-
mation with butjTic acid or with foreign serum or with the extract of sperm or of
foreign cells; if the eggs are not treated after the membrane formation with a
hypertonic solution or a suppression of oxidations. A indicates the area where
the disintegration begins.

modification of its surface, which in many cases leads to the
formation of a membrane. If we cause membrane formation in
an unfertilized sea-urchin egg by artificial means, it begins to
develop, but very soon perishes; much more rapidly than
if it is not exposed to any treatment. I was able to show that

160 The Mechanistic Conception of Life

this rapid death of the sea-urchin egg, after artificial membrane
formation, can be prevented either by withdrawing the oxygen
from the egg or by inhibiting the oxidations in the egg by the
addition of a trace of potassium cyanide. The membrane
formation, therefore, causes the rapid death of the egg through
an acceleration of oxidations. Warburg has recently shown
that the artificial membrane formation in the unfertilized sea-
urchin egg causes the same increase in the rapidity of oxidations
as the entrance of a spermatozoon.

If we wish to cause the unfertilized eggs to develop to the
pluteus stage after the membrane formation, we have to subject
them to a second treatment. This may consist in putting them
about fifteen minutes after the membrane formation into a
hypertonic solution of a certain osmotic pressure (for instance,
50 c.c. of sea-water +8 c.c. n/2j NaCl) for one-half to one hour.
If, after this time, they are put back into normal sea- water they
no longer perish, but develop into normal larvae. I ventured
the hypothesis that the artificial membrane formation causes
a rapid increase of the oxidations in the egg and in this way
causes it to develop, but that these oxidations lead to the rapid
decay of the eggs at room temperature for the reason that the
egg contains a toxic substance, or a toxic complex of conditions,
which in the presence of oxidations leads to the rapid death of
the egg. The second treatment serves the purpose of rendering
the egg immune against the toxic effects of the oxidations.

If we first cause the artificial membrane formation in the
unfertilized egg by any of the various means which I have
described in former papers, and if we afterward treat the eggs
for a short time with a hypertonic solution, they develop after
being transferred to normal sea-water in the same way as if a
spermatozoon had entered them. They reach the successive
larval stages, develop into a blastula, gastrula, and pluteus, and
live as long as the larvae produced from eggs fertilized by a
spermatozoon.

Prevention of Death by Fertilization 161

Hence the physico-chemical activation of the unfertiUzed
egg of the sea-urchin consists of two kinds of treatment. The
one is a change in the surface of the egg which may or may
not result in the so-called formation of the membrane. This
change causes the acceleration of oxidations which in my opinion
is the essential feature of the process of fertilization. The
second treatment consists in abolishing the faulty condition
which makes oxidations fatal to the egg. This second treat-
ment may consist in exposing the eggs for about half an hour
or a little more to a hypertonic solution. We can substitute,
however, for this treatment another treatment, namely, the
deprivation of the egg for three hours from oxidations, either
by removing the oxygen from the solution or by adding a trace
of potassium cyanide to the solution. If, after the treatment
with the hypertonic solution for half an hour, or the treat-
ment with lack of oxygen for about three hours, the eggs are
put back into normal sea-water they can develop into normal
larvae.

We can show that the spermatozoon also causes the develop-
ment of the egg by two different agencies comparable in their
action to the agencies used in the methods of chemical fertiliza-
tion which we have just described.

For this purpose we must fertilize the egg of the sea-urchin
with a sperm different from its own, and for the following
reason: The spermatozoon of the sea-urchin enters so rapidly
into the egg that it is impossible to show that it causes the
development of the egg by two different substances.

If, however, we fertilize the sea-urchin egg with the sperm
of star-fish, it takes from ten to fifty minutes to cause the
membrane formation in the eggs, the reason bemg that the
star-fish sperm can penetrate only very slowly into the egg of
the sea-urchin.

It is, as a rule, not possible to fertilize the egg of the sea-
urchin by star-fish sperm in normal sea-water. But I found

162 The Mechanistic Conception of Life

eight years ago that if we make the sea- water slightly more
alkaline than it naturally is the eggs of the sea-urchin can be
fertilized by the sperm of the star-fish. For the fertilization
of the Californian sea-urchin, Strongylocentrotus purpuratus,
with the sperm of Asterias, the best results were obtained when
0.6 c.c. of n/10 NaOH were added to 50 c.c. of sea-water. In
this case, with active sperm, in about fifty minutes all the eggs
form the typical fertilization membrane.

If we watch the further development of sea-urchin eggs
fertilized by star-fish sperm we notice very soon that there are
two different kinds of eggs present; the one kind of eggs
behave as if they had been fertilized with sperm of their own
kind. That is to say, they segment regularly and develop into
swimming blastulae and gastrulae. The other kind of eggs,
however, act as if they had been treated with one of the agencies
which cause the membrane formation in the unfertilized sea-
urchin egg; these eggs begin to segment, but at room tempera-
ture they slowly perish by cytolysis. If, however, these eggs
are treated for half an hour with a hypertonic solution they
develop into larvae.

If we examine the eggs of a sea-urchin which have been
treated in an alkaline medium with the sperm of the star-fish,
we find that only a certain percentage of these eggs contain
the sperm nucleus, and this percentage seems to be identical
with the percentage of the eggs which develop into larvae.
As far as the other eggs are concerned, which form only a
membrane and then disintegrate, no sperm nucleus can be
found inside of them. I am inclined to draw the following
conclusion from these observations: The spermatozoon of the
star-fish penetrates very slowly through the surface film of
the sea-urchin egg. When it lingers for some time partially
imbedded in the surface film, one of the substances of the
spermatozoon is dissolved in the superficial layer of the egg
and causes the membrane formation. Through the act of

Prevention of Death by Fertilization 163

membrane formation the further entrance of the spermatozoon
mto the egg is prevented, since the fertilization membrane is
impermeable to sperm. This membrane formation leads to an
increase in the rate of oxidations and the beginning of the de-
velopment of the egg. The latter, however, contains a toxic
substance, or a faulty complex of conditions which has to be
abolished, before the oxidations necessary for development can
take place T\dthout the egg being destroyed by them. The
spermatozoon carries a second substance into the egg which
renders it immune against the fatal actions of the oxidations.
While the membrane-forming substance of the spermatozoon
may be situated at its surface, or superficially at least, the
second substance which transforms the egg from an anaerobe
into an aerobe must be situated in the interior of the sperma-
tozoon; since it can only act if the spermatozoon penetrates
into the egg. We see in these observations, concerning the
fertilization of the sea-urchin egg by the star-fish sperm, a
proof that the activation of the egg by the spermatozoon is
also caused by two different substances, one of which causes
the membrane formation, while the second renders the egg
immune against the toxic action of the oxidations. These
data support the assumption made above that the life-saving
action of the spermatozoon is due to the fact that it carries a
substance into the egg which renders the latter immmie against
the toxic action of oxidations.

Ill

Seven years ago I fomid that a number of agencies destroy
the fertilized egg much more rapidly than the unfertilized egg.
Thus, for instance, while in a pure sodium chloride solution the
unfertilized egg of the Californian sea-urchin may be kept alive
for several days, the fertilized egg is destroyed in such a solu-
tion in less than twenty-four hours. If we use slightly alka-
line solutions of sodium chloride the greater resistance of the

164 The Mechanistic Conception of Life

unfertilized egg is perhaps still more striking. The egg of the
Atlantic form of sea-urchin, Arbacia, is cytolyzed in a neutral
sodium chloride solution in a few hours, while the unfertilized egg
may live for a considerably longer period of time. When we
put fertilized and unfertilized eggs into hypertonic solutions,
we find also that the fertilized eggs suffer much more than the
unfertilized. What causes this difference of sensitiveness
between fertilized and unfertilized eggs? It is possible that
the permeabihty of the fertilized eggs is greater than that
of the unfertilized. While this is probably to some extent
true, yet it is not the whole explanation of the difference in the
behavior of the two kinds of eggs. I have been able to show for
a number of toxic solutions that their effect can be either com-
pletely annihilated or at least diminished if we take the oxygen
away from the solution. Thus, for instance, fertilized eggs of
the sea-urchin which perish very rapidly in pure salt solutions,
or a solution of sodium + calcium, or a solution of sodium +
barium, can be kept alive for a considerable period of time in
the same solutions if we either carefully remove the oxygen from
the solutions, or if we diminish the rate of the oxidations in the
eggs by adding a trace of sodium cyanide. In this case we have
the direct proof that solutions which are fatal for the egg when
the oxidations are allowed to go on are rendered completely, or
at least partially, harmless if we stop the oxidations in the egg.
Not only the toxic action of salt solutions upon the fertilized
egg could be inhibited by the suppression of the oxidations in the
egg, but also the toxic action of sugar solutions, or of solutions of
alcohol in the sea-water, or of a solution of chloral hydrate.^

These observations prove directly that in the presence
of certain toxic substances or mixtures of substances the
oxidations in the egg lead to its rapid destruction; while a
suppression of the oxidation saves the life of the egg.

1 Or of phenylurethane. This observation does not agree very well with the
assumption that the narcotic action of these substances is due to a retardation of
oxidation.

Prevention of Death by Fertilization 165

We therefore believe that we may conclude that the rapid
death of the unfertilized egg of certain species is caused by the
oxidations which take place in these eggs; and that the life-
saving action of the spermatozoon consists in the fact that the
latter, in addition to the membrane-forming substance, carries
a second substance, or group of substances, into the egg which
renders it immune against the harmful effect or consequences
of oxidations.

IX. THE ROLE OF SALTS IN THE PRESERVATION

OF LIFE

THE ROLE OF SALTS IN THE PRESERVATION OF LIFE^

Less is known of the role of the salts in the animal body
than of the role of the three other main food-stuffs, namely,
carbohydrates, fats, and proteins. As far as the latter are con-
cerned, we know at least that through oxidation they are
capable of furnishing heat and other forms of energy. The
neutral salts, however, are not oxidizable. Yet it seems to be a
fact that no animal can live on an ash-free diet indefinitely,
although no one can say why this should be so. We have a
point of attack for the investigation of the role of the salts in
the fact that the cells of our body live longest in a liquid which
contains the three salts, NaCl, KCl, and CaCU in a definite
proportion, namely, 100 molecules NaCl, 2.2 molecules KCl,
and 1 . 5 molecules of CaCl.^. This proportion is identical with
the proportion in which these salts are contained in sea-water;
but the concentration of the three salts is not the same in both
cases. It is about three times as high in the sea-water as in our
blood serum.

Biologists have long been aware of the fact that the ocean
has an incomparably richer fauna than fresh-water lakes or
streams and it is often assumed that life on our planet originated
in the ocean. The fact that the salts of Na, Ca, and K exist in
about the same proportion in our blood serum as in the ocean
has led some authors to the conclusion that our ancestors were
marine animals, and that, as a kind of inheritance, we still carry
diluted sea-water in our blood. Statements of this kind have

1 Carpenter lecture delivered at the Academy of Medicine of New York,
October 19. 1911. Reprinted from Science, N.S., XXXIV, No. 881, 653-65,
November 17, 1911, by courtesy of Professor James McKeen Cattell.

169

170 The Mechanistic Conception of Life

mainly a metaphorical value, but they serve to emphasize the
two facts, that the three salts, NaCl, KCl, and CaCl,, exist in
our blood in the same relative proportion as in the ocean and that
they seem to play an important role in the maintenance of life.
I intend to put before you a series of experiments which
seem to throw some light on the mechanism by which the
solutions surrounding living cells influence their duration of life.

In order to give a picture of the extent to which the life of
many animals depends upon the cooperation of the three salts I
may mention experiments made on a small marine crustacean,
Gammarus, of the Bay of San Francisco. If these animals are
suddenly thrown into distilled water, their respiration stops
(at a temperature of 20° C.) in about half an hour. If they are
put back immediately after the cessation of respiration into
sea- water, they can recuperate. If ten minutes or more are
allowed to elapse before bringing them back into the sea-water,
no recuperation is possible. Since in this case death is caused
obviously through the entrance of distilled water into the
tissues of the animals, one would expect that the deadly effect
of distilled water would be inhibited if enough cane sugar were
added to the distilled water to make the osmotic pressure of
the solution equal to that of the sea-water. If, however, the
animals are put into cane-sugar solution, the osmotic pressure
of which is equal to that of sea-water, the animals die just
about as rapidly as in distilled water. The same is true if the
osmotic pressure of the sugar solution is higher or lower than
that of the sea-water. The sugar solution is, therefore, about
as toxic for these animals as the distilled water, although in the
latter case water enters into the tissues of the animal, while
in the former case it does not.

If the sea-water is diluted with an equal quantity of distilled
water in one case, and of isotonic cane-sugar solution in the

Role of Salts in Preservation of Life 171

other, in both cases the duration of life is shortened by practi-
cally the same amount.

If the crustaceans are brought into a pure solution of NaCl,
of the same osmotic pressure as the sea-water, they also die in
about half an hour. If to this solution a little calcium chloride
be added in the proportion in which it is contained in the sea-
water the animals die as rapidly as without it. If, however,
both CaClo and KCl are added to the sodium chloride solution,
the animals can live for several days. The addition of KCl
alone to the NaCl prolongs their life but little.

If KCl and CaCl, are added to a cane-sugar solution isotonic
with sea-water, the animals die as quickly or more so than in the
pure cane-sugar solution.

If other salts be substituted for the three salts the animals
die. The only substitution possible is that of SrCl, for CaCl.,.
We find also that the proportion in which the three salts of
sodium, calcium, and potassium have to exist in the solution
cannot be altered to any extent. All this leads us to the con-
clusion, that in order to preserve the life of the crustacean
Gammarus, the solution must not only have a definite concentra-
tion or osmotic pressure but that this osmotic pressure must be
furnished by definite salts, namely, sodium chloride, calcium
chloride, and potassium chloride in the proportion in which
these three salts exist in the sea-water (and in the blood) ; this
fact could also be demonstrated for many other marine animals.
The relative tolerance of various cells and animals for abnormal
salt solutions is, however, not the same, a point which we shall

discuss later on.

Ill

What is the role of the salts in these cases ? The botanists
have always considered salt solutions as nutritive solutions.
It is a well-kno^\^l fact that plants require definite salts, e.g.,
nitrates and potassium salts, for their nutrition, and the ques-
tion now arises whether the three salts XaCl, KCl, and CaCl,,

172 The Mechanistic Conception of Life

which are needed for the preservation of animal life, play the
role of nutritive salts. Experiments which I made on a small
marine fish, Fundulus, proved beyond question that this is not
the case. If the young, newly hatched fish are put into a pure
solution of sodium chloride of the concentration in which this
salt is contained in sea-water, the animals very soon die. If,
however, KCl and CaCl, be added to the solution in the right
proportion, the animals can live indefinitely. These fish,
therefore, behave in this respect like Gammarus and the tissues
of the higher animals, but they differ from Gammarus and the
majority of marine animals inasmuch as the fish can live long,
and in some cases, indefinitely, in distilled and fresh water,
and certainly in a very dilute solution of sodium chloride.
From this fact I drew the conclusion that KCl and CaClg do not
act as nutritive substances for these animals, that they only
serve to render NaCl harmless if the concentration of the latter
salt is too high. I succeeded in showing that as long as the
sodium chloride solution is very dilute and does not exceed the
concentration of m/8, the addition of KCl and CaCl, is not
required. Only when the solution of NaCl has a concentration
above m/8 does it become harmful and require the addition of
KCl and CaCl.

