U> ' ill iiiti ®I|f i. % ^xli Siibraru Jforttj ffiarolttia g'tatf Qlnllege QH31 W3 J QH31 1 Li T.np'h 121852 This book may be kept out TWO WEEKS ONLY, and is subject to a fine of 191^ f CENTS a day thereafter. It is due on the day indicated below: 7 Oct Jlj i\m 5 \i V 57 196B 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 BY JACQUES LOEB; M.D., PH.D., SC.D. MEMBER OF THE ROCKEFELLER INSTITUTE FOR MEDICAL RESEARCH THE UNIVERSITY OF CHICAGO PRESS CHICAGO, ILLINOIS Copyright 1912 By The University of Chicago All rights reserved Published July 1912 Composed and Printed By The University of Chicago Press Chicago, Illinois, U.S.A. w.C PREFACE The essays contained in this volume were wTitten 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 121852 TABLE OF CONTENTS PAGE I. The Mechanistic Conception of Life - - - - 3 11. The Significance of Tropisms for Psychology - - 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 Natm*e of the Process of Fertilization - - 113 VII. On the Nature of Formative Stimulation (Artificial Par- thenogenesis) - - - - - - - -127 VIII. The Prevention of the Death of the Egg thi'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 jMonists at Ham- burg, September 10, 1911; reprinted from Popular Science Monthly, January, 1912, by courtesy of Professor J. McKeen Cattell. 3 0. H. Hia UBRARY Morth Carolina Stale Colltat 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 ''riddle 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 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 through the action of the spermatozoon. This formation of a fertilization membrane can be induced by a pm-ely 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 mollusks. 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 m 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 h}' pertonic 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 9 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 Figs. 6 and 7. respectively. Fig. 7 -The sea-urchin egg divided into four and eight cells 10 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 11 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 flsh 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 0. 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 what 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 which 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, under normal conditions, that exhibit marked differences in structure. A careful consideration will suggest that nothing 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. 18 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 iinfertilized eggs. Each contains one sex cliromosome marlced X: the other six dark spots are the chromo- somes which are supposed to transmit hereditary 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 will 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 w^hether 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 j'ear 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 born 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 showm 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 with 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 {Drosopkila) 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-emmently in one sex. We say pre-eminently, smce theoretically we can predict cases in which color blindness or white eyes must appear also in the female. Breeding experiments have sho\Mi 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 :\Iendel already intimated. X o.is 0*2 o sS?^? SoS ® £ O tc S ego 2-0 eS eS u S O — 0;= ^ S o _ u © O 6X1 — a ©^ ce © © S- C rC C C S«w © -. ■ t^ •^-S- © cS 3 O t- tc'+^ ^ O — < •!-! 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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 biologj', 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 embrj^os 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 number 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 established 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 heterocUtus) 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., Ctenolabrus 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 originnte, but which escape our notice because they cannot live 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 kno"«Ti 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 ^vithin wide Hmits 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. Consequenth' 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 intiuenee of only one source of light and positively heliotropic.) 28 The Mechanistic Conception of Life In a series of experiments I have shown that the heUotropic reactions of animals are identical with the heliotropic reactions of plants. It was knoTNTi 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 figiire. (From nature.) These animals bend in the same way to the light as the stems of positively heliotropic plants kept imder siniilar 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 29 heliotropic away from it), and the same is observed in motile animals. In plants only the more refrangible raj^s 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 natm"e.) 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 imder the same conditions. constant; and the same result was obtained simultaneousl}' 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 hehotropism 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 with the Bimsen-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 ''will" 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 })ody. 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 with ethics: it seems the only con- ception of life which can lead to an understanding of the source of ethics. II. THE SIGNIFICANCE OF TROPISIVIS FOR PSYCHOLOGY II THE SIGNIFICANCE OF TROPISxMS FOR PSYCIIOLOGY^ I A mechanistic conception of life is not complete unless 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 ''truly psychical" unexplained. 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 grouj) 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 ''will." Through the writings of Schopenhauer and E. von Hart- mann I became interested in the problem of will. 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. jMunk stated that he had succeeded in proving that every memory image in a dog's brain is localized 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 jMunk 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 d>iianiics of brain processes. A better opportunity seemed to offer itself in the study of the comparative psychology of the lower animals in wliich 1 Lecture delivered at the Sixth International Psychological Connress at Geneva. 1909. (After a translation in Popular Science Monthly by Miss CJraco D. Watkinson.) Reprinted by courtesy of Professor James McKwn Cattell. 35 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" w^as 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 w^hich 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 imder certain conditions seems to a lay- man to be the will 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 physical 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. II 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 twentj^-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 Hght 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 tlie interference of the glass stops their furthiT progressive niove- ments.i It can be demonstrated in these animals that t\w 1 Loeb, Der Heliotropismus der Ticrc unci seine Uebereinstimmuno mil dem Heliotropismus der Pflanzen. Wiirzburg. 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 symmetrical 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., XIII, 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 symmetry of the two retinae are chemically alike, the non-symmetrical points chemically imlike. Now if an unequal amount 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 ^\^ll take place in one optic nerve than in the other. S. S. Maxwell and C. D. 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 synnnetrical groups of muscles of both halves of the body will receive equal chemical stimuU and thus reach equal states of contractiun, 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 inequaUty 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 -svinged 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 hiunan 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 i)he- 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. Heliotrupi.sm is unusually common among the larvae of marine animals anrl 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 sntii- 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 hiw of Bunsen and Roscoe which states that the chemical etYect 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 1 Whether an animal is positively or negatively heliotropic deponds upon the fact whether the light causes an increase or a decrease in the tension of the inuseles. Why light should have these opposite effects is as yet unknown. 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, under 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 antl 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 aj)hi(ls. The mass of photochemical substance which is acted uj)()ii by the light is often relatively small, so that even when tho 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 bj' increasing the active mass of the photosensitive substance. In this way the same intensity of light which before produced no heliotrojiic reaction now may cause a very pronounced positively heliotro])i(' reae- 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 dilTer- ence in the action of the muscles of both sides of the body and a turning of the head tow^ard 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, I Loeb, Pflugers Archiv, CXV, 564. 1906. 44 The Mechanistic Conception of Life this may often be accomplished by simultaneously reducing the temperature. IV The animals which are strongly positively heliotropic 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 with 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 'Hrial 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 rays, 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 intensity 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 i.s there- fore erroneous to assert that holiotropism doterminos 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. A\'e have seen that acid increases the sensitiveness of certain animals to light, possibly by increasing the active mass of the photocheniical 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 heliotropically. 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 sufhcieiitly to cause it to move for the period of a few seconds directly toward the source of light. Later the production of carl)onic 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 writer 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 'Hrial 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 sufficiently 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-caUed "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 V As far back as 1889 I pointod out that the photosonsitivo- ness of an animal is different in different physiological conditions and that, therefore, under natural conditions, heliotropisin 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 cliangcs 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 l)y an increase in temperature. When driven out of the nest in this condition they are strongly positively heliotropic and I have never fomid 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 hungry.^ 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 heliotroi)ic 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-knowTi fact that during sexual maturity special substances are formed which influence various organs. For 1 Loeb, op. ciL, p. 24. (This latter fact has Ijoon 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 folhcle cause a special sensi- tiveness in the non-pregnant uterus, so that ever}' mechanical stimulus causes the latter to form a decidua. In this waj' 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 young 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 hehotropism. They can now creep clo^^Tl- 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 imknown. Significance of Tropisms for Psychology 49 vital economy of these creatures. As is well known, tlic mating of these insects takes place during the so-called nui)tial 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 lii^iit follows the direction of the rays of the sun. I gained the iinj)ression that this nuptial flight is merely the consecjuence of a very highly developed heliotropic sensitiveness. The case seeuLs 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 raj's, 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, RadV and myself. VI 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, fomided 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 nuist die out. 1 Radl, Der Phototropismus der Tiers, 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 with 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 within the cell is retarded. Significance of Tropisms for Psychology 51 reactions of animals are determined by their needs or In* natural selection could hardly be found. One might be led to suppose that galvanotropism and lieli- 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 ui)()n the surface of the skin, while the electric current influences all the cells of the body. As already mentioned, the disturbing rom- plications arising from this latter circumstance di.sapi)ear for the most part when we work with unicellular organisms, anrl 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 why 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, whicii li\-e under the bark of trees, may show positive heliotroi)isin. 1 found, moreover, that the crab, Cuma Rathkd, which lives in the nuid of 1 Claparede. "Les tropismes devant la psychoiogie.- Joum. f. Psychologit und Neurologic, XIII, 150. 1908. 52 The Mechanistic Conception of Life the harbor at Kiel, when brought into the Ught and removed from the mud shows positive heliotropism. It is, therefore, just as incorrect to assert that the heUotropic 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 heliotropic animals move toward the source of light even if by so doing they go from the sun- light into the shade. W W is b. window through which sunlight S falls into the room. By a piece of board d e the sunlight S is prevented from striking the region fe c of a table near the window and this part of the table is in the shade. Only the daylight 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 sunlight into the shade toward the source of light luitil 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 exjxTiiiH'nt, I have never found a single indication of such an adaptation. In every case it has been sho\Mi that positively hcliotropic animals are positive to any intensity of light above the threshold. Thus winged plant lice or wingless larvae of Chrysorrhoca or copepods, which have been made heliotroi)ic 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 retjuired 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 iih(jt(j- chemical action of light may inhibit the positive h('liotroi)isin. 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 heliotroi)ic. 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 hehotropic. This is the case, for instance, with the larvae of Polygordius. But it would be wrong in this case to sjM-ak of an adaptation of the animal to a certain intensity of light. 1 Quite often without even stopping for a moment. In animals sonsltivr to differences (see next chapter) a stopping occurs in ihis cxp.rinn-nt in tlir pavsinn from the light into the shadow, but they go. nevertheless, ininu-dlately on in the direction of the source of light. The reader will And a furtlu-r account of this experiment in my book on The Dynamics of Livinu MntUr. 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 shown 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 C'Unterschiedsempfindlichkeit") differential sensi- bility, in order to distinguish them from tropisms.^ 1 Loeb, " Ueber die Umwandliing positiv heliotropischer Tiere, u.s.w.," Pflugera Archiv, 1893. See also the recent investigations of Georges Bohxi, La naissance de V intelligence, Paris, 1909; "Les essais et les erreurs chez les etioles de mer," Bull. Significance of Tropisms for Psychology 55 It is hardly necessary to point out Ihtc that tiie effects of rapid changes in intensity, when they are very marked, can easily complicate and entirely obscure tiic iK-liotropic 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 tuU-- worms they thereupon draw back very (luickly. 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 heliotro})ism, 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 fintl 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 onl>- 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 ])lace. The reader is referred to my book^ as well as to the mure rt'cenl works of Bohn, La naissance de V intelligence^ and Ln nouvelle psychologic animale.'^ Let us bear in mind that "ideas" also Inst. gen. psychol., 1907; "Intervention des reactions osclUatoircs dans Irs tro- pismes," Ass. franc, d. Sciences, 1907. 1 Jennings, The Behavior of Lower Organisms. 190G. 2 Comparative Physiology of the Brain and Comparatire Ptychology. Now York and London, 1900. 3 Paris, "Bibliothequede philosophic sclentlflQue." 1909. < Paris, " Bibliotheque de philosopliio contemporaino." 1911. 56 The Mechanistic Conception of Life can act, much as acids do for the heliotropism 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. IX 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 geotropic 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 behaviur, 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-f)rpans of the sensory nerves and every change of pressure mi^ht 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 exi)eriments 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. K. P. Lyon carried out these experiments, which showed that no disturbance of the orientation resulted from this operati(jn. 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 eartli is regu- lated by the pressure of the otoliths upon the nerve . We find that there are two classes of investigators in biology, grouped according to their attitude toward such simj)le 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 under such a law offers an ojjportunity for them to point out the inadequacy of the latter. The other group of investigators aims to discover antl not to disj)rove laws. When such investigators have discovered a simi)le law which is generally apphcable, they know that an apparent exception does not necessarily overt lirow the law, but that possibly an opportunity is offered for a n<>w discovery and an extension of the old law. Mendel's laws have been brilliantly confirmed in a number of cases. In some cases of ai)parent 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, l)ut that they are uwrvW 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 writers, 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. Jennings 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 01 indicate that the role ascribed to symmetr}' has no iinportanrn for the theory of tropisms. I am, however, iiichned to draw another conclusion, namely, that in the fiddlor crabs in the first place there is an entirely different connection between the retina and the locomotor muscles from tliat in otlier crustaceans, and that, secondly, there is a special peculiarity in regard \u the function of the two retinae whereby they do not act Hkc symmetrical surface elements. I believe that a new discovery may be made here.' XI These data may suffice to explain my point of view. To me it is a question of making the facts of psj-chology 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, l)ut 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 1 am of tiie opinion that the contents of comparative psychology will under the influence of the above-mentioned endeavors be diffcn^nt from the contents of speculative