The experiments on Fitndi^ZiiS, therefore, prove that a mixture
of NaCl + KCl +CaCl2 does not act as a nutritive solution, but
as a -protective solution. KCl and CaCl,^ are only necessary in
order to prevent the harmful effects which NaCl produces if
it is alone in solution and if its concentration is too high. We
are dealing, in other words, with a case of antagonistic salt
action ; an antagonism between NaCl on the one hand and KCl
and CaClo on the other. The discovery of antagonistic salt
action was made by Ringer, who found that there is a certain
antagonism between K and Ca in their action on the heart.
When he put the heart of a frog into a mixture of NaCl + KCl
he found that the contractions of the heart were not normal,

Role of Salts in Preservation of Life 173

but they were rendered normal b}' the addition of a Uttle CaClo.
A mixture of NaCl+CaCl. also caused abnormal contractions
of the heart, but these were rendered normal by the addition
of KCl. Ringer drew the conclusion that there existed an
antagonism between potassium and calcium, similar to that
which Schmiedeberg had found between different heart poisons,
e.g., atropin and muscarin. Biedermann had found that alka-
line salt solutions cause twitchings in the muscle and Ringer
found that the addition of Ca inhibited these twitchings.
Since these experiments were made many examples of the
antagonistic action of salts have become kno^^Ti.

It had generally been assumed that the antagonistic action
of two salts was based on the fact that each salt, when applied
singly, acted in the opposite way from that of its antagonist.
We shall see that in certain cases of antagonistic salt action at
least this view is not supported bj' fact.

What is the mechanism of antagonistic salt action? I
believe that an answer to this question lies in the following
observations on the eggs of Fundidus, If these eggs are put
immediately after fertilization into a pure sodium chloride
solution which is isotonic with sea-water, they usuall}' die
without forming an embryo. If, however, only a trace of a
calcium salt, or of any other salt with a bivalent metal (with
the exception of Hg, Cu, or Ag) is added to the m/2 NaCl
solution, the toxicity of the solution is diminished or even
abolished. Even salts which are very poisonous, namely, salts
of Ba, Zn, Pb, Ko, Ni, JVIn, and other bivalent metals, are able
to render the pure solution of sodium chloride harmless, at
least to the extent that the eggs can live long enough to form
an embryo. The fact that a substance as poisonous as Zn or
lead can render harmless a substance as indifferent as sodium
chloride seems so paradoxical that it demanded an explanation,

174 The Mechanistic Conception of Life

and this explanation casts light on the nature of the protective
or antagonistic action of salts. For the antagonistic action of
a salt of lead or zinc against the toxic action of sodium chloride
can only consist in the lead salt protecting the embryo against
the toxic action of the NaCl. But how is this protective
action possible?

We have mentioned that if we put the young fish, imme-
diately after hatching, into a pure m/2 solution of sodium
chloride the animals die very quickly, but that they live
indefinitely in the sodium chloride solution if we add both
CaCl., and KCl. How does it happen that for the embryo, as
long as it is in the egg shell, the addition of CaCl, to the NaCl
solution suffices, while if the fish is out of the shell the addition
of CaCl., alone is no longer sufficient and the addition of KCl
also becomes necessary ? Moreover, if we try to preserve the
life of the fish after it is taken out of the egg in an m/2 sodium
chloride solution by adding ZnSO^, or lead acetate, to the solu-
tion we find that the fish die even much more quickly than
w^ithout the addition.^

If we look for the cause of this difference our attention
is called to the fact that the fish, as long as it is in the egg, is
separated from the surrounding solution by the egg membrane.
This egg membrane possesses a small opening, the so-called
micropyle, through which the spermatozoon enters into the egg.
I have gained the impression that this micropyle is not closed
as tightly immediately after fertilization as later on, since the
newly fertilized egg is killed more rapidly by an m/2 solution of
NaCl than it is killed by the same solution one or two days
after fertihzation. One can imagine that the micropyle con-
tains a wad of a colloidal substance which is hardened gradually
to a leathery consistency if the egg remains in the sea-water.

1 R. Lillie has found that in the larvae of Arenicola a slight antagonism
between NaCl and ZnS04 can be proved. This shows that the general laws of
antagonism between two salts differ in degree but not in principle in the living
organism and the dead envelop of the fish egg.

Role of Salts in Preservation of Life 175

With the process of hardening, or tanning, it becomes more
impermeable for the NaCl solution. This process of hardening
is brought about apparently very rapidly if we add to the m/2
NaCl solution a trace of a salt of a bivalent metal like Ca, Sr,
Ba, Zn, Pb, Mn, Ko, and Xi, etc. It is also possible that similar
changes take place in the whole membrane. The process of
rendering the m/2 Na solution harmless for the embryo of the
fish, therefore, depends apparently upon the fact that the addi-
tion of the bivalent metals renders the micropyle or perhaps-
the whole membrane of the egg more impermeable to NaCl
than was the case before.

But these are only one part of the facts which throw a light
upon the protective or antagonistic action of salts. Further
data are furnished by experiments which I made together with
Professor Gies, also on the eggs of Fundulus. Gies and I were
able to show that not only are the bivalent metals able to render
the sodium chloride solution harmless, but that the reverse is
also the case, namely, that NaCl is required to render the solu-
tions of many of the bivalent metals, for instance ZnSO^, harm-
less. (That the SO^ ion has nothing to do with the result was-
shown before by experiments with Na^SO^.)

If the eggs of Fundulus are put immediately after fertiliza-
tion into distilled w^ater, a large percentage of the eggs develop^
often as many as 100 per cent, and the larvae and embryos
formed in the distilled water are able to hatch. If we add^
however, to 100 c.c. of distilled water that quantity of ZnSO^
which is required to render the NaCl solution harmless, all the
eggs are killed rapidly and not a single one is able to form an
embryo. If we add varjdng amounts of NaCl we find that,
beginning with, a certain concentration of NaCl, this salt
inhibits the toxic effects of ZnSO^ and many eggs are able to
form an embryo. This can be illustrated by the foUo^Wng
table :

176 The Mechanistic Conception of Life

TABLE I

Percentage of
Nature of the Solution the Eggs Forming

an Embryo

100 c.c. distilled water 49

100 c.c. distilled water+8 c.c. m/32 ZnS04 0

100 c.c. m/64 NaCl+S c.c. m/32 ZnSOi 0

100 c.c. m/32 NaCl+8 c.c. m/32 ZnSO, 3

100 c.c. m/16 NaCl+8 c.c. m/32 ZnS04 8

100 c.c. m/8 NaCl+8 c.c. m/32 ZnSOi 44

100 c.c. m/4 NaCl+8 c.c. m/32 ZnSO, 38

100 c.c. 3/8 NaCl+8 c.c. m/32 ZnS04 37

100 c.c. m/2 NaCl+8 c.c. m/32 ZnSO, 34

100 c.c. 5/8 NaCl+8 c.c. m/32 ZnSO^ 29

100 c.c. 6/8 NaCl+8 c.c. m/32 ZnSO^ '. 8

100 c.c. 7/8 NaCl+8 c.c. m/32 ZnSO, 6

100 c.c. m NaCl+8 c.c. m/32 ZnSOi 1

This table shows that the addition of NaCl, if its concentra-
tion exceeds a certain limit, namely, m/8, is able to render the
ZnSO^ in the solution comparatively harmless.

If we now assume that ZnSO^ renders the 5/8 m NaCl solu-
tion harmless by rendering the egg membrane comparatively
impermeable for NaCl we must also draw the opposite conclu-
sion, namely, that NaCl renders the egg membrane compara-
tively impermeable for ZnSO^. We therefore arrive at a new
conception of the mutual antagonism of two salts, namely,
that this antagonism depends, in this case at least, upon a
common, cooperative action of both salts on the egg membrane,
by which action this membrane becomes completely or com-
paratively impermeable for both salts. And from this we must
draw the further conclusion that the fact that each of these
salts, if it is alone in the solution, is toxic, is due to its com-
paratively rapid diffusion through the membrane, so that it
comes into direct contact with the protoplasm of the germ.

As long as we assumed that each of the two antagonistic
salts acted, if applied singly, in the opposite way from its
antagonist, it was impossible to understand these experiments

Role of Salts in Preservation of Life 177

or find an analogue for them in colloid chemistry. But if we
realize that NaCl alone is toxic because it is not able to render
the egg membrane impermeable; and that ZnSO^ if alone in
solution is toxic for the same reason; while both combined are
harmless (since for the 'banning" of the membrane the action
of the two salts is required) these experiments become clear.

We may, for the sake of completeness, still mention that
salts alone have such antagonistic effects; glycerin, urea, and
alcohol have no such action. On the other hand, ZnSO^ was
not only able to render NaCl harmless, but also LiCl, NH^Cl,
CaCl2, and others; and vice versa.

These experiments on the egg of Fundulus are theoretically
of importance, since they leave no doubt that in this case at
least the "antagonistic" action of salts consists in a modification
of the egg membrane by a combined action of two salts, whereby
the membrane becomes less permeable for both salts.

It is not easy to find examples of experiments in the litera-
ture which are equally unequivocal in regard to the character
of antagonistic salt action ; but I think that some recent experi-
ments by Osterhout satisfy this demand.

It has long been a question whether or not cells are at all
permeable for salts. Nobody denies that salts diffuse much
more slowly into the cells than water; but some authors,
especially Overton and Hoeber, deny categorically that they
can diffuse at all into the cells. Overton's view is based partly
on experiments on plasmolysis in the cells of plants. If the
cells of plants, for example, those of Spirogyra, are put into a
solution of NaCl or some other salt of sufficiently high osmotic
pressure, the volume of the contents of the cell decreases
through loss of water and the protoplasm retracts, especially
from corners of the rigid cellulose walls. Overton maintains
that this plasmolysis is permanent, and concludes from this

178 The Mechanistic Conception of Life

that only water but no salt can diffuse through the cell-wall;
since otherwise salt should gradually diffuse from the solution
into the cell, and through this increase in the osmotic pressure
of the cell the water should finally diffuse back into the cell and
restitute the normal volume of the cell. According to Overton
this does not happen.

Osterhout has recently shown that Overton's observations
were incomplete in a very essential point and that in reality
the plasmolysis, which occurs in this case when the cell is put
into the h^q^ertonic solution, disappears again in a time which
varies wdth the nature of the salt in solution. This stage of
reversion of plasmolysis had been overlooked by Overton. If
the cell, however, remains permanently in the h^^jertonic
sodium chloride solution, a shrinking of the contents of the
cell takes place again, which superficially resembles plasmolysis,
but which in reality has nothing to do with plasmolysis, but
is a phenomenon of death. That this second ''false plas-
molysis," as Osterhout calls it, has nothing to do with the hyper-
tonic character of the solution was proved by the fact that
hypotonic solutions of toxic substances may produce the same
phenomenon.

In one experiment which Osterhout describes,

a portion of a Spirogyra filament was plasmolyzed in .2 m CaCl2,
but not in . 195 m CaClo. A .29 m NaCl solution has approximately
the same osmotic pressure as a .2m CaClo solution. But on placing
another portion of the same Spirogyra filament in a . 29 m NaCl solu-
tion the expected plasmolysis does not occur and it is impossible to
plasmolyze the cells until they are placed in . 4 m NaCl.

Osterhout explains this difference in the concentration of the
two salts required for plasmolysis by the assumption that NaCl
diffuses more rapidly into the cell than CaCl.,, a conclusion which
I reached also on the basis of my earlier experiments on animals.
Osterhout's experiments also show that the antagonism of
NaCl and CaClg depends partly on the facts that the two salts

Role of Salts in Preservation of Life 179

inhibit each other from diffusing into the cells, and this conclu-
sion is based among others upon the following experiment.

By dividing a Spirogyra filament into several portions it was found
that it was plasmolyzed in .2 m CaCL and in .38 m XaCl, but neither
in .195 m CaCla nor in .375 m NaCl. On mixing 100 c.c. .375 m
XaCl with 10 c.c. . 195 m CaClo and placing other portions of the same
filament in it, prompt and very marked plasmolysis occurred.

The explanation for this observation lies in the fact that in
the mixture of NaCl and CaCl, the two salts render their
diffusion into the cell mutually more difficult. After a longer
period of time the plasmolyzed cells can expand again in a
mixture of NaCl and CaCl,, but that occurs much later than if
they are in the pure NaCl solution.

These experiments are the analogue of the observation on the
embryo of the eggs of Fundulus in which a pure solution of
ZnSO^ diffused rapidly through the membrane or micropyle,
while, if both salts were present, the diffusion was inhibited
or considerably retarded.

While the observations of Osterhout show that Overton was
not justified in using the experiments on plasmolj^sis to prove
that the neutral salts cannot diffuse into the cells, yet they do
not prove that these salts diffuse into the cell under normal con-
ditions. In Osterhout 's experiments the cells are in strongly
h^T^ertonic solutions and it does not follow that such solutions
act like isotonic, perfectly balanced solutions.

Wasteneys and I have recently sho^Mi that the toxic action
of acids upon Fundulus can be annihilated by salts. If we
add 0.5 c.c. n/10 butyTic acid to 100 c.c. of distilled water these
fish die in 2| hours or less. In solutions which contain 0.4 c.c.
or less acid they can live for a week or more. If we add,
however, 0 . 5 c.c. of butyric acid to 100 c.c. of solutions of NaCl
of various concentration, we find that above a certain limit

180

The Mechanistic Conception of Life

the NaCl can render the acid harmless. It is needless to
say that the NaCl used in these experiments was strictly
neutral and that the amount of acid present in the mixture of
acid and salt was measured. The following experiment may
serve as an example. Six fish were put into 500 c.c. of each
of the following seven mixtures, namely,

1) 100 c.c. HoO +0.5 c.c. n/10 butyric acid

2) 96 c.c. H.2O+ 4 c.c. m/2 NaCl+0.5 c.c. n/10 butyric acid

3) 94 c.c. H.2O+ 6 c.c. m/2 NaCl+0.5 c.c. n/10 butyric acid

4) 92 c.c. H0O+ 8 c.c. m/2 NaCl+0.5 c.c. n/10 butyric acid

5) 90 c.c. HoO+10 c.c. m/2 NaCl-f-0.5 c.c. n/10 butyric acid

6) 88 c.c. HoO+12 c.c. m/2 NaCH-0.5 c.c. n/10 butyric acid

7) 85 c.c. H2O+I5 c.c. m/2 NaCl+0.5 c.c. n/10 butyric acid

After certain intervals the number of surviving fish was
ascertained. The result is given in Table II.

TABLE II

After

2 hours
4 hours

1 day . .

2 days.

3 days.

4 days.

Number of Surviving Fish in 0.5 c.c. n/10 Butyric Acid

+ 0

4.0

6.0

8.0

10.0

12.0

c.c. m/2 NaCl in 100 c.c. of the Solution

15.0

, ,

6
5
5
5
5
5

If the amount of acid was increased, the amount of NaCl
also had to be increased to render the acid harmless. In order
to render 0.5 c.c. n/10 but^Tic acid pro 100 c.c. solution harm-
less, 10 c.c. m/2 NaCl had to be added; while 0.8 c.c. butyric
acid required 20 c.c. and 1.0 c.c. butyric acid required about
28 c.c. m/2 NaCl in 100 c.c. of the solution.