psychology. But 1 believe also that the further development of this subject will fall more to the lot of biologists trained in physical chemistry than to the specialists in psychology or zoology, for it is in general hanlly 1 From which I expect, furthermore, that they will only conMrin still moro the laws of heliotropism. This expectation is based upon anuloK'ous rolatlons In the plem-onectids, which I cannot, however, discuss furtlu-r here. However, probably no one will maintain that the existence of the pleuroni>ctlds 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- pomt 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 COXCKP- TIONS CONCERNING THE C0MPARAT1\ K PHYS- IOLOGY OF THE CENTRAL NERVOUS SYSTEM Ill SOME FUNDAMENTAL FACTS AND CONCIIPTIDNS COX- CERNING THE COMPARATIVE PHVSIoLoCV OF THE CENTRAL NERVOUS SVSTKM' 1. The understanding of complicated i)li(n(>in(iiu dciH'nd.s 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 coordinatetl movement, the closing of the eyelid, for example, when the conjuctiva is touched by a foreign body, or the narrowing of the pui)il 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 physiology to make reflexes the basis of the analysis of the functions of the central nervous system, and consequently much imiiortanco has been attached to the processes underlying tlu-m 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 thi^ muscles; destruction of tlie spinal cord would, according to this, make the reflex impossible, 1 Reprinted from Loeb. J.. Comparative Phyniology of the Brain and Compara- tive Psychology (1899). By courtesy of G. W Putnam's Sons of Now ^ ..rk an.l London. 65 66 The Mechanistic Conception of Life just as the destruction of the mirror might prevent the reflection of light. 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 eminently 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 phj^siolo- 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 G7 same time offered a different conception of reflexes. The Hijijht of the moth into the flame is a typical reflex proce<<. The light stimulates the peripheral s(*nse-orjj;ans. the stimulus |)asse8 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 with the heliotrujjic effects of light on plant organs. Since plants possess no nerves anc! no ganglia, this identity of animal with i)lant heliotroj)ism can force but one inference — these heliotrojiic effects must depend upon conditions which are common to both animals and plants. At the end of my book on heliotropism' 1 exi)resse(l 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 cTnmetrical elements have a different irritability. Those nearer the oral i^ole 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 stinui- lation in such a way that symuK^trical 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 surrounded 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 unknowTi 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 Uexkuell 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 irritabihty. Since Physiology of Central Nehvous System 5. We must also deprive the ganglion-cells of all specific significance in spontaneous movements, just as wc Ikivo rlon*' in the case of simple reflexes and instincts. Hy sj)()ntaneous movements we mean movements which arc ai)parcntly 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 j)reserve the activity beyond a short period of time. We must discriminate between simple and conscious sj)on- taneity. In simple spontaneity we nmst 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, antl from this the con- clusion has been drawn that all automatic movement6 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 mori)hoK)gy into that of physical chemistry. The peculiar qualities of each tissue are partly due to the fact that it contains certain ions (Xa, 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 th»- Xa ions be increased and the Ca ions be reduced, the muscles are able to contract rhythmically like 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 pow(>r of rhythmical spontaneous contractions is not dut^ to the specific the contraction of one group of muscles noces.sitate.s the stretching of their antaRO- nists the coordinated character of locomotive action seems to Inxnime IntelllKlMe (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 rhythm. 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 following 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 Xkuvous System 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 anri of organization in general show a degree of purposefulness whicli may even surpass that of any reflex or instinctive or conscious act. And yet we do not consi(h'r the plienomena of develop- ment to be dependent upon consciousness. On the other hand, physiologists who have api)reciated tlie untenable character of such metaphysical speculations have held that the only alternative is to droj) the search for the mechanisms underlying consciousness and study exclusiv<'ly 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 proces-es which make consciousness possible, it abandons its best i)ossi- bilities. But to obtain results, the errors of the metaphysician must be avoided and explanations must rest upon facts, not words. The method should l)e the same for animal psychology that it is for brain physiology. It should consist in the right understanding of the fundamental i^rucess which ri'curs in all psychic phenomena as the elemental component. This process, according to my opinion, is the activity of the assocuitirc memory, or of association. Consciousness is only a metaphysical term for phenomena which are determined by a.ssociative memory. By associative memory I mean that int'chanism 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 (luite 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 showTi 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 memorj^, 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 whicli may he produced wlirii onr variahh* is changed; it is not surprising tliat tlicre should Im- sudden changes in the ontogenetic and phylogrnetic development of organisms when then* are so many variaMes 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 (hscovery to he made in the field of brain physiology and psycholog>'. But at the same time it is evident that this mechanism cannot be unrav<'led 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 chang«*s occurring in colloidal substances. The question is. Which peculiarities of the colloidal substances can make the phenom<'non of asso- ciative memory possible? For the solution of this prol^lem the experience of phj'sical chemistry and of the i)hysi()log>- of the protoplasm must be combined. From the sam<' sourc(»s 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 AXP THE MECHANIS:\I OF VISION IV PATTERN ADAPTATION OF FISIIKS AM) THi: MLLllAX- ISM OF VLSIOX^ The mechanism of the action of the brain is cntin-ly unkno\vn to us. We are unable to look into the active brain and the objective results of brain action are in general so difTcr- 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 \isual si)here and that the extirpation of definite parts of this sphere caused blindness in definite parts of the retina. I repeated thes<' 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 ]\Iunk had believetl 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 ])ossibility 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 Physiologischcs CcntralUalf, XXV. No. JJ. l*.»l-'. This note is given merely as a suggestion concerning ihv nurlumism un.I. rl> ing c.rtain brain processes. 71) 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 disturbinj^ fac-tor.s 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 (Ufferent 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 th<' 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 uj)on 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 telei)hotograpliy. 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 liavo 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 hj'droid, Antennulana 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 i)olyjrs on their upper sides. In this animal we never find a root originating at the ajx'x, or in place of a branch, or polyps originating on the under side of a branch. In observing these phenomena the question arose: \Vhat 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 phiw. 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 Lectures delivered at the Marine HloloKleal Laboratory of Woods Hole, 1893. by courtesy of Glnn & Co. 2 Untersuchungen zur physiologischvn Morpholotjic dcr Tier,. I. Hrt«TO- morphosis. Wurzburg, 1891. II. Organbildung uiul Wuchsthuin, Wilrzhurg. 1SU2. Translated in Studies in General Physiology. 85 86 The Mechanistic Conception of Life ^y \ Fig. 23. — Diagram of normal regenera- tion if a piece o 6 of Antennularia is hung 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. p.5 b .CL ^ w Fig. 24. — Diagram of heteromorphic re- generation in A7iten- nularia. A piece o h 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 o 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 ah (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 h below. It was found that after a few days the root end b had formed little roots, W, which >/ ^S Fig. 22. — A piece of the normal stem of Antennularia an- tennina,& hydroid of the Bay of Naples. Approximately natu- ral size. 5 S, stem. W, stolons or roots. Physiological Morphology 87 grew downward, and the apical end, a, had formed a now stem, S. A similar piece was cut out from another specimen and was hung upside down in the aquarium (Fig. 24). The njut 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 a.s in the normal animal, namely, the branches which were growing I V 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 tlu- lowt-r side which had ceased to grow, grow downward as stolon.s and atta<'h tlu'insolvt-s to solid bodies. On the upper side a new stem c d grows vertically upward. obliquely upward bore polj'ps 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 exci.sed 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 do\\iiward, and these downward growing elements were no longer branches but roots (Fig. 25). Tiii.s could be proved by their physiological reactions, for the roots attach themselves to the surface of solid Ixulies, e.g., the glas.s 88 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 oblique 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, vdth. the root end above, the branches on the under side grew out as roots, and at the upper end a stem arose as usual. Fig. 26. — Diagrammatic regeneration in a piece o 6 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 connnon ? The stems always originated from the uj^ixt end or side of an element, and roots always from the lower side or end uf 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 geotroi)ism, the stem grows vertically upward. Gravitation does not p7)s. 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 i)olyps which appear at the end of long stems. Fig. 27, which Mr. 90 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 b, 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 b. 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 6 of tWs hydroid was cut out and put into a jar with sea-water. The ends o 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 ho produeod. Another form, Pennaria, is just as favorabh'. 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 with 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 pol^-ps, if tliossiblc to reconcile these facts with the principles of natural selection, causal or physl«)loplcal morphology would not gain therel^y, as the circunustances that determine the forms of animals and plants are only the different forms of enerKy 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 > Since this was written phenomena of heteromorphosis have been produced in many animals. Herbst found that in crustaceans an antenna covild 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 93 which it was directed in the normal cumlitiun. We may speak in such cases of polarization. Thr ohvirest example of this I found in an actinian, Ceriatithus membranacem. If we cut a rectangular piece, cdef, 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), whicli was directed toward the oral end of the animal. Nothing of the sort oc- curs in the side ce, or dc , or fd. 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, be, 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. Thev endeavor to forei' them into the new oral disc, where the mouth should l)e, and only after a struggle of some minutes do they give up the futile attenipt. I tried in every possible way to proiluce tentacles in tlie al)()ral end of a piece which had been cut out. but without success. Hydra behaves, as regards i)olarization. a little differently from Cerianthus. If we make an incision in the stem, a Fio. 29.— DiaKram- matic. If a pli-cr r d ef Is cut out from the wall of Cenanthun. a si'a aiu'inoiu". iM've tt'iUacli's ar»' fornird only at till' upiHTCut ef. yj 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. 94 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 nature. 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 Avhere a mouth ought to be, showing 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 unknown 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 95 are exhibited by the same animal. If we cut out a sufficiently large piece of the stem of Tubularia mesemhryantheinum , 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 pol}^:), 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 polyp. 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 pol^'ps.^ Ki< 32. III. THE MECHANICS OF GROWTH IN ANIMALS In order to arrive at an explanation of the phenomena of organization we must ask w^hat 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 n«'toro- inorphosis in TuKuta- ria. From uaturr. Thcnormal Tittntlunn (Muls at oin' mil In a stolon, at tlu> olIuT in a IuukI or |K)lyp. If a pitfi' (1 f> Is rut out and susprndt'*! in watiT a nrw ' " r polj pi-aml/i- 1 at lM)th inds. U r ran thus product- an animal whirii t«Tnil- nat«'s in a head at innh t-ndsof it,s iHKly; wliilf in Vln. 2H an animal was n-pn-smt- roth rnds in a stolon or fiK)t. 1 It was found later independently by lx)Mi Clodlowski and myself that If wo ligature the stem of a Tubularian the polyps at both ends are fornietl slmultam*- 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 thej^ 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 mthout. 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 •J7 osmotic changes are the sources for the energy wliicli is needed to overcome the resistance to growth.^ In order to ascertain whether I could determine wliat are the mechanical causes of growth in animals, I began at Naples some experiments on Tubularia ynesembryanthemum. I chose long stems belonging to the same colony and distributed theui in a series of dishes contaming sea-water of differt^nt concentra- tions. In some of the dishes the concentration had })een 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 certam 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. 33j will give an idea of the dependence of gro\\'th upon the concentration of the sea-water in Tubularia. The values for the amount of sodium chloride, in 100 cubic centimeters of sea-water, are represented on the axis of the abscissa, the valu(»s 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 »h« an acid. I found that acids cause a swelling of nuiscles and It has since boon shown that this is a general plienomeuoii. 98 The Mechai^istic 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 Fig. 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 known pro- cesses, it was necessary to find out whether, in case of growi:h, 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 growth 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 exact h' the case. Ranke had already sho\\Ti that the blood of a tetanizcd 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 found 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 growth of the seed. I tried to ascertain whether segmentation, like growth 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 plays 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 pubhcation 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 inequality 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 simply 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 growth 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 sho\\Ti 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 standpoint. According to him, the different regions of the unsegmented egg correspond alread}' 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 showTi^ 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 in 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-urchin (Arbacia) put into dilute sea-water. The protoplasm swells until the membrane m bursts and part of the protoplasm b flows out; each of the two droplets may develop into a blastula, so that from such an egg two larvae may arise, as indicated in 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 b 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^vn 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 cannot 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 full}' 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 t^'pical a manner that it seems as if there were a prearrangement of some kmd. 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 1 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, "that 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 Pflamenorgane, 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 unkno^vn 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 l^lood- 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 heterocUtus). 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 cells 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 proving 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 possibilities. It is possible that the hypotheti- 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 outgrowth 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 w^e 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 ma}' 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 I 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 solel}' 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 physicist and chemist form new combinations from the elements of non-living nature. VI. ON THE NATURE OF THE PROCESS OF FERTILIZATION VI ON THE NATURE OF THE PROCESS OF FERTILIZATION^ I 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- lized although the ''aura seminahs" 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 l^eheve 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 showTi 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 enzj^mes. 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 womid 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 unfertilized egg of the silkworm can develop parthenogenetically. It was, moreover, known 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 by the experiment. It was not until very recently that such experiments were made. II Eight years ago I observed that if the fertilized eggs of the sea-urchin were put into sea-water Avhose 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 3J 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. Eutwickelungs- mechanik, VIII, 1899. 3 Norman, W. W., " Segmentation of the Nucleus without Segmentation of the Protoplasm," Arch. f. Entwickelungsmechanik, III, 1890. ^ Mead, A., "The Rate of Cell-Division and the Function of the Centrosome," Woods Hole Biol. Led., 1898. 118 The Mechanistic Conception of Life well known, 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. Journ. of Phys., Ill, 1900. 