Not only butyric acid, but any kind of acid, could be
rendered harmless by neutral salts, e.g., HCl by NaCl.

Role of Salts in Preservation of Life 181

Wasteneys and I could show that the rate of the absorption
of acid by the fish is the same in solutions with and without salt.
This proves that the action of the salts consisted in this case
not in preventing the diffusion or absorption of the acid, but in
modifying the deleterious effect of the absorbed acid.

We can state a little more definitely the cause of death by
acid. If we put the fish into a weak acid solution in distilled
water just strong enough to kill the fish in from one to two hours
(e.g., 500 c.c. H,O+2.0 c.c. n/10 HCl), we notice that the acid
very soon makes the normally transparent epidermis of the
fish opaque, and a little later the epidermis falls off in pieces
and shreds. This, however, is probably not the direct cause
of the death, but I am inclined to assume that the fish die
from suffocation caused by a similar action of the acid upon
the gills.

The action of the acid upon the epidermis of the body as
well as upon the gills is prevented through the addition of
neutral salts.

It is well known that the action of acids upon proteins can
be inhibited by neutral salts. ^ Thus the internal friction of
certain protein solutions is increased by acids while the addition
of neutral salts inhibits this effect (Pauli). The swelling of
gelatin caused by acid is inhibited by salts (Procter). ^

It is possible that in the experiments with acid the fish is
killed in the following way. The acid causes certain proteins
in the surface layer of the epithelial cells of the gills and of the
skin to swell, whereby this surface layer becomes more perme-
able for the acid. The acid can now diffuse into the epithelial
cells and act on the protoplasm, whereby the cells are killed.
If salts are present in the right concentration, the combined
action of acid and salt causes a dehydration of the surface film

1 It seems that the first, experiments on the antagonism between acids and
salts were published by the author in PflUgers Archiv, Vol. LXXV, p. 308, 1899.

2 The beautiful osmometric experiments of R. Lillie should also be mentioned
in this connection.

182 The Mechanistic Conception of Life

of these cells, as it does in the experiments on gelatin or as in
the cases of tanning of hides by the combined action of acids
and salt solutions. This combined dehydrating or "tanning"
action of acid and salts on the surface of the epithelial cells of the
gills diminishes the permeability of this layer for the acids and
prevents them from diffusing into the cells and thus destroying
the protoplasm. In this way the gills are kept intact and the
life of the fish is saved.

As long as the amount of acid is small the amount absorbed
is not essentially diminished by the presence of salts ; but while
in the presence of salts the acid is consumed in the tanning
action of the surface layer of the cells, or is absorbed in this
layer; if no salt is present part of the acid diffuses into the
epithelial cells and kills the latter.

VII

We have thus far considered the cases of antagonism between
two electrolytes only. The case of the antagonism between three
electrolytes is a little more complicated.

We choose as an example the antagonism between NaCl,
KCl, and CaCl, — the antagonism which is most important in
life phenomena. If the mechanism of the antagonism between
NaCl, on the one hand, and KCl and CaCl,, on the other, is of
the same nature as that between NaCl and ZnSO^ in the case of
the eggs of Fundulus, it must be possible to show that not only
is NaCl toxic if it is alone in solution, and that it is rendered
harmless by the two other salts, but that the reverse is true
also. This can be proved in the case of KCl. To demonstrate
it, we have again to experiment on organisms which are, in wide
limits, independent of the osmotic pressure of the surrounding
solution since the concentration of the KCl in sea-water is very
low. The experiments were carried out by Mr. Wasteneys
and myself on Fundulus. The method consisted in putting six
fish, after washing them twice with distilled water, into 500 c.c.

Role of Salts in Preservation of Life 183

of the solution. It was ascertained from day to day how many
fish survived.

When the fish were put into pure solutions of KCl of the
concentration in which this salt is contained in the sea-water
(2.2 c.c. m/2 KCl in 100 c.c. of the solution) they died mostly
in less than two days. This is not due to the low concentration
of the KCl solution, which is only 1/50 of that of the sea-water,
since the fish can live indefinitely in a pure NaCl solution of the
same concentration as that in which the KCl exists in the sea-
water.

If we add to the toxic quantities of KCl increasing quantities
of NaCl, we find that as soon as the solution contains 17 or more
molecules of NaCl to one molecule of KCl, the toxic action of
KCl is considerably diminished, if not completely comiteracted.
The following table may serve as an example :

TABLE III

After Days

Number of S

URViviNG Fish in 2.2 c.

c. m/2 KCl IX 100 c.c.

H.O

m/lOO

m/20

m/8

m/4

3 m/8

m/2

NaCl

1
0

3
0

4
0

6
6
6
5
5
5
5
4

6
5
4
3
3
3
3
3

6
6
6
5
4
1
0

More accurate determinations showed that already a 3/16 m
NaCl solution renders the solution of 2 . 2 c.c. m/2 KCl in 100 c.c.
of the solution harmless.

It was next determined whether different concentrations of
KCl required different concentrations of NaCl. It was found
that the coefficient of antagonization KCl /NaCl has an approxi-
mately constant value, namely, about 1/17, as the following
table shows.

184 The Mechanistic Conception of Life

TABLE IV

Coefficient

of Antago-

nization,

0.6 c.c. m/2 KCl rendered harmless in 100 c.c. 3/64 m NaCl ... 1/16

0.7 c.c. m/2 KCl rendered harmless in 100 c.c. 4/64 m NaCl. . . 1/18

0.9 c.c. m/2 KCl rendered harmless in 100 c.c. 5/64 m NaCl. . . 1/17

1.0 c.c. m/2 KCl rendered harmless in 100 c.c. 5/64-6/64 m

NaCl 1/16-1/19

1 . 1 c.c. m/2 KCl rendered harmless in 100 c.c. 6/64 m NaCl. . . 1/17
1 .65 c.c. m/2 KCl rendered harmless in 100 c.c. 5/32 m NaCl. . . 1/19

2 . 2 c.c. m/2 KCl rendered harmless in 100 c.c. 6/32 m NaCl . . . 1/17
2.75 c.c. m/2 KCl rendered harmless in 100 c.c. 7/32 m NaCl. . . 1/16

3 . 3 c.c. m/2 KCl rendered harmless in 100 c.c. 9/32 m NaCl . . . 1/17

What happens if we vary this ratio ? If we add too little
NaCl to the KCl solution, namely, only 1 to 10 molecules NaCl
to 1 molecule of KCl, the solution becomes more harmful than
if KCl is alone in solution; if we add considerably more than 17
molecules NaCl, e.g., 50 molecules to one molecule of KCl, the
solution becomes toxic again; and the more so the higher the
concentration of NaCl. This indicates that the antagonistic
effect requires a rather definite ratio of the two salts. This
furnishes the reason why an m/2 solution of NaCl can, as a rule,
not be rendered completely harmless by the mere addition of
KCl, but that in addition CaClg is needed.

If we add to 100 c.c. m/2 NaCl enough KCl to make the
ratio KCl: NaCl = 1/17 we find that the antagonization of KCl:
NaCl becomes incomplete. If the amount of KCl in 100 c.c.
of the solution exceeds 2.2 c.c. m/2 KCl, antagonization is still
to some extent possible, but it becomes more incomplete the
higher the concentration of KCl. For this reason it is not
possible to render an m/2 solution of NaCl harmless by the
mere addition of KCl.

CaClg acts upon KCl similarly as does NaCl, but it
acts more powerfully; i.e., the coefficient of antagonization,
KCl/CaCla, is several hundred or a thousand times as great
as that of KCl/NaCl, as the following tables shows.

Role of Salts in Preservation of Life 185

TABLE V

Coefficient of Antago-
nization KCl/CaCL

1 . 1 c.c. m/2 KCl in 100 c.c. H.O require 0.1 m/lOO CaCl^. . . .550
1 .65 c.c. m/2 KCl in 100 c.c. H,0 require 0.5 m/100 CaCL. ... 165

2.2 c.c. m/2 KCl in 100 c.c. H.O require 0.3 m/100 CaCL. . . .366
2.75 c.c. m/2 KCl in 100 c.c. H^O require 1.0 m/100 CaCU. .. .137.5

3.3 c.c. m/2 KCl in 100 c.c. H^O require 1.6 m/100 CaCL. . . . 103

The coefficients are not as regular as in the case of antagoni-
zation of KCl by NaCl. This is due to the fact that the minimal
value of CaCl^ at which it renders the KCl harmless cannot
be determined as sharply as the limit for NaCl. Why is less
CaCl, required than NaCl ? We can only answer with a sug-
gestion first offered by T. B. Robertson, namely, that CaCIg
produces its protective effect through the formation of a com-
paratively insoluble compound (in this case on the gills or the
rest of the surface of the animal) while NaCl acts through the
formation of a compound which is more soluble. This view
is corroborated by the observation which we made, that Sr is
just as effective to antagonize KCl as CaCl,, but that Mg
is much less efficient. This would correspond with the well-
known fact that many strontium salts are just as insoluble, if
not more insoluble, than the calcium salts, while the magnesium
salts are often incomparably more soluble, for instance, in the
case of the sulphates. BaCl, antagonizes KCl also powerfully,
but, probably, in consequence of the fact that the substances
formed at the surface of the animal or the gills, diffuse slowly
into the cells, the fish do not remain alive as long if Ba is used as
if the more harmless Ca and Sr are used.

It is very remarkable that CaCl, renders harmless any given
concentration of KCl below 6.6 c.c. m/2 KCl in 100 c.c. of the
solution, but not above this limit. This limit is exactly the
same which we found in the case of antagonization of KCl by
NaCl. Even the combination of NaCl and CaCl, does not
permit us to render harmless more than 6.6 c.c. m/2 KCl in
100 c.c. of the solution.

186 The Mechanistic Conception of Life

If we try to render NaCl harmless by KCl and CaCL, we
find that CaCl, can antagonize even a 6/8 m and a 7/8 m solu-
tion of NaCl, while KCl ceases to show any antagonistic effect
if the NaCl solution exceeds m/2 or 5/8 m.

Experiments with pure CaClg solutions give the result that
this substance is harmless in a solution of that concentration in
which this salt is contained in the sea-water. Fundulus can
live indefinitely in a solution of 1.5 c.c. m/2 CaClg in 100 c.c.
Botanists have also found that weak solutions of CaCl^ are
comparatively little toxic. This gives us the impression that
the effect upon the surface film of protoplasm produced by
CaCla is especially important for the protection of the proto-
plasm. This conclusion receives an indirect support by the
well-known experiments of Herbst, who found that in sea-water
deprived of calcium the segmentation cells of a sea-urchin
embryo fall apart through the disintegration or liquefaction of
a film which surrounds the embryo and keeps the cells together.
If such eggs are brought back into solution containing calcium
the film is restored and the cells come into close contact again.

It is therefore not impossible that the mechanism of the
antagonism between KCl and NaCl is similar to that found
between NaCl and ZnSO^. It seems only due to the high con-
centration of the NaCl in the sea-water and in the blood that,
in addition to KCl and NaCl, CaClj is needed. But the case
is not so unequivocal as the previously mentioned cases of
antagonism between only two electrolytes.

VIII

It is necessary for our understanding of the life-preserving
action of salts that we do not depend merely on conclusions
drawn from experiments, but that we must be able to see
directly in which way abnormal salt solutions cause the death
of the cell. Such an opportunity is offered us through the

Role of Salts in Preservation of Life 187

observation of the eggs of the sea-urchin. If we put the ferti-
Hzed eggs of the sea-urchin into an abnormal salt solution, a
destruction of the cell gradually takes place. The destruction,
as a rule, begins on the surface of the protoplasm, and consists
very often in the formation and falling off of small granules or
droplets. This process gradually continues from the periphery
toward the center until the whole egg is disintegrated. For
different salt solutions the picture of the disintegration is a little
different, but sufficiently characteristic for a given solution, so
that if one become familiar with these pictures, one is able to
diagnose to some extent the nature of the solution from the way
in which the cell disintegrates.

This process of disintegration can be observed if the eggs
are put into a pure solution of sodium chloride, or in a mixture of
sodium chloride and calcium chloride, or in a mixture of sodium
chloride and potassium chloride. If, however, all three salts
are used in the proportion in which they occur in the sea-water
no disintegration takes place and the surface of the egg remains
perfectly smooth and normal. One gains the impression as if
the protoplasm of the egg were held together by a continuous
surface film of a definite texture. If we put the egg into an
abnormal solution this surface film is modified and changed,
and the change of the surface film is often followed by a gradual
process of disintegration of the rest of the cell.

These observations on the sea-urchin egg, therefore, sug-
gest the possibility that the combination of the three salts in
their definite proportion and concentration has the function of
forming a surface film of a definite structure or texture,
around the protoplasm of each cell, by which the protoplasm
is kept together, protected against and separated from the
surrounding media.

The previously mentioned observation of Herbst again
shows the important role of calcium in this process.

188 The Mechanistic Conception of Life

IX
The objection might be raised that the beneficial action of
the three salts could only be proved on marine animals or on
tissues of higher animals, which are said to be '^ adapted" to a
mixture of NaCl, KCl, and CaCl., in definite proportions.
Experiments on fresh-water organisms, for which ^'adaptation"
to a mixture of NaCl, KCl, and CaCl, in these definite propor-
tions cannot be claimed, show that this objection is not valid.
Ostwald worked with fresh-water crustaceans which he put into
mixtures of various salts. It was found that these animals
live longer in a mixture of NaCl + KCl +CaClo than in a solu-
tion of NaCl, or NaCl+KCl, or NaCl+CaCl, of the same
osmotic pressure.

Osterhout was able to show that the spores of a certain
variety of Vaucheria die in a pure 3/32 m solution of NaCl in
10 to 20 minutes, while they live in 100 c.c. 3/32 m NaCl + 1 c.c.
3/32 CaClg 2 to 4 weeks, and in 100 c.c. 3/32 m NaCl + 1 c.c.
3/32 m CaCl,+2.2 c.c. 3/32 m KCl 6 to 8 weeks. The reac-
tion of the solution was strictly neutral and the NaCl the purest
obtainable. The results remained the same after the NaCl
had been recrystallized six times. Experiments with Spirogyra
gave a similar result. The solutions were all 3/32 m. In
NaCl the Spirogyra died in 18 hours; in NaCl+KCl in two
days; in NaCl+KCl+CaCl, they lived 65 days. Osterhout
caused wheat grains to develop in such solutions and measured
the total length of the roots formed.

Total Length of
Nature of the Solution Roots after 40 Days

H2O 740 mm.

100 c.c. 3/25 NaCl 59 mm.

100 c.c. 3/25 NaCl+2.0 3/25 CaCL 254 mm.

100 c.c. 3/25 NaCl+2.0 3/25 CaCl2+2.2 3/25 m KCl 324 mm.

These cases, to which many other similar observations might
be added, prove that the life-preserving effect of the combina-
tion of NaCl + KCl +CaCL in definite proportions is not due

Role of Salts in Preservation of Life 189

to the fact that organisms are ''adapted" to this mixture but

to a specific protective effect of the combination of the three

salts upon the cells.