2 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 |n NaCl solution or a V^n MgClg solution. I found, after a number of experiments, that by putting the unfertilized 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 ahve 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 \"-n MgClg 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 fertilized 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 soiu"ces 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^Miing 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 which 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. Journ. of Phys., III. 2 Gemmill, "The Vitality of the Ova and Spermatozoa of Certain Animals," Journ. 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 tlie artificial production of a fertilization membrane, which is descrilx'd 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 lirought 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 \0-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 (tr>T3sine, 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 MgClj 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 enzymes 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 Marking 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 FORIVIATIVE 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 stimuh 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 dra\Mi 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 likewise 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 Medical 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. I 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 ma}^ 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 accomphshed 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: By 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 unfertilized 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 i> 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. II We will begin with a description of the method of artificial parthenogenesis in the egg of the Calif ornian 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. Briefiy .^tated 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 in the sea-urchm egg. Fig. 1 represents the unfertiUzed 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+3 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 bv fattv 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 hypertonic 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 hydrox>'l-ions in the solution. If after this time the eggs are transferred into normal sea-water they develop at room temperature in a way 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 tlian 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 matvu-e. 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, Ijut before the 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 sohition 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 saiKinin. The following figures 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-hoiu-, 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 imdergoes the rapid cvtolvsis 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 reaclud 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 with 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 h^'drocarbons like amylen, benzol, toluol, and in a much lesser degree chloroform, etc. Hertwig had already 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 c}i:olysis 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 without subsequent cytolysis. If such eggs were afterward treated with 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 cytolytic destruction (Figs. 44-47). Bases can also call forth membrane formation, but their action is rather slow and depends on the presence of free oxygen. 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 salicyialdehyde to sea-water. Camera drawings. Fig. 44, unfertilized egg at the beginning of tlie experiment. Fig. 45, membrane formation in the sahcyialdeliyde-sea-water. Fig. 46, beginning of the cytolysis. Fig. 47, cytolysis completed. Tlie cytolyzed egg has in tliis case an entirely different appearance from that of an egg cytolyzed in saponin. of fertilization. Such a temperature kills these eggs almo.st instantly and consequently they are no longer able to develop after this treatment. The eggs of the star-fish Aster ias for- besii 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 cytolysis and are transformed into '' ghosts." We have been able to convince ourselves, therefore, that all the agencies which cause cytolysis 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 cytolysis 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 cytolysis of this cortical layer of the egg and this cytolysis is caused by the spermatozoon. IV We have already mentioned that the cj^tolysis 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 hypothetical substance can be brought about in two ways: first, by treating the egg for a short time with 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.^ V 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 » 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 sul)sequent treatment with 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 butyric 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 butyric acid, since in the formation of the membrane the spermatozoon is thro^^^l 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 possibility 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. VI 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 Sipunculides , 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 var}^ 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 h\'pertonic solu- tion if we wish to call forth a normal development of the egg treated with serum. The lysin of the blood is Uke 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.2 It is curious that SrCl, and Babl, 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 bo precipitated with acetone (Loeb, Pflugers Archiv, CXXIV, 37, I'JOSj. 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 o\^ti 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 coeciun of the sea-urchin, though this was true only 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 own 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 known 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 wliich in artificial i)ar- 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 Calif ornian 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 b\' Biitschli, that the cytoplasm is an emulsion. It would then follow that the membrane formation as well as the c>i:olysis 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 de])ends upon the destruction of the emulsion in the cortical layer of the egg only. The lysin of the egg destroj's 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 cj'tolysis of the whole egg. If we go back to the idea of Blitschli 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 c}i:olytic agencies which destroy only the cortical layer; while all general cytolytic agencies destroy the cortical layer as well as the rest of the egg. IX 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 absori:)tion of water This confirms Loeb's view that meinl)rane formation is caused by the Uquef action 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." X 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 unfertilized 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 unfertihzed 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 fihn 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-wat(T (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 c>i:oplasin and its round shape. The membrane is, therefore, impermeai)le 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 under 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 rnembrane 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. XI 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 cjd^olytic 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 ox>'gen 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 c^'tolysis 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 (Ij'sin) 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 C3i:olysis of the whole egg they can be used only if the eggs are withdra\Mi from their influence after the cortical layer is destroyed but before the rest of the egg has midergone destruction. Cytolysis of the cortical layer leads often but not always to the formation of the membrane of fertilization. Since all cytol>i-ic 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 V VIII THE PREVENTIOX OF THE DEATH OF THE EGG THROUGH THE ACT OF FERTILIZATIOX^ I 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 diiniui>h- ing the rate of oxidations in the egg through the addition of a trace of potassium cyanide. The life-saving action of lack of . » Reprinted from the. Harvey Lectures, 1911, by courtesy of Messrs. J. B. Lippincott & Co. 155 156 The Mechanistic Conception of Life oxygen can be sho^ai in various ways. The maturation of the egg itself depends upon oxidations. If the oxygen 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 ox>^gen; 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 oxygen 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 way 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 fertiUzation saves the hfe 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, we find 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. II 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 rendering the egg immune against the fatal action of oxidations. I have sho^vn 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 butyric 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 unfertilized egg of the sea-urchin consists of two kinds of treatment. The one is a change in the surface of the egg whicli 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-urchm egg with the sperm of star-fish, it takes from ten to fifty minutes to cause the membrane formation in the eggs, the reason being 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 ta 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 into 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 t(jxic substance, or a faulty complex of conditions which has to be abolished, before the oxidations necessary for development can take place without 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 immune against the toxic action of oxidations. Ill Seven years ago I found 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 hj'pertonic 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 permeabiUty 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 -f- 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 IGo 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 IX THE ROLE OF SALTS IN THE PRESERVATION OF LIFE^ I 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 CaCl, in a definite proportion, namely, 100 molecules NaCl, 2.2 molecules KCl, and 1.5 molecules of CaCl2. 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-G5, 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. II 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 Hfe is shortened hy practi- cally the same amount. If the crustaceans are brought into a pure solution of XaCl, of the same osmotic pressure as the sea-water, they also die in about half an hour. If to this solution a Httle 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 CaCU are added to a cane-sugar solution isotonic wdth 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, how^ever, 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\\Ti 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 NaCl, 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 CaCU 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 CaClg 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 Fw?2c?w7ws, therefore, prove that a mixture of NaCl + KCl H-CaClo does not act as a nutritive solution, but as a protective solution. KCl and CaCla 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 CaCl^ 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 by the addition of a Uttle CaCl.. 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 Ring<'r found that the addition of Ca inhibited these twitchings. Since these experiments were made many examples of the antagonistic action of salts have become known. 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 by fact. IV 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 Fundulns. If these eggs are put immediately after fertilization into a pure sodium chloride solution which is isotonic with sea-water, they usually 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 in 2 NaCl solution, the toxicity of the solution is dimmished or even abolished. Even salts which are very poisonous, namely, salts of Ba, Zn, Pb, Ko, Ni, Mn, 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 without 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 fertilization. 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 ZnSOj 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 Ni, etc. It is also possible that similar changes take place in the whole membrane. The i)rocess of rendering the m/2 Na solution harmless for the em})ryo 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 XaCl 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 vnih. the result was shown before by experiments with Na^SO^.) If the eggs of Fundulus are put immediately after fertiliza- tion into distilled water, 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 w^ater 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 varying 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 following 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 ZnSOi 0 100 c.c. m/64 NaCl+8 c.c. m/32 ZnSO^ 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 ZnSO, 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 ZnSOi 37 100 c.c. m/2 NaCl+8 c.c. m/32 ZnSOi 34 100 c.c. 5/8 NaCl+8 c.c. m/32 ZnS04 29 100 c.c. 6/8 NaCH-8 c.c. m/32 ZnSOi 8 100 c.c. 7/8 NaCl+8 c.c. m/32 ZnSOi 6 100 c.c. m NaCl+8 c.c. m/32 ZnSO, 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 comparative!}' 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 ''tanning" 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, CaCl,, 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. V 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 partl\' 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 w^alls. 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 showTi 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^-pertonic solution, disappears again in a time which varies with 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 hypertonic sodium chloride solution, a shrinking of the contents of the cell takes place again, which superficial^ 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 CaClo, 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 XaCl. Osterhout explams this difference in the concentration of the two salts required for plasmolysis by the assumption that NaCl diffuses more rapidly into the cell than CaCla, 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 CaCl2 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 .2m CaCli and in . 38 m XaCl, hut neither in .195 m CaClo nor in .375 m NaCl. On niixin ])ut tlic ferti- lized 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. P'or 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 v/ith 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 i)rotoi)lasm 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 CaClg in definite proportions. Experiments on fresh-water organisms, for which ''adaptation" to a mixture of NaCl, KCl, and CaClg 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 H-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 CaCl^ 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+CaCl2 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 CaCL+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 H-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. X 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 CaCl2) . What causes this difference ? Six years ago I found that the unfertilized eggs of the sea- urchin {Strongylocentrotus purpuratiis) can keep alive and remam 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 CaCl., 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 surromided 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 found 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 \\ithdrawal of oxygen or the addition of KCN or NaCN. Warburg and Meyerhof have drawn the conclusion that in a pure NaCl solution the rate of oxidations of the egg of Strongylocentrotus 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 fertiUzed egg just as well in solutions of NaCl+CaCl^ 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. XI 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 X 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 knowTi that the majority of marine animals lay their unfertilized eggs in the ocean and that the males shed their sperm also into the sea-w^ater. 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 belief. As a rule all or the majority of individuals of a species in a given region spa^\Tl on the same day, and when this occurs the sea-water constitutes a veritable suspension of sperm. It has recently been shown 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 bcconics ()})vious when we bring together mature eggs and equally mature antl active sperm of different families. When this is done no ep:g is, as a rule, fertilized. The eggs of a sea-urchin can he fertilized by sperm of their o\vn 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 hydroxyl 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 showTi 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-fi^h, l)rit tie- stars, and holothurians; while in normal sea-water or with less sodium hydroxide not a single egg of the same female coukl be fertilized with the star-fish sperm which provcil ctTective 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 faima? In other words, are the hybrids between sea-urchin and star-fish, or better still, between sea-urchin and moUusks, 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 mollusks 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 moUusk form 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 moUusk 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 unless 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 accomphsh 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 with 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 cytolytic 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-water for from thirty-five to fifty-five minutes, they develop into larvae. If, however, they are left in the sea-w^ater con- taining the saponin they undergo, a few minutes after membrane formation, the disintegration knowTi 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 cji^olysis is due to a liquefaction of certain fatty or fat-like compounds, the so-called lipoids, in the cell. If this view is correct, it w^ould 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 known that the blood corpuscles, as a rule, midergo 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'pertonic 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 tliis ditTcrence 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 with hyper- 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 cytolytic 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 tlegree of cytolysis which leads to membrane formation, it is i)r()l)able 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 underlying or following membrane formation. The question may be raised whether tlie larvae produced by artificial parthenogenesis can reach the mature stage. This I question may be answered in the affirmative, since Delng(^ 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 change in the chemical constitution of the sea-water. — The reader is probably familiar with the fact that there exist two different tj^pes 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. Twins 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 with 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 fertilized 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 j)()tassium chloride, or of sodium chloride and calcium chloride, or of sodium chloride and magnesium chloride. The e^^s 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 purpui'atus 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 coUoidal substance which seems to serve the purpose of keeping aU 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 mto a neutral mixture of salts in which either KCl, or CaCli, 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 imtU its long axis touches the fertilization membrane. The two daughter-cells formed (Fig. 57) are separated from each other, instead of remaming 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 cells 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 {Fundulm heteroditus) to fuse into a single cyclopean eye through the addition of magnesium chloride to the sea-water. When he added al)out G grams of magnesium chloride to 100 c.c. of sea-water and i)laced 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-ej'-ed condition was found to result from the union or fusion of the ''Anhigen" 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. Tliis 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 iuipossible 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 comj^are 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, 11)07. 