It seems, therefore, to be a general fact that wherever tissues
or animals require a medium of a comparatively high osmotic
pressure — like our tissues — their life lasts much longer in a
mixture of NaCl+KCl+CaCl, in the proportion in which these
salts exist in the blood and in the ocean, than in any other
osmotic solution, even a pure solution of NaCl. But the reader
has noticed that there are considerable differences in the resist-
ance of various organisms to abnormal solutions . While a marine
Gammarus dies in half an hour in an isotonic solution of NaCl or
cane sugar, red blood corpuscles or even the muscle of a frog can
be kept for a day or longer in such a solution (of course even
the muscle of a frog lives longer if the NaCl solution contains in
addition KCl or CaClg). What causes this difference?

Six years ago I found that the unfertilized eggs of the sea-
urchin (Strongylocentrotus purpuratus) can keep alive and
remain apparently intact in a pure neutral solution of CaCl,
or of NaCl for several days at a temperature of 15°, while the
fertilized eggs of the same female are killed in a pure neutral
solution of CaClo in a few hours. The same difference is found
for other salts also. What causes this difference? Several
authors have suggested that it is due to the fact that the
fertilized egg is more permeable to salts than the unfertilized
egg. But recent experiments by Warburg, which were con-
firmed and amplified by Harvey, make it doubtful whether the
salts which are not soluble in fats can enter the fertilized egg
at all. I believe that the explanation of the difference is much
more simple. The unfertilized egg is surrounded by a cortical
layer and this layer is destroyed or modified in the process of
fertilization. One result of this modification is the formation
of the fertilization membrane, for which I have been able to

190 The Mechanistic Conception of Life

show that it is readily permeable for salts. As long as the
cortical layer of the unfertilized egg is intact, it prevents the
surrounding salt solution from coming in contact with the proto-
plasm or at least it retards this process. If, however, the
cortical layer is destroyed by fertilization the surrounding
salt solution comes directly in contact with the protoplasm and
if the solution is abnormal it can cause the disintegration of the
surface layer of the protoplasm.

I am inclined to believe that differences in the resisting
power of various cells or organisms to abnormal salt solutions are
primarily due to differences in the constitution of the protective
envelopes of the animals or the cells. Microorganisms which
can live in strong organic acids or salt solutions of a high
concentration probably possess a surface layer which shuts off
their protoplasm from contact with the solution. For the
protoplasm of muscle the rather tough sarcolemma forms not
an absolute but nevertheless an effective wall against the
surrounding solution.

But aside from differences of this kind there are other condi-
tions which influence the degree of resistance of cells to various
solutions. I have foimd that the fertilized eggs of the sea-
urchin will live longer in abnormal salt solutions if the oxida-
tions in the egg are stopped, either by the withdrawal of oxygen
or the addition of KCN or NaCN. Warburg and Meyerhof
have dra^\Tl the conclusion that in a pure NaCl solution the
rate of oxidations of the egg of Strongylocentfotus is increased
and that it is this increase in the rate of oxidations which kills
the eggs. But this increase of oxidations cannot be observed
in the eggs of Arbacia when they are put into a pure NaCl solu-
tion and, moreover, lack of oxygen prolongs the life of the
fertilized egg just as well in solutions of NaCl-f-CaClg or of
NaCl+BaCl,, for which salts these authors do not claim that
they can raise the rate of oxidations of the egg. I am inclined
to believe that during or previously to cell-division, besides

Role of Salts in Preservation of Life 191

phenomena of streaming inside the cell, changes in the surface
film of the protoplasm occur, whereby this film is more easily
injured by the salts. If we suppress the oxidations we suppress
also the processes leading to cell-division and thereby retard
the deleterious action of the abnormal salt solution upon the
surface layer of the protoplasm of the egg.

If we now raise the question as to why salts are necessary
for the preservation of the life of the cell we can point to a
number of cases in which this answer seems clear. Each cell
may be considered a chemical factory, in which the work can
only go on in the proper way, if the diffusion of substances
through the cell-wall is restricted. This diffusion depends on
the nature of the surface layer of the cell. Overton and others
assume that this layer consists of a continuous membrane of
fat or lipoids. This assumption is not compatible with two
facts, namely, that water diffuses very rapidly into the cell,
and second, that life depends upon an exchange of water-
soluble and not of fat-soluble substances between the cells
and the surrounding liquid. The above-mentioned facts of the
antagonism between acids and salts suggest the idea that the
surface film of cells consists exclusively or essentially of certain
proteins.

The experiments mentioned in this paper indicate that the
role of salts in the preservation of life consists in the ^'tanning"
effect which they have upon the surface films of the cells,
whereby these films acquire those physical qualities of dura-
bility and comparative impermeability, without which the cell
cannot exist.

On this assumption we can understand that neutral salts
should be necessary for the preservation of life although they
do not furnish energy.

As far as the dynamical effects of salts are concerned it is

192 The Mechanistic Conception of Life

not impossible that some of them belong also to the type of
those mentioned in this paper. The fact that the addition of
calcium to an NaCl solution prevents the twitchings of the
muscle, which occur in the pure NaCl solution, suggests the
possibility that the CaCl, merely prevents or retards the diffu-
sion of NaCl through the sarcolemma. But other effects of
salts, e.g., the apparent dependence of contractility of the
muscle upon the presence of NaCl, or the role of PO^, do not
find their explanation in the facts discussed here.

X. EXPERIMENTAL STUDY OF THE INFLUENCE
OF ENVIRONMENT ON ANIMALS

EXPERIMENTAL STUDY OF THE INFLUENCE OF
ENVIRONMENT ON ANIMALS^

I. INTRODUCTORY REMARKS

What the biologist calls the natural environment of an
animal is from a physical point of view a rather rigid combina-
tion of definite forces. It is obvious that by a purposeful and
systematic variation of these and by the application of other
forces in the laboratory, results must be obtainable which do
not appear in the natural environment. This is the reasoning
underlying the modern development of the study of the effect
of environment upon animal life. It was perhaps not the least
important of Darwin's services to science that the boldness of
his conceptions gave to the experimental biologist courage to
enter upon the attempt of controlling at will the life phenomena
of animals, and of bringing about effects which cannot be
expected in nature.

The systematic physico-chemical analysis of the effect of
outside forces upon the form and reactions of animals is also
our only means of unraveling the mechanism of heredity
beyond the results which can be obtained by a mere cytological
investigation. The manner in which a germ cell can force upon
the adult certain characters will not be understood until we
succeed in varying and controlling hereditary characteristics;
and this can only be accomplished on the basis of a systematic
study of the effects of chemical and physical forces upon living
matter.

Owing to limitation of space this sketch is necessarily very
incomplete, and it must not be inferred that studies which are

1 Reprinted from Darwin and Modern Science (1909), by courtesy of Professor
A. C. Seward, of the University of Cambridge, England.

195

196 The Mechanistic Conception of Life

not mentioned here were considered to be of minor importance.
All the writer could hope to do was to bring together a few
instances of the experimental analysis of the effect of environ-
ment, which indicate the nature and extent of our control
over life phenomena and which also have some relation to the
work of Darwin. In the selection of these instances preference
is given to those problems which are not too technical for the
general reader.

The forces, the influence of which we shall discuss, are in
succession chemical agencies, temperature, light, and gravita-
tion. We shall also treat separately the effect of these forces
upon form and instinctive reactions.

II. the effects of chemical agencies

a) Heterogeneous hybridization. — It was held until recently
that hybridization is not possible except between closely related
species and that even among these a successful hybridization
cannot always be counted upon. This view was well supported
by experience. It is, for instance, well known that the majority
of marine animals lay their unfertilized eggs in the ocean and
that the males shed their sperm also into the sea-water. The
numerical excess of the spermatozoa over the ova in the sea-
water is the only guaranty that the eggs are fertilized, for the
spermatozoa are carried to the eggs by chance and are not
attracted by the latter. This statement is the result of numer-
ous experiments by various authors, and is contrary to common
beUef. As a rule all or the majority of individuals of a species in
a given region spawn on the same day, and when this occurs the
sea-water constitutes a veritable suspension of sperm. It has
recently been showni by experiment that in fresh sea-water the
sperm may live and retain its fertilizing power for several days.
It is thus unavoidable that at certain periods more than one kind
of spermatozoa is suspended in the sea-water and it is a matter
of surprise that the most heterogeneous hybridizations do not

Influence of Environment on Animals 197

constantly occur. The reason for this becomes obvious when
we bring together mature eggs and equally mature and active
sperm of different families. When this is done no egg is, as a
rule, fertilized. The eggs of a sea-urchin can be fertilized by
sperm of their o^\^l species, or, though in smaller numbers, by
the sperm of other species of sea-urchins, but not by the sperm
of other groups of echinoderms, e.g., star-fish, brittle-stars,
holothurians, or crinoids, and still less by the sperm of more
distant groups of animals. The consensus of opinion seemed
to be that the spermatozoon must enter the egg through a
narrow opening or canal, the so-called micropyle, and that the
micropyle allowed only the spermatozoa of the same or of a
closely related species to enter the egg.

It seemed to the writer that the cause of this limitation of
hybridization might be of another kind and that by a change
in the constitution of the sea-water it might be possible to
bring about heterogeneous hybridizations, which in normal
sea- water are impossible. This assumption proved correct.
Sea-water has a faintly alkaline reaction (in terms of the physi-
cal chemist its concentration of hydroxy 1 ions is about 10~^ n
at Pacific Grove, California, and about 10~^ n at Woods Hole,
Massachusetts). If we slightly raise the alkalinity of the sea-
water by adding to it a small but definite quantity of sodium
hydroxide or some other alkali, the eggs of the sea-urchin
can be fertilized with the sperm of widely different groups of
animals. In 1903 it was sho\\Ti that if we add from about 0.5
to 0.8 c.c. n/10 sodium hydroxide to 50 c.c. of sea-water, the
eggs of Strongylocentrotus purpuratus (a sea-urchin which is
found on the coast of California) can be fertilized in large
quantities by the sperm of various kinds of star-fish, brittle-
stars, and holothurians; while in normal sea-water or with
less sodium hydroxide not a single egg of the same female could
be fertilized with the star-fish sperm which proved effective
in the hyperalkaline sea-water. The sperm of the various forms

198 The Mechanistic Conception of Life

of star-fish was not equally effective for these hybridizations;
the sperm of Asterias ochracea and A. capitata gave the best
results, since it was possible to fertilize from 50 per cent to
100 per cent of the sea-urchin eggs, while the sperm of Pycno-
podia and Asterina fertilized only 10 or 2 per cent respectively
of the same eggs.

Godlewski used the same method for the hybridization of
the sea-urchin eggs with the sperm of a crinoid {Antedon
rosacea) . Kupelwieser afterward obtained results which seemed
to indicate the possibility of fertilizing the eggs of Strongylo-
centrotus with the sperm of a mollusk (Mytilus) . Recently, the
writer succeeded in fertilizing the eggs of Strongylocentrotus
franciscanus with the sperm of a mollusk — Chlorostoma. This
result could only be obtained in sea-water the alkalinity of
which had been increased (through the addition of 0.8 c.c. n/10
sodium hydroxide to 50 c.c. of sea- water). We thus see that
by increasing the alkalinity of the sea-water it is possible to
effect heterogeneous hybridizations which are at present impos-
sible in the natural environment of these animals.

It is, however, conceivable that in former periods of the
earth's history such heterogeneous hybridizations were possible.
It is known that in solutions like sea- water the degree of alkalin-
ity must increase when the amount of carbon dioxide in the
atmosphere is diminished. If it be true, as Arrhenius assumes,
that the Ice age was caused or preceded by a diminution in the
amount of carbon dioxide in the air, such a diminution must also
have resulted in an increase of the alkalinity of the sea-water,
and one result of such an increase must have been to render
possible heterogeneous hybridizations in the ocean which in the
present state of alkalinity are practically excluded.

But granted that such hybridizations were possible, would
they have influenced the character of the fauna ? In other
words, are the hybrids between sea-urchin and star-fish, or better
still, between sea-urchin and mollusks, capable of development.

Influence of Environment on Animals 199

and if so, what is their character ? In all cases of heterogeneous
hybridization the vitality of the egg or the embryo seems
weakened and it is still doubtful whether any heterogeneous
hybrid can reach maturity. The number of experiments is still
limited and this statement is therefore not yet final.

So far as the question of heredity is concerned, all the
experiments on heterogeneous hybridization of the egg of
the sea-urchin with the sperm of star-fish, brittle-stars, crinoids,
and moUusks have led to the same result, namely, that the larvae
have purely maternal characteristics and differ in no way from
the pure breed of the form from which the egg is taken. By way
of illustration it may be said that the larvae of the sea-urchin
reach on the third day or earlier (according to species and
temperature) the so-called pluteus stage, in which they possess
a typical skeleton (Fig. 10, p. 11); while neither the larvae
of the star-fish nor those of the molluskform a skeleton at the
corresponding stage. It was, therefore, a matter of some
interest to find out whether or not the larvae produced by the
fertilization of the sea-urchin egg with the sperm of star-fish
or mollusk would form the normal and typical pluteus skeleton.
This was invariably the case in the experiments of Godlewski,
Kupelwieser, Hagedoorn, and the writer. These hybrid larvae
were exclusively maternal in character.

It might be argued that in the case of heterogeneous hybridi-
zation the sperm nucleus does not fuse with the egg nucleus, and
that, therefore, the spermatozoon cannot transmit its hereditary
substances to the larvae. But these objections are refuted
by Godlewski's experiments, in which he showed definitely that
if the egg of the sea-urchin is fertilized with the sperm of a
crinoid the fusion of the egg nucleus and sperm nucleus takes
place in the normal way.

h) Artificial parthenogenesis. — Possibly in no other field of
biology has our ability to control life phenomena by outside
conditions been proved to such an extent as in the domain of

200 The Mechanistic Conception of Life

fertilization. The reader knows that the eggs of the over-
whelming majority of animals cannot develop miless a sperma-
tozoon enters them. In this case a living agency is the cause of
development and the problem arises whether it is possible to
accomplish the same result through the application of well-
knowTi physico-chemical agencies. This is, indeed, true, and
during the last ten years living larvae have been produced by
chemical agencies from the unfertilized eggs of sea-urchins,
star-fish, holothurians, and a number of annelids and mollusks;
in fact this holds true in regard to the eggs of practically all
forms of animals with which such experiments have been tried
long enough. In each form the method of procedure is some-
what different and a long series of experiments is often required
before the successful method is found.

The facts of artificial parthenogenesis, as the chemical
fertilization or activation of the egg is called, have, perhaps,
some bearing on the problem of evolution. If we wish to form
a mental image of the process of evolution we have to reckon
with the possibility that parthenogenetic propagation may have
preceded sexual reproduction. This suggests also the possi-
bility that at that period outside forces may have supplied the
conditions for the development of the egg which at present the
spermatozoon has to supply. For this, if for no other reason, a
brief consideration of the means of artificial parthenogenesis
may be of interest to the student of evolution.

It seemed necessary in these experiments to imitate as
completely as possible by chemical agencies the effects of the
spermatozoon upon the egg. When a spermatozoon enters the
egg of a sea-urchin or certain star-fish or annelids, the immediate
effect is a characteristic change of the surface of the egg, namely,
the formation of the so-called membrane of fertilization (Figs.
1 and 2). The writer found that we can produce this mem-
brane in the unfertilized egg by certain acids, especially the
monobasic acids of the fatty series, e.g., formic, acetic, propionic,

Influence of Environment on Animals 201

butyric, etc. Carbon dioxide is also very efficient in this
direction. It was also found that the higher acids are more
efficient than the lower ones, and it is possible that the sperma-
tozoon induces membrane formation by carrying into the egg a
higher fatty acid, namely oleic acid or one of its salts or esters.

The physico-chemical process which underlies the formation
of the membrane seems to be the cause of the development of the
egg. In all cases in which the unfertilized egg has been treated
in such a way as to cause it to form a membrane it begins to
develop. For the eggs of certain animals membrane formation
is all that is required to induce a complete development of the
unfertilized egg, e.g., in the star-fish and certain annelids.
For the eggs of other animals a second treatment is necessary.
Thus the unfertilized eggs of the sea-urchin Strongylocentrotus
purpuratus of the Californian coast begin to develop when
membrane formation has been induced by treatment ^vith a
fatty acid, e.g., butyric acid; but the development soon ceases
and the eggs perish in the early stages of segmentation, or after
the first nuclear division. But if we treat the same eggs after
membrane formation, for from thirty-five to fifty-five minutes
(at 15° C.) with sea-water the concentration (osmotic pressure)
of which has been raised through the addition of a definite
amount of some salt or sugar, the eggs will segment and develop
normally, when transferred back to normal sea-water. If care
is taken, practically all the eggs can be caused to develop into
plutei, the majority of which may be perfectly normal and may
live as long as larvae produced from eggs fertilized with sperm.

It is possible that the sea-urchin egg is injured in the process
of membrane formation. The nature of this injury became
clear when it was discovered that all the agencies which cause
hemolysis, i.e., the destruction of the red blood corpuscles, also
cause membrane formation in unfertilized eggs, e.g., fatty acids
or ether, alcohols or chloroform, etc., or saponin, solanin,
digitalin, bile salts, and alkali. It thus happens that the

202 The Mechanistic Conception of Life

phenomena of artificial parthenogenesis are linked together with
the phenomena of hemolysis which at present play so important
a role in the study of immunity. The difference between
cytolysis (or hemolysis) and fertilization seems to be this, that
the latter is caused by a superficial cytolysis of the egg, while if
the cytol>i:ic agencies have time to act on the whole egg the
latter is completely destroyed. If we put unfertilized eggs of a
sea-urchin into sea-water which contains a trace of saponin we
notice that, after a few minutes, all the eggs form the typical
membrane of fertilization. If the eggs are then taken out of the
saponin solution, freed from all traces of saponin by repeated
washing in normal sea-water, and transferred to the hypertonic
sea-Avater for from thirty-five to fifty-five minutes, they develop
into larvae. If, however, they are left in the sea-water con-
taining the saponin they undergo, a few minutes after membrane
formation, the disintegration knoTvn in pathology as cytolysis.
Membrane formation is, therefore, caused by a superficial or
incomplete cytolysis. It is possible that the subsequent treat-
ment of the egg with hypertonic sea-water is partly needed to
overcome the destructive effects of this cytolysis of the cortical
layer.

Many pathologists assume that hemolysis or cytolysis is
due to a liquefaction of certain fatty or fat-hke compounds,
the so-called lipoids, in the cell. If this view is correct, it
would be necessary to ascribe the fertilization of the egg to the
same process.

The analogy between hemolysis and fertilization throws,
possibly, some light on a curious observation. It is well knoAATi
that the blood corpuscles, as a rule, undergo cytolysis if injected
into the blood of an animal which belongs to a different family.
The writer found last year that the blood of mammals, e.g., the
rabbit, pig, and cattle, causes the egg of Strongylocentrotus to
form a typical fertilization membrane. If such eggs are after-
ward treated for a short period with hj^Dertonic sea-water they

Influence of Environment on Animals 203

develop into normal larvae (plutei). Some substance contained
in the blood causes, presumably, a superficial cytolysis of the
egg and thus starts its development.

We can also cause the development of the sea-urchin egg
without membrane formation. The early experiments of the
writer were done in this way and many experimenters still use
such methods. It is probable that in this case the mechanism
of fertilization is essentially the same as in the case where the
membrane formation is brought about, with this difference
only, that the cytolytic effect is less when no fertilization
membrane is formed. This inference is corroborated by
observations on the fertilization of the sea-urchin egg with ox
blood. It very frequently happens that not all of the eggs form
membranes in this process. Those eggs which form membranes
begin to develop, but perish if they are not treated ^vith h^'per-
tonic sea-water. Some of the other eggs, however, which do not
form membranes, develop directly into normal larvae without
any treatment with hypertonic sea-water, provided they are
exposed to the blood for only a few minutes. Presumably some
blood enters the eggs and causes the cj'tolytic effects in a less
degree than is necessary for membrane formation, but in a
sufficient degree to cause their development. The slightness
of the cytolytic effect allows the egg to develop without treat-
ment with hypertonic sea-water.

Since the entrance of the spermatozoon causes that degree
of cytolysis which leads to membrane formation, it is probable
that, in addition to the cytolytic or membrane-forming sub-
stance (presumably a higher fatty acid), it carries another
substance into the egg which counteracts the deleterious effects
underh'ing or following membrane formation.

The question may be raised whether the larvae produced
by artificial parthenogenesis can reach the mature stage. This
question may be answered in the affirmative, since Delage has
succeeded in raising several parthenogenetic sea-urchin larvae

204 The Mechanistic Conception of Life

beyond the metamorphosis into the adult stage and since in all
the experiments made by the writer the parthenogenetic plutei
lived as long as the plutei produced from fertilized eggs.

c) On the production of twins from one egg through a charige
in the chemical constitution of the sea-water. — The reader is
probably familiar with the fact that there exist two different
t>T)es of human twins. In the one type the twins differ as
much as two children of the same parents born at different
periods ; they may or may not have the same sex. In the second
type the twins have invariably the same sex and resemble each
other most closely. T^\'ins of the latter type are produced from
the same egg, while twins of the former type are produced
from two different eggs.

The experiments of Driesch and others have taught us that
twins originate from one egg in this manner, namely, that the
first two cells into which the egg divides after fertilization
become separated from each other. This separation can be
brought about by a change in the chemical constitution of
the sea-water. Herbst observed that if the fertilized eggs of the
sea-urchin are put into sea-water which is freed from calcium,
the cells into which the egg divides have a tendency to fall
apart. Driesch afterward noticed that eggs of the sea-urchin
treated \\dth sea-water which is free from lime have a tendency
to give rise to twins. The writer has recently found that twins
can be produced not only by the absence of lime, but also
through the absence of sodium or of potassium ; in other words,
through the absence of one or two of the three important
metals in the sea-water. There is, however, a second condition,
namely that the solution used for the production of twins must
have a neutral or at least not an alkaline reaction.

The procedure for the production of twins in the sea-urchin
egg consists simply in this: the eggs are fertiUzed as usual in
normal sea-water and then, after repeated washing in a neutral
solution of sodium chloride (of the concentration of the sea-

Influence of Environment on Animals 205

water), are placed in a neutral mixture of potassium chloride
and calcium chloride, or of sodium chloride and potassium
chloride, or of sodium chloride and calcium chloride, or of
sodium chloride and magnesium chloride. The eggs must
remain in this solution until half an hour or an hour after they
have reached the two-cell stage. They are then transferred
into normal sea- water and allowed to develop. From 50 to 90
per cent of the eggs of Strongylocentrotus purpuratus treated in
this manner may develop into twins. These twins may remain
separate or grow partially together and form double monsters,
or heal together so completely that only slight or even no
imperfections indicate that the individual started its career as a
pair of twins. It is also possible to control the tendency of such
twins to grow together by a change in the constitution of the
sea-water. If we use as a twin-producing solution a mixture
of sodium, magnesium, and potassium chlorides (in the propor-
tion in which these salts exist in the sea-water) the tendency
of the twins to grow together is much more pronounced than if
we use simply a mixture of sodium chloride and magnesium
chloride.

The mechanism of the origin of twins, as the result of alter-
ing the composition of the sea-water, is revealed by observation
of the first segmentation of the egg in these solutions. This
cell-division is modified in a way which leads to a separation
of the first two cells (see Figs. 55 to 57). If the egg is afterward
transferred back into normal sea-water, each of these two cells
develops into an independent embryo. Since normal sea-water
contains all three metals, sodium, calcium, and potassium, and
since it has besides an alkaline reaction, we perceive the reason
why twins are not normally produced from one egg. These
experiments suggest the possibility of a chemical cause for the
origin of twins from one egg or of double monstrosities in mam-
mals. If, for some reason, the liquids which surround the
human egg a short time before and after the first cell-division

206

The Mechanistic Conception of Life

are slightly acid, and at the same time lacking in one of the
three important metals, the conditions for the separation of

Fig. 51

Fig. 52

Fig. 53

Fig. 54

Figs. 51-54. — Cell-division in a sea-urchin egg, Strongylocentrotus purpura-
tus, in normal sea-water. This type of cell-division leads to the formation of one
embryo from an egg. M is the fertilization membrane, P a layer of colloidal
substance which seems to serve the purpose of keeping all the cells of an egg
together.

the first two cells and the formation of identical twins are
provided.

Fig. 55

Fig. 56

Fig. 57

Fig. 58

Figs. 55-58. — Cell-division in the egg of Strongylocentrotus purpuratus
which leads to the formation of twins. This cell-division can be observed if the
egg is put after fertilization into a neutral mixture of salts in which either KCl,
or CaClj, or NaCl is lacking.

In such a neutral solution the substance which forms the elastic layer (PM,
Fig. 51) is dissolved. During the segmentation the protoplasm of the egg spreads
imtil its long axis touches the fertilization membrane. The two daughter-cells
formed (Fig. 57) are separated from each other, instead of remaining connected
as in the normal cell-division (Fig. 53). If about one hour later the eggs are put
back into normal sea-water each of the two ceUs develops into an embryo (Fig. 58),
and the egg thus gives rise to two instead of to one embryo.

In conclusion it may be pointed out that the reverse result,
namely, the fusion of normally double organs, can also be
brought about experimentally through a change in the chemical
constitution of the sea-water. Stockard succeeded in causing

Influence of Environment on Animals 207

the two eyes of a fish embryo {Fundulus heteroclitus) to fuse into
a single cyclopean eye through the addition of magnesium
chloride to the sea-water. When he added about 6 grams of
magnesium chloride to 100 c.c. of sea-water and placed the
fertilized eggs in the mixture, about 50 per cent of the eggs gave
rise to one-eyed embryos.

When the embryos were studied the one-eyed condition was found
to result from the union or fusion of the ''Anlagen" of the two eyes.
Cases were observed which showed various degrees in this fusion; it
appeared as though the optic vesicles were formed too far forward
and ventral, so that their antero-ventro-median surfaces fused. This
produces one large optic cup, which in all cases gives more or less
evidence of its double nature.^

We have confined ourselves to a discussion of rather simple
effects of the change in the constitution of the sea-water upon
development. It is a priori obvious, however, that an unlimited
number of pathological variations might be produced by a
variation in the concentration and constitution of the sea-water,
and experience confirms this statement. As an example we
may mention the abnormalities observed by Herbst in the
development of sea-urchins through the addition of lithium to
sea-water. It is, however, as yet impossible to connect in a
rational way the effects produced in this and similar cases with
the cause which produced them; and it is also impossible to
define in a simple way the character of the change produced.

III. THE influence OF TEMPERATURE

a) The influence of temperature upon the density of pelagic
organisms and the duration of life. — It has often been noticed
by explorers who have had a chance to compare the faunas in
different climates that in the polar seas such species as thrive
at all in those regions occur, as a rule, in much greater density
than they do in the moderate or warmer regions of the ocean.
This refers to those members of the fauna which live at or near

1 Stockard, Archiv f. Entwicklungsmechanik, XXIII, 249, 1907.

208 The Mechanistic Conception of Life

the surface, since they alone lend themselves to a statistical
comparison. In his account of the Valdivia expedition, Chun^
calls especial attention to this quantitative difference in the
surface fauna and flora of different regions. "In the icy water
of the Antarctic, the temperature of which is below 0° C, we
find an astonishingly rich animal and plant life. The same
condition ^\^th which we are familiar in the Arctic seas is
repeated here, namely, that the quantity of plankton material
exceeds that of the temperate and warm seas." And again, in
regard to the pelagic fauna in the region of the Kerguelen
Islands, he states: ''The ocean is alive with transparent jelly
fish, Ctenophores (Bolina and Callianira) and of Siphonophore
colonies of the genus Agalma.''

The paradoxical character of this general observation lies
in the fact that a low temperature retards development, and
hence should be expected to have the opposite effect from that
mentioned by Chun. Recent investigations have led to the
conclusion that life phenomena are affected by temperature in
the same sense as the velocity of chemical reactions. In the
case of the latter van't Hoff had shown that a decrease in
temperature by 10 degrees reduces their velocity to one-half
or less, and the same has been found for the influence of tempera-
ture on the velocity of physiological processes. Thus Snyder
and T. B. Robertson found that the rate of heart beat in the
tortoise and in Daphnia is reduced to about one-half if the
temperature is lowered 10° C, and Maxwell, Keith Lucas, and
Snyder found the same influence of temperature for the rate
with which an impulse travels in the nerve. Peter observed
that the rate of development in a sea-urchin's egg is reduced
to less than one-half if the temperature (within certain limits)
is reduced by 10 degrees. The same effect of temperature upon
the rate of development holds for the egg of the frog, as Cohen
and Peter calculated from the experiments of 0. Hertwig.

1 Chun, Aus den Tie/en des Weltmeeres, p. 225, Jena, 1903.

Influence of Environment on Animals 209

The writer found the same temperature coefficient for the rate
of maturation of the egg of a mollusk (Lottia).

All these facts prove that the velocity of development of
animal life in Arctic regions, where the temperature is near the
freezing point of water, must be from two to three times smaller
than in regions where the temperature of the ocean is about
10° C, and from four to nine times smaller than in seas the
temperature of which is about 20° C. It is, therefore, exactly
the reverse of what we should expect when authors state that
the density of organisms at or near the surfac e of the ocean in
polar regions is greater than in more temperate regions.

The writer believes that this paradox finds its explanation
in experiments which he has recently made on the influence
of temperature on the duration of life of cold-blooded marine
animals. The experiments were made on the fertilized and
unfertilized eggs of the sea-urchin, and yielded the result that
for the lowering of temperature by 1° C, the duration of life
was about doubled. Lowering the temperature by 10 degrees
therefore prolongs the life of the organism 2^^, i.e., over a thou-
sand times, and a lowering by 20 degrees prolongs it about one
million times. Since this prolongation of life is far in excess
of the retardation of development through a lowering of
temperature, it is obvious that, in spite of the retardation
of development in Arctic seas, animal life must be denser
there than in temperate or tropical seas. The excessive
increase of the duration of life at the poles will necessitate
the simultaneous existence of more successive generations of
the same species in these regions than in the temperate or
tropical regions.^

The writer is inclined to believe that these results have
some bearing upon a problem which plays an important role
in theories of evolution, namely, the cause of natural death.