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 with 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 showTi 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 Tiefen des Weltmeeres, p. 225, Jena, 1903. Influence of Environment on AxNimals 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 temj)erature 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 a})out 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 surface of the ocean in polar regions is greater than in more temperate regions. The writer beUeves 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 nuist 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 tem]iorato or tropical regions.^ The writer is inclined to believe that these results have some bearing upon a problem which \)\wy^ an imjiortant role in theories of evolution, namely, the cause of natural death. 1 The high coefficient of temperature of the duration of life may possibly only be foxmd near the upper temperature limit for tlie life of organisms. But this is sufficient for otir 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 imderlie development also determine natural death. Physical chemistry has taught us to identify two chemi- cal processes even if only certain of their features are kno^vn. 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 unicellular organisms in general are immortal, while he assumed 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 imlimited 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 Weismann 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 natural 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 found to be immortal. Influence of Environment on AnixMals 211 h) Changes in the color of butterflies produced through the influence of temperature.— The experiments of Dorfnicister, 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 dimori)hism 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 oi tlie same species. By keeping the pupae of Vanessa prorsa several weeks at a temperature of from 0° to 1° Weismami 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 fomid 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 colunm 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 + 3r)» to + 42" to + 10° C. Forms) + 37° C. + 41° C. + 46° C. ichnusoides polaris urticae ichnusa polaris irhmisoidos (nigrita) (nigrita) antigone fischeri 10 fischori antigone (iokaste) (i(jkaste) testudo dixeyi poly- chloros erythrome- ias tlixoyi tpstiulo hygiaea artemis antiopa epione artomis hygiara elymi wiskotti cardui wiskotti elvnii klymene merri- fieldi atalaiita nicrn- fit'ldi klynu'ne weismanni porima prorsa poriina weisnianni 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 ^\dth 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, (5) (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 aljcrrations 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 poh'ps in a hydroid, Eudendrium racemosum, at Woods Hole, may l)e 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 a(iuarium where light strikes them during the day, a regeneration of numerous polyps 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 daylight suffices for this effect. Goldfarb, who repeated these exi)eri- ments, states that an exposure of comparatively short duration is sufficient to produce this effect. It is possible that tlie light favors the formation of substances which are a prere(iuisite 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 body 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 which 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 fomid l)y Keeble and (lam])l(* in other crustaceans, e.g., in Hippolyte cranchii, but the iiifluciiee of the surroundings upon the coloration of tliis 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 exjKTi- ments are so well known that a short reference to them will suffice. 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 photogra])hy by mechanical color adaptation, has ever l)een 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 iu a cell two different non-miscible liquids (or a liquid and a solid) 1 Poulton, E. B., Colours of Animals ("International Sciontillc Sories"). 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 down 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 down 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 down 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 do'\\'nward. 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 (Avhen 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 naturally sensitive, or if they 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 arrangomont in n mnp:notir fi^ld. 'Pliis can be proved very nicely in the case of starving caterpillars of I'or- thesia. The writer put such caterpiUars into a ghiss tube the axis of which was at right angles to the ])lane of the window: the caterpillars went to the window side of the tubi- and remained there, even if leaves of their food plant wcn^ 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 shown 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 tlie following theory of these instinctive reactions. Animals of the type of those mentioned are automatically oriented b}^ the light in such n 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 which the symmetrical muscles i)articipate e(iually in locomotion, the symmetrical muscles work with vq\u\\ 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 une(iually and in positively hehotropic animals those muscles will work with greater energy which brings the plan(^ 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 water 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, viiih. 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 Dujjh/iiac is cuuled and at the same time carbon (Hoxide added, the aiiininls wliieli were before indifferent to Hght now ix'coinc most strikinj^ly positively heliotropic. Marhie eo])ei)ods ean be made posi- tively heliotropic by the lowering of the tem])erature alone, or by a sudden increase in the concentration of the sea-water. These data have a bearing upon the dej^th migration.s of pelagic animals, as was pointed out years ago by Theo. T. Clroom and the writer. It is well kno^^^l that many animals living near the surface of the ocean or fresh-water lakes, have a tendency to migrate upward toward evening and downward in the morning and during the day. These periodic motioiLs are determined to a large extent, if not exclusively. })y the heliotropism of these animals. Since the consumi)tion of carbon dioxide by the green plants ceases toward evening, the tension of this gas in the water must rise and this nmst have the effect of inducing positive heliotropism or increasing its mtensity. At the same time the temperature of the water near the surface is lowered and this also increases the positive heliotroiiism 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 sho\Mi. 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 down or become negatively heliotropic and migrate actively downward. These experiments have also a bearing ui^mi the problem of the inheritance of instincts. The character which is tr:uis- mitted in this case is not the tendency to migrate iK^riodically upward and do^vnward, but the positive heliotropism. Tin- tendency to migrate is the outcome of the fact that lu-riodically varying external conditions induce a jieriodic 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 hehotropic 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 with the heli- otropism of plants. The writer has shown 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 sjTithesis 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 around the vessel, until finally all the chromatophores form a sheath aromid 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 WTiter 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 shown 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 periphery 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 ox ANiMALa 225 the opinion that the formation of the Ijhi.stuia may be cauhcd generally by a tropic reaction of the })lastomeros, tho latter being forced by an outside influence to crecj) to the surface of the egg. These examples may suffice to indicate that tlic arrangement of definite groups of cells and the mor])liol()gical elTects resulting therefrom may be determined by forces lying outside the cells. Since these forces are ubiquitous and constant it a])pears 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 fur 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 yomig larvae. When a piece of meat and of fat of the same animal are placed side by side, the fly will deposit its eggs ujxjn the meat on which the larvae can grow, and not upon the fat, on which they would starve. Here we are dealing witli the effect of a volatile nitrogenous substance which reflexly causes the peri- staltic motions for the laying of the egg in the female Hy. Kammerer has investigated the conditions for the laying of eggs in two forms of salamanders,' e.g., Salamaudra atra and >S. maculosa. In both forms the eggs are fertilized in the body and begin to develop in the uterus. Since there is room only for a few larvae in the uterus, a large number of eggs ])erish 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 yomig ones are bom; 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, many eggs (from twelve to 226 The Mechanistic Conception of Life seventy-two) are produced and the larvae are of course in an early 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 dryness 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 maculosa 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. VII. 