1 The high coeflEicient of temperature of the duration of life may possibly only
be found near the upper temperature limit for the life of organisms. But this is
suflScient for our theory.

210 The Mechanistic Conception of Life

It has been stated that the processes of differentiation and
development lead also to the natural death of the individual.^
If we express this in chemical terms it means that the chemical
processes which underlie development also determine natural
death. Physical chemistry has taught us to identify two chemi-
cal processes even if only certain of their features are knoA\Ti.
One of these means of identification is the temperature
coefficient. When two chemical processes are identical, their
velocity must be reduced by the same amount if the tempera-
ture is lowered to the same extent. The temperature coefficient
for the duration of life of cold-blooded organisms seems, how-
ever, to differ enormously from the temperature coefficient for
their rate of development. For a difference in temperature of
10° C, the duration of life is altered five hundred times as much
as the rate of development; and, for a change of 20° C, it is
altered more than a hundred thousand times as much. From
this we may conclude that, at least for the sea-urchin eggs and
embryo, the chemical processes which determine natural
death are certainly not identical with the processes which
underlie their development. T. B. Robertson has also arrived
at the conclusion, for quite different reasons, that the process
of senile decay is essentially different from that of growth and
development.

1 Weismann showed that infusorians or Tinicellular organisms m general are
immortal, while he assiimed that all the other organisms with the exception of their
germ-plasm are mortal. Leo Loeb first called attention to the fact that the
transplantation of a cancer can be repeated to an milimited series of generations,
and since it is the originally transplanted cancer-cell and the cells derived from it
by multiplication that survive, he pointed out that this proved that the principle
of immortality must also be granted to cancer-cells (1901). Later he generalized
this idea and stated that other cells may be considered immortal in the same
sense in which AVeismann claimed this for the unicellular organisms. One can
indeed well imagine that the same piece of skin might be transplanted through an
indefinite series of generations of mice and that such a transplanted piece might
outlive an indefinite number of generations of mice in exactly the same way as a
cancer cell does.

The natiiral death of the metazoa is perhaps a secondary phenomenon due
to the cessation of respiratory motions or of the heart beat. This leads to the
death of the cells through lack of oxygen. If respiratory motions and circulation
could be maintained indefinitely even the metazoa might be foimd to be immortal.

Influence of Environment on Animals 211

h) Changes in the color of butterflies produced through the
influence of temperature. — The experiments of Dorfmeister,
Weismann, Merrifield, Standfuss, and Fischer on seasonal
dimorphism and the aberration of color in butterflies have so
often been discussed in biological literature that a short refer-
ence to them will suffice. By seasonal dimorphism is meant
the fact that species may appear at different seasons of the year
in a somewhat different form or color. Vanessa prorsa is the
summer form, Vanessa levana the winter form of the same
species. By keeping the pupae of Vanessa prorsa several
weeks at a temperature of from 0° to 1° Weismann succeeded
in obtaining from the summer chrysalids specimens which
resembled the winter variety, Vanessa levana.

If we wish to get a clear understanding of the causes of
variation in the color and pattern of butterflies, we must direct
our attention to the experiments of Fischer, who worked with
more extreme temperatures than his predecessors, and foimd
that almost identical aberrations of color could be produced
by both extremely high and extremely low temperatures. This
can be seen clearly from the following tabulated results of his
observations. At the head of each column the temperature
to which Fischer submitted the pupae is given, and in the
vertical column below are found the varieties that were pro-
duced. In the vertical column A are given the normal forms:

0°to-20°C.

0° to

A (Normal

+ 35° to

+ 36° to

+ 42° to

+ 10° C.

Forms)

+ 37° C.

+ 41° C.

+ 46° C.

ichnusoides

polaris

urticae

ichnusa

polaris

ichnusoides

(nigrita)

antigone

fischeri

antigone

(iokaste)

testudo

dixeyi

poly-
chloros

erythrome-
las

dixeyi

testudo

hygiaea

artemis

antiopa

epione

artemis

hygiaea

elymi

wiskotti

cardui

wiskotti

elymi

klymene

merri-
fieldi

atalanta

merri-
fieldi

klymene

weismanni

porima

prorsa

porima

weismanm

212 The Mechanistic Conception of Life

The reader will notice that the aberrations produced at a
very low temperature (from 0° to —20° C.) are absolutely
identical with the aberrations produced by exposing the pupae
to extremely high temperatures (from 42° to 46° C.) • Moreover
the aberrations produced by a moderately low temperature
(from 0° to 10° C.) are dentical with the aberrations produced
by a moderately high temperature (from 36° to 41° C).

From these observations Fischer concludes that it is errone-
ous to speak of a specific effect of high and of low temperatures,
but that there must be a common cause for the aberration found
at the high as well as at the low temperature limits. This cause
he seems to find in the inhibiting effects of extreme temperatures
upon development.

If we try to analyze such results as Fischer's from a physico-
chemical point of view, we must realize that what we call life
consists of a series of chemical reactions, which are connected
in a catenary way; inasmuch as one reaction or group of reac-
tions (a) (e.g., hydrolyses) causes or furnishes the material for
a second reaction or group of reactions, (6) (e.g., oxidations).
We know that the temperature coefficient for physiological
processes varies slightly at various parts of the scale; as a rule
it is higher near 0° and lower near 30°. But we know also that
the temperature coefficients do not vary equally for the various
physiological processes. It is, therefore, to be expected that
the temperature coefficients for the group of reactions of the
type (a) will not be identical through the whole scale with the
temperature coefficients for the reactions of the type (6). If,
therefore, a certain substance is formed at the normal tempera-
ture of the animal in such quantities as are needed for the
catenary reaction (6), it is not to be expected that this same
perfect balance will be maintained for extremely high or
extremely low temperatures ; it is more probable that one group
of reactions will exceed the other and thus produce aberrant

Influence of Environment on Animals 213

chemical effects, which may miderlie the color aberrations
observed by Fischer and other experimenters.

It is important to notice that Fischer was also able to produce
aberrations through the application of narcotics. Wolfgang
Ostwald has produced experimentally, through variation of
temperature, dimorphism of form in Daphnia.

IV. THE EFFECTS OF LIGHT

At the present day nobody seriously questions the statement
that the action of light upon organisms is primarily one of a
chemical character. While this chemical action is of the
utmost importance for organisms, the nutrition of which
depends upon the action of chlorophyll, it becomes of less
importance for organisms devoid of chlorophyll. Nevertheless,
we find animals in which the formation of organs by regenera-
tion is not possible unless they are exposed to light. An
observation made by the writer on the regeneration of polyps
in a hydroid, Eudendrium racemosum, at Woods Hole, may be
mentioned as an instance of this. If the stem of this hydroid,
which is usually covered with polyps, is put into an aquarium the
polyps soon fall off. If the stems are kept in an aquarium
where light strikes them during the day, a regeneration of
numerous pol^'ps takes place in a few days. If, however, the
stems of Eudendrium are kept permanently in the dark, no
polyps are formed even after an interval of some weeks; but
they are formed in a few days after the same stems have been
transferred from the dark to the light. Diffused dayhght
suffices for this effect. Goldfarb, who repeated these experi-
ments, states that an exposure of comparatively short duration
is sufficient to produce this effect. It is possible that the light
favors the formation of substances which are a prerequisite
for the origin of polyps and their growth.

Of much greater significance than this observation are the

214 The Mechanistic Conception of Life

facts which show that a large number of animals assume, to
some extent, the color of the ground on which they are placed.
Pouchet found through experiments upon crustaceans and fish
that this influence of the ground on the color of animals is
produced through the medium of the eyes. If the eyes are
removed or the animals made blind in another way these
phenomena cease. The second general fact found by Pouchet
was that the variation in the color of the animal is brought
about through an action of the nerves on the pigment cells of
the skin; the nerve action being induced through the agency
of the eye.

The mechanism and the conditions for the change in colora-
tion were made clear through the beautiful investigations of
Keeble and Gamble, on the color change in crustaceans.
According to these authors the pigment cells can, as a rule, be
considered as consisting of a central bodj^ from which a system
of more or less complicated ramifications or processes spreads
out in all directions. As a rule, the center of the cell contains
one or more different pigments w^hich under the influence of
nerves can spread out separately or together into the ramifica-
tions. These phenomena of spreading and retraction of the
pigments into or from the ramifications of the pigment cells
form on the whole the basis for the color changes under the
influence of environment. Thus Keeble and Gamble observed
that Macromysis flexuosa appears transparent and colorless
or gray on sandy ground. On a dark ground their color
becomes darker. These animals have two pigments in their
chromatophores, a brown pigment and a whitish or yellow
pigment; the former is much more plentiful than the latter.
When the animal appears transparent all the pigment is con-
tained in the center of the cells, while the ramifications are free
from pigment. When the animal appears brown both pigments
are spread out into ramifications. In the condition of maximal
spreading the animals appear black.

Influence of Environment on Animals 215

This is a comparatively simple case. Much more compli-
cated conditions were found by Keeble and Gamble in other
crustaceans, e.g., in Hippolyte cranchii, but the influence of the
surroundings upon the coloration of this form was also satis-
factorily analyzed by these authors.

While many animals show transitory changes in color under
the influence of their surroundings, in a few cases permanent
changes can be produced. The best examples of this are those
which were observed by Poulton in the chrysalids of various
butterflies, especially the small tortoise-shell. These experi-
ments are so well known that a short reference to them will
suflfice. Poulton^ found that in gilt or white surroundings the
pupae became light colored and there was often an immense
development of the golden spots, ''so that in many cases the
whole surface of the pupae glittered with an apparent metallic
luster. So remarkable was the appearance that a physicist,
to whom I showed the chrysalids, suggested that I had played a
trick and had covered them with goldleaf." When black sur-
roundings were used, "the pupae were as a rule extremely dark,
with only the smallest trace, and often no trace at all, of the
golden spots which are so conspicuous in the lighter form."
The susceptibility of the animal to this influence of its surround-
ings was found to be greatest during a definite period when the
caterpillar undergoes the metamorphosis into the chrysalis stage.
As far as the writer is aware, no physico-chemical explanation,
except possibly Wieners' suggestion of color photography by
mechanical color adaptation, has ever been offered for the
results of the type of those observed by Poulton.

V. EFFECTS OF GRAVITATION

a) Experiments on the egg of the frog. — Gravitation can only
indirectly affect life phenomena; namely, when we have in a
cell two different non-miscible liquids (or a liquid and a solid)

1 Poulton, E. B., Colours of Animals ("International Scientific Series"),
London, 1890, p. 121.

216 The Mechanistic Conception of Life

of different specific gravity, so that a change in the position of
the cell or the organ may give results which can be traced to a
change in the position of the two substances. This is very
nicely illustrated by the frog's egg, which has two layers of very
viscous protoplasm one of which is black and one white. The
dark one occupies normally the upper position in the egg and
may therefore be assumed to possess a smaller specific gravity
than the white substance. When the egg is turned with the
white pole upward a tendency of the white protoplasm to flow
dowTi again manifests itself. It is, however, possible to prevent
or retard this rotation of the highly viscous protoplasm, by
compressing the eggs between horizontal glass plates. Such
compression experiments may lead to rather interesting results,
as 0. Schultze first pointed out. Pfliiger had already shown
that the first plane of division in a fertilized frog's egg is vertical
and Roux established the fact that the first plane of division is
identical with the plane of symmetry of the later embryo.
Schultze found that if the frog's egg is turned upside do\Mi at the
time of its first division and kept in this abnormal position,
through compression between two glass plates for about twenty
hours, a small number of eggs may give rise to twins. It is
possible, in this case, that the tendency of the black part of the
egg to rotate upward along the surface of the egg leads to a
separation of its first cells, such a separation leading to the
formation of twins.

T. H. Morgan made an interesting additional observation.
He destroyed one-half of the egg after the first segmentation
and found that the half which remained alive gave rise to only
one-half of an embryo, thus confirming an older observation of
Roux. When, however, Morgan put the egg upside do^\^l after
the destruction of one of the first two cells, and compressed the
eggs between two glass plates, the surviving half of the egg gave
rise to a perfect embryo of half-size (and not to a half-embryo
of normal size as before). Obviously in this case the tendency

Influence of Environment on Animals 217

of the protoplasm to flow back to its normal position was
partially successful and led to a partial or complete separation
of the living from the dead half; whereby the former was
enabled to form a whole embryo, which, of course, possessed
only half the size of an embryo originating from a whole egg.

b) Experiments on hydroids. — A striking influence of gravita-
tion can be observed in a hydroid, Antennularia antennina,
from the Bay of Naples. This hydroid consists of a long,
straight, main stem which grows vertically upward and
which has at regular intervals very fine and short bristle-
like lateral branches, on the upper side of which the polyps
grow. The main stem is negatively geotropic, i.e., its apex
continues to grow vertically upward when we put it obliquely
into the aquarium, while the roots grow vertically downward.
The writer observed that when the stem is put horizontally
into the water the short lateral branches on the lower side
give rise to an altogether different kind of organ, namely, to
roots, and these roots grow indefinitely in length and attach
themselves to solid bodies; while if the stem had remained in its
normal position no further growth would have occurred in the
lateral branches. From the upper side of the horizontal stem
new stems grow out, mostly directly from the original stem,
occasionally also from the short lateral branches. It is thus
possible to force upon this hydroid an arrangement of organs
which is altogether different from the hereditary arrangement.
The writer had called the change in the hereditary arrange-
ment of organs or the transformation of organs by external
forces heteromorphosis. We cannot now go any farther into
this subject, which should, however, prove of interest in rela-
tion to the problem of heredity.

If it is correct to apply inferences drawn from the observa-
tion on the frog's egg to the behavior of Antennularia, one might
conclude that the cells of Antennularia also contain non-miscible
substances of different specific gravity, and that wherever

218 The Mechanistic Conception of Life

the specifically lighter substance comes in contact with the
sea-water (or gets near the surface of the cell) the growth of a
stem is favored; while contact with the sea-water of the
specifically heavier of the substances, will favor the formation
of roots.

VI. the experimental control of animal instincts

a) Experiments on the mechanism of heliotropic reactions in
animals. — Since the instinctive reactions of animals are as
hereditary as their morphological character, a discussion of
experiments on the physico-chemical character of the instinctive
reactions of animals should not be entirely omitted from this
sketch. It is obvious that such experiments must begin with
the simplest type of instincts, if they are expected to lead to
any results; and it is also obvious that only such animals must
be selected for this purpose, the reactions of which are not
complicated by associative memory or, as it may preferably be
termed, associative hysteresis.

The simplest type of instincts is represented by the purpose-
ful motions of animals to or from a source of energy, e.g., light;
and it is with some of these that we intend to deal here. When
we expose winged aphides (after they have flown away from the
plant), or young caterpillars of Porthesia chrysorrhoea (when
they are aroused from their winter sleep), or marine or fresh-
water copepods and many other animals, to diffused daylight
falling in from a window, we notice a tendency among these
animals to move toward the source of light. If the animals are
naturallv sensitive, or if thev are rendered sensitive through the
agencies which we shall mention later, and if the light is strong
enough, they move toward the source of light in as straight a
line as the imperfections and peculiarities of their locomotor
apparatus will permit. It is also obvious that we are here
dealing with a forced reaction in which the animals have no
more choice in the direction of their motion than have the iron

Influence of Environment on Animals 219

filings in their arrangement in a magnetic field. This can be
proved very nicely in the case of starving caterpillars of Por-
thesia. The writer put such caterpillars into a glass tube the
axis of which was at right angles to the plane of the window:
the caterpillars went to the window side of the tube and
remained there, even if leaves of their food plant were put into
the tube directly behind them. Under such conditions the
animals actually died from starvation, the light preventing
them from turning to the food, which they eagerly ate when the
light allowed them to do so. One cannot say that these animals,
which we call positively heliotropic, are attracted by the light,
since it can be showTi that they go toward the source of light
even if in so doing they move from places of a higher to places
of a lower degree of illumination.

The writer has advanced the following theory of these
instinctive reactions. Animals of the type of those mentioned
are automatically oriented by the light in such a way that
symmetrical elements of their retina (or skin) are struck by
the rays of light at the same angle. In this case the intensity
of light is the same for both retinae or symmetrical parts of the
skin.

This automatic orientation is determined by two factors,
first a peculiar photosensitiveness of the retina (or skin), and
second a peculiar nervous connection between the retina and
the muscular apparatus. In symmetrically built heliotropic
animals in Avhich the symmetrical muscles participate equally
in locomotion, the symmetrical muscles work with equal energy
as long as the photochemical processes in both eyes are identi-
cal. If, however, one eye is struck by stronger light than the
other, the symmetrical muscles will work unequall}' and in
positively heliotropic animals those muscles will work with
greater energy which brings the plane of symmetry back into
the direction of the rays of light and the head toward the source
of light. As soon as both eyes are struck by the rays of light

220 The Mechanistic Conception of Life

at the same angle, there is no more reason for the animal to
deviate from this direction and it will move in a straight line.
All this holds good on the supposition that the animals are
exposed to only one source of light and are very sensitive to
light.

Additional proof for the correctness of this theory was
furnished through the experiments of G. H. Parker and S. J.
Holmes. The former worked on a butterfly, Vanessa antiope,
the latter on other arthropods. All the animals were in a
marked degree positively heliotropic. These authors found
that if one cornea is blackened in such an animal, it moves
continually in a circle when it is exposed to a source of light,
and in these motions the eye which is not covered with paint is
directed toward the center of the circle. The animal behaves,
therefore, as if the darkened eye were in the shade.

h) The production of positive heliotropism hy acids and other
means and the periodic depth migrations of pelagic animals. —
When we observe a dense mass of copepods collected from a
fresh-water pond, we notice that some have a tendency to go to
the light while others go in the opposite direction and many,
if not the majority, are indifferent to light. It is an easy matter
to make the negatively heliotropic or the indifferent copepods
almost instantly positively heliotropic by adding a small but
definite amount of carbon dioxide in the form of carbonated
water to the w^ater in which the animals are contained. If the
animals are contained in 50 c.c. of water it suffices to add from
3 to 6 c.c. of carbonated water to make all the copepods energeti-
cally positively heliotropic. This heliotropism lasts about
half an hour (probably until all the carbon dioxide has again
diffused into the air). Similar results may be obtained with
any other acid.

The same experiments may be made with another fresh-
water crustacean, namely Daphnia, with this difference, how-
ever, that it is as a rule necessary to lower the temperature of

Influence of Environment on Animals 221

the water also. If the water containing the Daphniae is cooled
and at the same time carbon dioxide added, the animals which
were before indifferent to light now become most strikingly
positively heliotropic. IMarine copepods can be made posi-
tively heliotropic by the lowering of the temperature alone, or
by a sudden increase in the concentration of the sea-water.

These data have a bearing upon the depth migrations of
pelagic animals, as was pointed out years ago by Theo. T. Groom
and the writer. It is well kno\Mi that many animals living
near the surface of the ocean or fresh-water lakes, have a
tendency to migrate upward toward evening and do'^Tiward
in the morning and during the day. These periodic motions
are determined to a large extent, if not exclusively, by the
heliotropism of these animals. Since the consumption of carbon
dioxide by the green plants ceases toward evening, the tension
of this gas in the water must rise and this must have the effect
of inducing positive heliotropism or increasing its intensity.
At the same time the temperature of the water near the surface
is lowered and this also increases the positive heliotropism in the
organisms.

The faint light from the sky is sufficient to cause animals
which are in a high degree positively heliotropic to move
vertically upward toward the light, as experiments with such
pelagic animals, e.g., copepods, have shoAMi. When, in the
morning, the absorption of carbon dioxide by the green algae
begins again and the temperature of the water rises, the animals
lose their positive heliotropism, and slowly sink doAMi or become
negatively heliotropic and migrate actively do^^•nward.

These experiments have also a bearing upon the problem
of the inheritance of instincts. The character which is trans-
mitted in this case is not the tendency to migrate periodically
upward and do\\Tiward, but the positive heliotropism. The
tendency to migrate is the outcome of the fact that periodically
varjdng external conditions induce a periodic change in the

222 The Mechanistic Conception of Life

sense and intensity of the heliotropism of these animals. It
is of course immaterial for the result, whether the carbon
dioxide or any other acid diffuse into the animal from the out-
side or whether they are produced inside in the tissue-cells of
the animals. Davenport and Cannon found that Daphniae,
which at the beginning of the experiment react sluggishly to
light, react much more quickly after they have been made to
go to the light a few times. The writer is inclined to attribute
this result to the effect of acids, e.g., carbon dioxide, produced
in the animals themselves in consequence of their motion.
A similar effect of the acids was shown by A. D. Waller in the
case of the response of a nerve to stimuli.

The writer observed many years ago that winged male and
female ants are positively heliotropic and that their heliotropic
sensitiveness increases and reaches its maximum toward the
period of nuptial flight. Since the workers show no heliotropism
it looks as if an internal secretion from the sexual glands were
the cause of their heliotropic sensitiveness. V. Kellogg has
observed that bees also become intensely positively heliotropic
at the period of their wedding flight, in fact so much so that by
letting light fall into the observation hive from above, the bees
are prevented from leaving the hive through the exit at the
lower end.

We notice also the reverse phenomenon, namely, that
chemical changes produced in the animal destroy its heli-
otropism. The caterpillars of Porthesia chrysorrhoea are very
strongly positively heliotropic when they are first aroused from
their winter sleep. This heliotropic sensitiveness lasts only as
long as they are not fed. If they are kept permanently without
food they remain permanently positively heliotropic until
they die from starvation. It is to be inferred that as soon as
these animals take up food, the formation of a substance or
substances in their bodies takes place, diminishing or annihilat-
ing their heliotropic sensitiveness.

Influence of Environment on Animals 223

The heliotropism of animals is identical ^\^th the heli-
otropism of plants. The writer has sho\\Ti that the experiments
on the effect of acids on the heliotropism of copepods can be
repeated with the same result in Volvox. It is, therefore,
erroneous to try to explain these heliotropic reactions of animals
on the basis of peculiarities (e.g., vision) which are not found
in plants.

We may briefly discuss the question of the transmission,
through the sex-cells of such instincts as are based upon heli-
otropism. This problem reduces itself simply to that of the
method whereby the gametes transmit heliotropism to the larvae
or to the adult. The writer has expressed the idea that all
that is necessary for this transmission is the presence of a pho-
tosensitive substance in the eyes (or in the skin) of the animal.
For the transmission of this the gametes need not contain
anything more than a catalyzer or ferment for the synthesis
of the photosensitive substance in the body of the animal.
What has been said in regard to animal heliotropism might,
if space permitted, be extended, mutatis mutandis, to geotropism
and stereotropism.

c) The tropic reactions of certain tissue-cells and the
morphogenetic effects of these reactions. — Since plant-cells show
heliotropic reactions identical with those of animals, it is not
surprising that certain tissue-cells also show reactions which
belong to the class of tropisms. These reactions of tissue-cells
are of special interest by reason of their bearing upon the
inheritance of morphological characters. An example of this
is found in the tiger-like marking of the yolk sac of the embryo
of Fundulus and in the marking of the young fish itself. The
writer found that the former is entirely, and the latter at least
in part, due to the creeping of the chromatophores upon
the blood-vessels. The chromatophores are at first scattered
irregularly over the yolk sac and show their characteristic
ramifications (Fig. 36, p. 106). There is at that time no definite

224 The Mechanistic Conception of Life

relation between blood-vessels and chromatophores. As soon
as a ramification of a chromatophore comes in contact with
a blood-vessel the whole mass of the chromatophore creeps
gradually on the blood-vessel (Fig. 37) and forms a complete
sheath aromid the vessel, until finally all the chromatophores
form a sheath around the vessels and no more pigment cells
are found in the meshes between the vessels (Fig. 38). Nobody
who has not actually watched the process of the creeping of
the chromatophores upon the blood-vessels would anticipate
that the tiger-like coloration of the yolk sac in the later
stages of development was brought about in this way. Similar
facts can be observed in regard to the first marking of
the embryo itself. The writer is inclined to believe that we
are here dealing with a case of chemotropism, and that the
oxygen of the blood may be the cause of the spreading of the
chromatophores around the blood-vessels. Certain observa-
tions seem to indicate the possibility that in the adult the
chromatophores have, in some forms at least, a more rigid
structure and are prevented from acting in the way indicated.
It seems to the writer that such observations as those made on
Fundulus might simplify the problem of the hereditary trans-
mission of certain markings.

Driesch has found that a tropism underlies the arrangement
of the skeleton in the pluteus larvae of the sea-urchin. The
position of this skeleton is predetermined by the arrangement
of the mesenchyme cells, and Driesch has showTi that these
cells migrate actively to the place of their destination, possibly
led there under the influence of certain chemical substances.
When Driesch scattered these cells mechanically before their
migration, they nevertheless reached their destination.

In the developing eggs of insects the nuclei, together with
some cytoplasm, migrate to the peripherj^ of the egg. Herbst
pointed out that this might be a case of chemotropism, caused
by the oxygen surrounding the egg. The writer has expressed

Influence of Environment on Animals 225

the opinion that the formation of the blastula may be caused
general^ by a tropic reaction of the blastomeres, the latter
being forced by an outside influence to creep to the surface of
the egg.

These examples maj^ suffice to indicate that the arrangement
of definite groups of cells and the morphological effects resulting
therefrom may be determined by forces lying outside the cells.
Since these forces are ubiquitous and constant it appears as if
we were dealing exclusively with the influence of a gamete;
while in reality all that is necessary for the gamete to transmit
is a certain form of irritability.

d) Factors which determine place and time for the deposition
of eggs. — For the preservation of species the instinct of animals
to lay their eggs in places in which the young larvae find their
food and can develop is of paramount importance. A simple
example of this instinct is the fact that the common fly lays
its eggs on putrid material which serves as food for the young
larvae. AVhen a piece of meat and of fat of the same animal are
placed side by side, the fly will deposit its eggs upon the meat
on which the larvae can grow, and not upon the fat, on which
they would starve. Here we are dealing with the effect of a
volatile nitrogenous substance which reflexly causes the peri-
staltic motions for the laying of the egg in the female fly.

Kammerer has investigated the conditions for the laj^ng of
eggs in two forms of salamanders, e.g., Salatnandra atra and
S. maculosa. In both forms the eggs are fertilized in the body
and begin to develop in the uterus. Smce there is room only
for a few larvae in the uterus, a large number of eggs perish
and this number is the greater the longer the period of gestation.
It thus happens that when the animals retain their eggs a long
time, very few young ones are born; and these are in a rather
advanced stage of development, owing to the long time which
elapsed since they were fertilized. When the animal lays its eggs
comparatively soon after copulation, man}- eggs (from twelve to

226 The Mechanistic Conception of Life

seventy-two) are produced and the larvae are of course in an
earty stage of development. In the early stage the larvae possess
gills and can therefore live in water, while in later stages they
have no gills and breathe through their lungs. Kammerer
showed that both forms of Salamandra can be induced to lay
their eggs early or late, according to the physical conditions
surrounding them. If they are kept in water or in proximity
to water and in a moist atmosphere they have a tendency to
lay their eggs earlier and a comparatively high temperature
enhances the tendency to shorten the period of gestation. If
the salamanders are kept in comparative dr^mess they show a
tendency to lay their eggs rather late and a low temperature
enhances this tendency.

Since Salamandra atra is found in rather dry alpine regions
with a relatively low temperature and Salamandra inaculosa in
lower regions with plenty of water and a higher temperature,
the fact that S. atra bears young which are already developed
and beyond the stage of aquatic life, while S. maculosa bears
young ones in an earlier stage, has been termed adaptation.
Kammerer's experiments, however, show that we are dealing
with the direct effects of definite outside forces. While we
may speak of adaptation when all or some of the variables
which determine a reaction are unknown, it is obviously in the
interest of further scientific progress to connect cause and
effect directly whenever our knowledge allows us to do so.

Vll. CONCLUDING REMARKS

The discovery of DeVries, that new species may arise by
mutation and the wide if not universal applicability of Mendel's
law to phenomena of heredity, as shown especially by Bateson
and his pupils, must, for the time being, if not permanently,
serve as a basis for theories of evolution. These discoveries
place before the experimental biologist the definite task of pro-
ducing mutations by physico-chemical means. It is true that

Influence of Environment on Animals 227

certain authors claim to have succeeded in this, but the writer
wishes to apologize to these authors for his inability to convince
himself of the validity of their claims at the present moment.
He thinks that only continued breeding of these apparent
mutants through several generations can afford convincing
evidence that we are here dealing with mutants rather than
with merely pathological variations.^

What was said in regard to the production of new species by
physico-chemical means may be repeated with still more
justification in regard to the second problem of transformation,
namely, the making of living from inanimate matter. The
purely morphological imitations of bacteria or cells which
physicists have now and then proclaimed as artificially produced
living beings, or the plays on words by which, e.g., the regenera-
tion of broken crystals and the regeneration of lost limbs by
a crustacean were declared identical will not appeal to the
biologist. We know that growth and development in animals
and plants are determined by definite although complicated
series of catenary chemical reactions, which result in the s^mthesis
of a definite compound or group of compounds, namely, nucleins.

The nucleins have the peculiarity of acting as ferments or
enzymes for their own synthesis. Thus a given type of nucleus
will continue to synthesize other nuclein of its o^vn kind. This
determines the continuity of a species; since each species has,
probably, its own specific nuclein or nuclear material. But it
also shows us that whoever claims to have succeeded in making
living matter from inanimate will have to prove that he has
succeeded in producing nuclear material which acts as a ferment
for its own synthesis and thus reproduces itself. Nobody has
thus far succeeded in this, although nothing warrants us in taking
it for granted that this task is beyond the power of science.

1 Since this was written the beautiful experiments of Kammerer as well as
those of Tower seem to have furnished proof that external conditions can cause
hereditary changes in animals.

INDEX

Abraxas, 17.

Acid poisoning, 18 ff .

Acids, influences of, on heliotropism,

42, 220; acids and salts, antagonism,

of, 179 ff.
Action of potassium cyanide, 156 flf.
Activation of the egg, 6 fif.
Agalma, 208.
Amphitrite, 144.
Animal instincts, experimental control

of, 218.
Antagonism of acids and salts, 179 flf.
Antagonism of three salts, 182 flf.
Antagonistic salt action, 172 flf.
Antedon rosacea, 198.
Antennularia antennina, 85, 91, 107,

217,
Ants, 47, 48.