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 Ammals 227 certain authors claim to have succeeded in tliis, but the writer wishes to apologize to these authors for liis iuahility to mnvinco himself of the validity of their claims at the present moment. He thinks that only continued breeding of these ai)parent mutants through several generations can afford convinciuK 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 apj^eal to the biologist. We know that growth and development in animals and plants are determmed by definite although complicated series of catenary chemical reactions, which result in the synthesis of a definite compound or group of compounds, namely, nuclei tus. The nucleins have the peculiarity of acting as ferments or enzymes for their owti synthesis. Thus a given type of nucleus will continue to synthesize other nuclein of its own kind. This determines the continuity of a species; since eacli species has, probably, its own specific nuclein or nuclear material. Hut 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 wiiich acts as a ferment for its own synthesis and thus reproduces itself. Nobody lia."^ thus far succeeded in this, although nothing warrants us in taking it for granted that this task is beyond the i)ower of science. 1 Since this was written the beautiful experiment.s of KaimneriT a.s wi«Il 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 tlie egg, 6 If. Agalma, 208. Amphitrite, 144. Animal instincts, experimental control of, 218. Antagonism of acids and salts. 179 ft. Antagonism of three salts. 182 ff. Antagonistic salt action, 172 S. 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 AT., 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. 20H. Cannon. 222. ('citalysis of t'stors. 43. Catalyzer. 4. .'"». "CrnUT of roordhiation," 72. Ceriarithun mfmhranaceus, 93. Chnrtoptrrus, 117. Chan^T in int«'nsity of li«ht. .'>4 fT. Changes in <*olor of hutt^Tlllcs pn>- ducfd through Infliu-nctJ of tenipera- turc, 211. Chemical agencies, effects of. 190 ff. Chemical synnnetry. 3H. 39. Chemotropisin. 224. Chlorostomn, 19.S. Chromosomes, Ki ff. Chun, 20S. Ciamician, 3S. Cinerarias, 37. Ciona intcslinalin, OS, 92. ClaparMe, 51. Cohen, 208. Color, changes hi. of butterflies. through infiuenre of t4'nip' 211; color a(lai)lation. sotf.. color blindness, 1«>. Compulsory movements, 38. Consciousnes.s. 72. Contents of life, 20 ff. Cooke, 99. Cooperative action of salts causing imi)ermeability of the I'gg mem- brane, 176 ff. Coordinated movements in reflexes. 70. Copepod, 43, 02. Correns, 21. Cortical layer, of unfertilized egg. 10. 189; mechanical destruction of. 10. 11. Co.sine law of illumination, 41. Crabs, fiddler .00. CtenolabruK. 2.5. Cumn Rnthkii, .'>1. Cytolysis. of the egg. 140; mivhanical causation of. 145. Cvtolvtic agents. 10. 132 ff.. 130. *144'ff.. 202. De Vries, 22t). Daphnia, 43. 2()S. 213. 220. Darwin. Charles. 195. 1«M1. Darwin, K.. .'>S. Davenport, 222. I)eatii and development. liiffertMit ciieinical proc«>sses. 21K» ff. Delagf, 11, i:U. 203. Depth migration. 220. Development, death anil. dlfftTent chemicul proces.s««s, 20".» IT. Difference of salt iMTineublllty of various nu-mbranj's. Is9 ff. Diffusion of saltti, 177 ff. 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 ff. ; 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. Emulsion theory of membrane forma- tion, 145, 147 ff. Enzymes, 5, 122, 123. Esters, catalysis of, 43. Ethics, 3, 5, 31 ff., 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 ff. Fiddler crabs, 60. Fischer, 211, 212, 213. Fischer, Emil, 115. Fovea centralis, 39. France, 58. Froschl, 29. Fundulus heteroclitus, 25, 105, 172 ff., 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 ff., 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 ff. Hartmann, E. von, 35. Harvey, 189. Heliotropic animals, 41; heliotropic reactions, experiments on the mech- anism of, 218. Heliotropism, 27 ff., 220 ff.; artificial causation of positive, 43, 220; influ- ence of acids on, 42, 220. Henking, 16. Herbst, 92, 134, 186, 187, 204, 207, 224. Heredity, 4, 15 ff., 49, 52, 101. Hertwig, O., 116, 134, 208. Hertwig, R., 7. Heterogeneous hybridization, 24, 25, 196 ff. Heteromorphosis, 85 ff., 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 ff. Hybrids, maternal character of hetero- geneous, 199. Hydra, 93 ff. Hydroids, Experiments on, 217. Hypertonic sea-water, 7, 116 ff., 131 ff. Hypotricha, 55. Illumination, Cosine law of, 41; inten- sity of, 44. Immunity, 142. Immunity of eggs to body extracts of same species, 142. Increased sensitiveness of unfertilized egg to cytolytic agents, 163 ff. 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 ff. Leeuwenhook, 113. Liebig, 115. Life, contents of, 26 ff. " Life principle," 14, 15. Light, change in intensity of, 54 ff. 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, 1.56. Maxwell, S. S., 39, 208. Mead, 7, 117. Mechanical causation of cytoly.sis, 145. ^lechanical destruction of cortical layer, 10, 11. Mechanics of growth in animals, 95. Membrane of fertilization, 8, 148 tf.; 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 ff . Memory, associative, 55, 73. Mendel, 4, 16. 20, 21, 49, 52. 59, 60. 226. Mendel's laws. 4. 15 flf.. 20, 49. 52, 59fT., 226. Menidia, 25. Merrifield, 211. Meyer hof, 190. Miescher, 114, 115. Minkowski, 79. Moenkhaus, 24. Monstrosities, artificial production of, in sea-urchins, 100 ff. Montgomery, 16. Morgan, 7, 17, 19, 21, 92, 117. 119. 216. Morphology, physiological. 109. Movements, compulsory, 38. Munk, 35, 79. Muscles, osmotic phenomena in, 99. Mytilus, 198. Natural selection, 50 ff. Nemec. 58. Nereis, 12. Neuberg, C 38. Norman, 117. Organisms, harmonious character of, 23 ff . Organs, fusion of normally double. 207. Osmotic phenomena in mu-scles, 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, KJOtT.. 164; in their relation to life and death, 14, 15; effect of retarded, on poisonous salt action, 190 ff. Parker. 39. 49. 220. PartlniiojfciH-sis, 117 ff.: artinrial. 7. 1 If. IT . 127 JT . 199 ff Purtliriiok'fiH'tic di'vi iopiiD-iit . varyitiK su.sceplihilit V of t'KKs to. 144. Paull. 99, I HI. Pawlow, (V2. I'm rifiriti, 91 . Peter. 2()S. Pettvnkoft-r. 4. Pfliik'tT. 2 Hi. IMiotcxIu iriieal action of IlKht. .'10. :W. Pliotocli.rniciil. effi-cLs. 27. 3H. 39. substaiiecs. :i9. Pli()tosrii.siti\ »• surfaces. .39. Pholo.sensiliveijes.s. vurvinn. In animah. 45. PhylloTrrn, 19. Physiological morphology. 109. Planarians. 39. .'V4. Pla.smoly.sis, 177. Poi.soning. acid, isff.; salt. ls»iff. Poi.sonous action of di.stitleil wal«T on (in m mtiriis, 1 7l) IT. Polarization, 92 ff. Polyyordius, .5.3. Polynoe. 132, 1.58. Porthfuia chrysorrhoea, 47. 4H. 21H 11.. 222. Potassium cyanidf, action of. 150 ft. Pouchet. so. 214. Poulton. 215. Prevost. 114. Procter, isi. Production of twins from one oRg through a chang*' in the chemical constitution of the sea-wator. 204 (T. Protective .solution. 172. Purposeful cliaraeti-r of reflexi's. 00. Pycnopudin, 19S. Pyrrharoriii, 10. Radl. 49. Ranke. 99. Hayleigh. 147. 152. Reflex, 05 ff. Reflexes, coordinated movements In. 70; purposeful character of. 00. Reinke. 24. " Kiddle of life." 5. Hiiigcr. 172. 173. Robertson. T. B.. 1S5. 20S. 210. Hole of water in segmentatltin. 99. Hoscot-. 27. 29. ;K), 41. 5.S. Kou.\. 24. 210. Rubner, 4. Sachs. 104. 107. SaltimarKira aira, 22.5. Siihimitinlrn mnrulns.i. 22.'). Salt poisoning. 1st. IT. : dlffen-n*-^- of permeabilitv of vurit»us meml>rane»i. 189 ff. ' . - - Salts, antagonism of *nd. 1.9fT: antagonism of tli: s isj fT ; (•ooperati\c action <.f. < pernu-ability of egg i 17(i ff. ; dilTusion of, 177 IT. Sclimiedeberg. 173. .*selio|)«'nhauer, 35. Seiuilt/.i'. 21<".. ^ ..,.-- Sea-wat«r. hypertonic. 7. llOff.. IM n. Segregation, law of. H*>. 20 ff. 232 The Mechanistic Conception of Life Selection, of intensity of light by ani- mals, 52flf.; natural, 50 flf. Sex determination, 16. Shearer, 11. Sipunculides, 141. Snyder, C. D., 39, 208. Solution, protective, 172. Spallanzani, 113. Spermatozoa, 113 fl. Spermatozoon, twofold action of, in fertilization, 12 ff., 132 flf., 161 flf. "Speziflsche Bildungsstoflfe," 104. Spontaneous movements, 71. Spyrogyra, 177 flf., 188. Standfuss, 211. Stereotropism, 91 flf., 223. Stevens, 17. Stieglitz, 43. Stockard, 206, 207. Strongylocentrotus franciscanus, 140, 198. Strongylocentrotus purpuratus, 8, 139 flf., 144, 162, 189, 197, 201, 205. Sumner, 80. Symmetry, chemical, 38, 39. Temperature as a cytolytic agent, 135 flf., 138 flf. Tower. 90, 227. Traube, 96. Tropisms, 26, 36 flf., 54, 55, 60, 62, 69, 70, 72. Tubularia mesembryanthemum, 95, 97. Tunicates, 92. Twin formation, 19, 205. Twins, production of, from egg, 204 flf. Twofold action of spermatozoon in fertilization, 12 flf., 132 flf., 161 flf. TjTosin, 23. Uexkuell. 70. " Unterschiedsempfindlichkeit," 54. Van Duyne, 92. van't Hoflf, 208. Vanessa antiope, 220. Vanessa levana, 211. Vanessa prorsa, 211. Varying photosensitiveness in animals, 45. Varying susceptibility of eggs of dififerent species to parthenogenetic development, 144. Vaucheria, 188. Virchow, 127, 128, Vision, 79 flf. Voit, 4. Volvox, 51, 223. Waller, A. D., 222. Warbiu-g, 13, 157, 160, 189. 190. Wasteneys, 13, 157, 181, 182. Water, distilled, poisonous action of distilled water on, 170 flf. ; role of, in segmentation, 99. Weismann, 91, 210, 211. Wiener, 215. "Will," 35 flf., 40. Wilson, 17, 18, 19. X-chromosomes, 18 flf. "Zielstrebigkeit," 24. Zuntz, 4. niffliiimnHiflff North Carolma State University Libraries' ''''"'7£S,S?.^!::i°'i°LyfEBI0L0GICALESS S02775722 H