Aphides, 19, 37 flf., 41 flf., 46.
Arabacia, 157, 164, 190.
Area striata, 79.
Arrhenius, 5, 198.

Artificial causation of positive heliotro-
pism, 43, 220.
Artificial parthenogenesis, 7, 116 flf.,

127 flf., 199 flf.
Artificial production of double and

multiple monstrosities in sea-urchins,

100 flf.
Artificial production of living matter, 5.
Ascidians, 68.

Associative memory, 55, 73.
Asterias capitata, 198.
Asterias forbesii, 135.
Asterias ochracea, 138, 162, 198.
Asterina, 132, 198.
Atwater, 4.
" Aura seminalis," 113.

Baer, K. E. von, 114.
Balanus perforatus, 46, 53.
Baltzer, 15.
Bancroft, 51.
Bardeen, 92.
Barry, 114.

Bases as membrane-forming sub-
stances, 134.
Bataillon, 11.
Bateson, 226,
Bees, 49.

Beginning of scientific biology, 4.
Berzelius, 4.
Biedermann, 173.
Biology, beginning of scientific, 4.
Bischof, 115.
Blaauw, 29.
Bohn, 40, 49, 54, 55.
Bolina, 208.
Boveri, 15.

Butschli, 145, 146, 147.
Bunsen, 27, 29, 30, 41, 58.
Bunsen-Roscoe law, 27, 28 flf., 40, 58.
Butyric acid, treatment of egg, 10.

Callianira, 208.

Cannon, 222.

Catalysis of esters, 43.

Catalyzer, 4, 5.

"Center of coordination," 72.

Cerianthus membranaceus, 93.

Chaetopterus, 117.

Change in intensity of light, 54 flf.

Changes in color of butterflies pro-
duced through influence of tempera-
ture, 211.

Chemical agencies, eflfects of, 196 flf.

Chemical symmetry, 38. 39.

Chemotropism, 224.

Chlorostoma, 198.

Chromosomes, 16 flf.

Chun, 208.

Ciamician, 38.

Cinerarias, 37.

Ciona intestinalis, 68, 92.

Claparede, 51.

Cohen, 208.

Color, changes in, of butterflies,
through influence of temperatiu*e,
211; color adaptation, 80 flf., 213 fif.;
color blindness, 16.

Compulsory movements, 38.

Consciousness, 72.

Contents of life, 26 flf.

Cooke, 99.

Cooperative action of salts causing
impermeability of the egg mem-
brane, 176 flf.

Coordinated movements in reflexes,
70.

Copepod, 43, 62.

Correns, 21.

Cortical layer, of unfertilized egg, 10,
189; mechanical destruction of,
10, 11.

Cosine law of illumination, 41.

Crabs, fiddler, 60.

Ctenolabrus, 25.

Cuma Rathkii, 51.

Cytolysis, of the egg, 146; mechanical
causation of, 145.

Cytolytic agents, 10, 132 flf., 136,
144 fif., 202.

De Vries, 226.

Daphnia, 43, 208, 213, 220.

Darwin, Charles, 195, 196.

Darwin, F., 58.

Davenport, 222.

Death and development, dififerent

chemical processes, 209 fif.
Delage, 11, 131, 203.
Depth migration, 220.
Development, death and, dififerent

chemical processes, 209 flf.
Difference of salt permeability of

various membranes, 189 flf.
Diflfusion of salts, 177 flf.

229

230

The Mechanistic Conception of Life

Doncaster, 21.

Dorfmeister, 211.

Driesch, 101, 108, 204, 224.

Drosophila, 21.

Dumas, 114.

Dzierzon, 116.

Ear, otoliths of, and orientation to
center of gravity of the earth, 57.

Effect of retarded oxidations on
poisonous salt action, 190 ff.

Effects of chemical agencies, 196 fl.;
of gravitation, 215; of light, 213.

Egg, activation of, 6ff. ; butyric acid,
treatment of, 10; cortical layer of
unfertilized, 10, 189; increased
sensitiveness of unfertilized, to
cytolytic agents, 163 ff.; produc-
tion of twins from, 204 ff .

Eggs, factors which determine time
and place for the deposition of, 225;
immunity of, to body extracts of
same species, 142; varying sus-
ceptibility of, 144.

EmiUsion theory of membrane forma-
tion, 145, 147 ff.

Enzymes, 5, 122, 123.

Esters, catalysis of, 43.

Ethics, 3, 5, 31 fl., 62.

Eudendrium racemosum, 213.

Experimental control of animal
instincts, 218.

Experiments, on hydroids, 217; on
the egg of the frog, 216; on the
mechanism of heliotropic reactions,
218; localization, 35.

Factors which determine time and
place for the deposition of eggs,
225.

Ferments of oxidation, 5.

Fertilization, 6; fertilization mem-
brane, 8, 148 fl.

Fiddler crabs, 60.

Fischer, 211, 212, 213.

Fischer, Emil, 115.

Fovea centralis, 39.

France, 58.

Froschl, 29.

Fundulus heteroclitus, 25, 105, 172 fl.,
175, 179 ff., 223.

Fusion of normally double organs, 207.

Galvanotropism, 50 ff.

Gamble, 214, 215.

Gammarus, 170 ff., 189; poisonous

action of distilled water on, 170 ff.
Gemmill, 121.

Geotropism, 56 fl., 89, 217, 223.
Gies, 175.

Godlewski, 15, 95, 198, 199.
Goldfarb, 213.
Gravitation, effects of, 215.
Groom, 221.
Growth, 96 ff., 100 ff.; mechanics of , in

animals, 95.
Growth and light, 213.
Guyer, 17, 18.

Haberlandt, 58.
Hagedoorn, 15, 199.
Handovski, 99.
Hardy, 148.

Harmonious character of organisms,
23 fl.

Hartmann, E. von, 35.

Harvey, 189.

Heliotropic animals, 41; heliotropic
reactions, experiments on the mech-
anism of, 218.

Heliotropism, 27 ff., 220 fl.; artificial
causation of positive, 43, 220; influ-
ence of acids on, 42, 220.

Henking, 16.

HgiiscIigii 79

Herbst, 92, 134, 186, 187, 204, 207, 224.

Heredity, 4, 15 fl., 49, 52, 101.

Hertwig, O., 116, 134, 208.

Hertwig, R., 7.

Heterogeneous hybridization, 24, 25,
196 fl.

Heteromorphosis, 85 fl., 217 fl.

Hippolyte cranchii, 215.

His, 100, 101, 115.

Hoeber, 177.

Holmes, 51, 60.

Holmes, S. J., 220.

Huxley, 15.

Huyghens, 104, 107.

Hybridization, heterogeneous, 24, 25,
196 fl.

Hybrids, maternal character of hetero-
geneous, 199.

Hydra, 93 fl.

Hydroids, Experiments on, 217.

Hypertonic sea- water, 7, 116 fl., 131 fl.

Hypotricha, 55.

Illumination, Cosine law of, 41 ; inten-
sity of, 44.

Immiuiity, 142.

Immunity of eggs to body extracts
of same species, 142.

Increased sensitiveness of unfertilized
egg to cytolytic agents, 163 fl.

Influence of membrane formation in
causing the egg to develop, 150 ff.

Influence of temperature, 207 ff.

Infusoria Coelenterates, 74.

Instincts, 69.

Intensity of illumination, 44.

Jacobi, 113.
Jennings, 55, 60.

Kammerer, 225, 226, 227.

Keeble, 214, 215.

Kellogg, v., 49, 222.

Knaffl, von, 136, 147.

Koeppe, 148.

Kupelwieser, 15, 21, 125, 198, 199.

Laplace, 4, 5.

Lavoisier, 4, 5.

Law of segregation, 16, 20 fl.

Leeuwenhook, 113.

Liebig, 115.

Life, contents of, 26 fl.

" Life principle," 14, 15.

Light, change in Intensity of, 54 fl.
effects of, 213; growth and, 213
photochemical action of, 30, 36
selection of intensity of, by animals
52 ff.

Lillie, F., 12.

Lillie, R., 10, 132, 136, 174, 181.

Index

231

Living matter, artificial production

of. 5.
Localization experiments, 35.
Loeb, Leo, 48, 210.
Lottia, 209.
Lucas, 208.
Luther, 38.
Lyon, 57.

"Lysin theory," 143.
Lysins, 139 ff.

McClung, 16, 17.

Mach, 57.

Macromysis flexuosa, 214.

Margelis, 89, 107.

Maternal character of heterogeneous
hybrids, 199.

Mathews, A. P., 11, 144, 156.

Maxwell, S. S., 39, 208.

Mead, 7, 117.

Mechanical causation of cytolysis, 145.

Mechanical destruction of cortical
layer, 10, 11.

Mechanics of growth in animals, 95.

Membrane of fertilization, 8, 148 flf.;
membrane-forming substances, bases
as, 134; membrane formation, emul-
sion theory of, 145, 147 flf. ; membrane
formation, influence of, in causing egg
to develop, 150 flf.

Memory, associative, 55, 73.

Mendel, 4, 16, 20, 21, 49, 52, 59, 60,
226.

Mendel's laws, 4, 15 flf., 20, 49, 52,
59 flf., 226.

Menidia, 25.

Merrifleld, 211.

Meyerhof, 190.

Miescher, 114, 115.

Minkowski, 79.

Moenkhaus, 24.

Monstrosities, artificial production of,
in sea-urchins. 100 flf.

Montgomery, 16.

Morgan. 7, 17, 19, 21, 92, 117, 119, 216.

Morphology, physiological, 109.

Movements, compulsory, 38.

Munk. 35. 79.

ISIuscles, osmotic phenomena in, 99.

Mytilus, 198.

Natural selection, 50 flf.
Nemec, 58.
Nereis, 12.
Neuberg, C, 38.
Norman, 117.

Organisms, harmonious character of,
23 flf.

Organs, fusion of normally double, 207.

Osmotic phenomena in muscles, 99.

Osterhout, 177, 178, 179, 188.

Ostwald, Wilhelm. 4.

Ostwald, Wolfgang, 38, 213.

Otoliths of ear and orientation to
center of gravity of the earth, 57.

Overton, 148, 177, 178, 179, 191.

Oxidation, ferments of, 5.

Oxidations, and their relation to the
egg after fertilization, 13, 157, 160 flf.,
164; in their relation to life and
death, 14, 15; eflfect of retarded, on
poisonous salt action, 190 flf.

Parker, 39, 49, 220.

Parthenogenesis, 117 flf.; artificial, 7,

116 flf., 127 flf., 199 flf.
Parthenogenetic development, varying

susceptibility of eggs to, 144.
Pauli, 99, 181.
Pawlow, 62.
Pennaria, 91.
Peter, 208.
Pettenkofer, 4.
Pfluger, 216.

Photochemical action of light, 30, 36.
Photochemical, eflfects, 27, 38, 39;

substances, 39.
Photosensitive surfaces, 39.
Photosensitiveness, varying, in animals,

45.
Phylloxera, 19.

Physiological morphology, 109.
Planarians, 39, .54.
Plasmolysis, 177.

Poisoning, acid, 18 ff.; salt, 186 flf.
Poisonous action of distilled water on

Gammarus, 170 flf.
Polarization, 92 flf.
Polygordius, 53.
Polynoe, 132, 158.
Porthesia chrysorrhoea, 47, 48, 218 flf.,

222.
Potassium cyanide, action of, 156 flf.
Pouchet, 80, 214.
Poulton, 215.
Prevost, 114.
Procter, 181.
Production of twins from one egg

through a change in the chemical

constitution of the sea-water, 204 flf.
Protective solution, 172.
Purposeful character of reflexes, 66.
Pycnopodia, 198.
Pyrrhocoris, 16.

Radl, 49.

Ranke, 99.

Rayleigh, 147, 152.

Reflex, 65 ff.

Reflexes, coordinated movements in,

70; purposeful character of, 66.
Reinke, 24.
"Riddle of life." 5.
Ringer, 172, 173.

Robertson, T. B., 185, 208, 210.
Role of water in segmentation, 99.
Roscoe, 27, 29, 30, 41, 58.
Roux, 24, 216.
Rubner, 4.

Sachs, 104, 107.

Salamandra atra, 225.

Salamandra maculosa, 225.

Salt poisoning, 186 flf. ; diflfercnce of
permeability of various membranes,
189 flf.

Salts, antagonism of acids and, 179 flf.;
antagonism of three salts. 182 ff.;
cooperative action of, causing im-
permeability of egg membrane,
176 ff.; diffusion of, 177 ff.

Schmiedeberg. 173.

Schopenhauer, 35.

Schultze, 216.

Sea-water, hypertonic, 7, 116 ff., 131 fl.

Segregation, law of, 16, 20 fl.

232

The Mechanistic Conception of Life

Selection, of intensity of light by ani-
mals, 52flf. ; natural, 50 fif.

Sex determination, 16.

Shearer, 11.

Sipunculides, 141.

Snyder, CD., 39, 208.

Solution, protective, 172.

Spallanzani, 113.

Spermatozoa, 113 ff.

Spermatozoon, twofold action of, in
fertilization, 12 ff., 132 ff., 161 fl.

"Speziflsche Bildungsstoffe," 104.

Spontaneous raovements, 71.

Spyrogyra, 177 fif., 188.

Standfuss, 211.

Stereotropism, 91 ff., 223.

Stevens, 17.

Stieghtz, 43.

Stockard, 206, 207.

Strongylocentrotus franciscanus, 140,
198.

Strongylocentrotus purpuratus, 8, 139 ff.,
144, 162, 189, 197, 201, 205.

Sumner, 80.

Symmetry, chemical, 38, 39.

Temperature as a cytolytic agent,

135ff., 138ff.
Tower. 90, 227.
Traube. 96.
Tropisms, 26, 36 ff., 54, 55, 60, 62,

69, 70, 72.
Tubularia mesemhryanthemum, 95, 97.
Tunicates, 92.
Twin formation, 19, 205.
Twins, production of, from egg, 204 ff.
Twofold action of spermatozoon in

fertilization, 12 ff., 132 ff., 161 ff.

Tyrosin, 23.

Uexkuell. 70.

" Unterschiedsempfindlichkeit," 54.

Van Duyne, 92.

van't Hoff, 208.

Vanessa antiope, 220.

Vanessa levana, 211.

Vanessa prorsa, 211.

Varying photosensitiveness in animals,

45.
Varying susceptibility of eggs of

different species to parthenogenetic

development, 144.
Vaucheria, 188.
Virchow, 127, 128.
Vision, 79 ff.
Voit, 4.
Volvox, 51, 223.

Waller, A. D., 222.

Warburg, 13, 157, 160, 189. 190.

Wasteneys, 13, 157, 181, 182.

Water, distilled, poisonous action of

distilled water on, 170 ff.; role of,

in segmentation, 99.
Weismann, 91, 210, 211.
Wiener, 215.
"Will," 35 ff., 40.
Wilson, 17, 18, 19.

X-chromosomes, 18 ff.

"Zielstrebigkeit," 24.
Zimtz, 4.

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