The Mechanistic Conception of Life

The essays contained in this volume were written on different 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 viewpoint. 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. IIL Some Fundamental Facts and Conceptions concerning the Comparative Physiology of the Central Nervous System ----^-----65

VII. On the Nature of Formative Stimulation (Artificial Parthenogenesis) - - - - - - - -127 VIII. The Prevention of the Death of the Egg tlii-ough the Act 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 probability to be considered untrue tomorrow. It happens in science, 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 descriptive 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 conditions of the experiment and the biological result (e.g.,

1 Address delivered at the First International Congress of jSIonists at Hamburg, September 10, 1911; reprinted from Popular Science Monthly, January, 1912, by coiu-tesy of Professor J. ZSIcKeen CatteU. Mendel's law of heredity). Biology as far as it is based on these two principles cannot retrogress, but must advance. 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 temperature 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

which accelerate the oxidations at body temperature sufficiently 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. 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 question. 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

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

worms, or even birds, show a tendency to a nuclear or even a cell division; and R. Hertwig, Mead, and Morgan had succeeded 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. — Unfertilized egg of the sea-urchin surrounded by spermatozoa. Only the heads of the spermatozoa are drawn, since at the magnification used the tails were not visible. Fig. 2. — The same egg immediately after the entrance of the spermatozoon. The egg is surrounded by a larger circle, the fertilization membrane, which is formed tlxrough the action of the spermatozoon. This formation of a fertilization membrane can be induced by a purely chemical treatment of the egg.

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

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, Strongylocentrotus purpuratus, I succeeded in finding a method of causing the unfertilized egg to form a membrane without injuring the egg. This method consists in treating the eggs for from one to two minutes with sea-water to which a definite amount of butyric acid (or some other monobasic fatty acid) has been added. If after that time the eggs are brought back into normal sea-water, all form a fertilization membrane in exactly the same way as if a spermatozoon had entered. This membrane formation or rather the modification of the surface of the egg which underlies the membrane formation starts the development. It does not allow it, however, to proceed very far at room temperature. In order to allow the development to go farther it is necessary to submit the eggs after the butyric acid treatment to a second operation. Here we have a choice between two methods. We can either put the eggs for about one half-hour into a hypertonic solution (which contains free oxygen) ; or we can put them for about three hours into sea-water deprived of oxygen. If the eggs are then returned to normal seawater 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 development 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 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.

Figs. 6 and 7. — The sea-urchin egg divided into four and eight cells respectively. 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. — 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 indication 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 superficial 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 destruction of this cortical layer. In the eggs of some animals this cortical layer can be destroyed mechanically by shaking the

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 destruction 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 development 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 physicochemical agencies has been accomplished.

I succeeded in showing that the spermatozoon causes the development of the sea-urchin egg in a way similar to that 1 This method does not work with the eggs of fish and is apparently as limited in its applicability as the causation of development by mechanical agitation. Fig. 10. — Pluteus stage of Strongylocentrotus purpuratus. S skeleton; D intestine. in my method of artificial parthenogenesis; namely, by carrying 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, starfish 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 hypertonic 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 formation resulted without the spermatozoon penetrating into the egg. This membrane formation led only to a beginning but not to a complete development. We may, therefore, conclude 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 hypertonic solution in the sea-urchin egg after the membrane formation.

The question arises as to how the destruction of the cortical 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 spermatozoon causes the development by accelerating the oxidations in the egg. This conclusion was confirmed by experiments by O. Warburg and by Wasteneys and myself in which it was found that through the process of fertilization the velocity of oxidations in the egg is increased to four or six times its original value. Warburg was able to show that the mere causation of the membrane formation by the butyric acid treatment has the same accelerating effect upon the oxidations as fertilization.

What remains unknown at present is the way in which the destruction of the cortical layer of the egg accelerates the oxidations. It is possible that the cortical layer acts like a solid crust and thus prevents the oxygen from reaching the surface of the egg or from penetrating into the latter sufficiently rapidly. The solution of these problems must be reserved for further investigation. 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 completely reduced to a physico-chemical explanation. Considering the youth of experimental biology we have a right to hope that whd.t has been accomplished in this problem will occur in rapid succession in those problems which today still appear as riddles.

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 acceleration 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 cessation 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 beginning 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

individual life is determined by the entrance of a metaphysical ''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.

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 seaurchin, 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 seaurchin 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 concerned, 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, 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 chromosomes 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 preeminently 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 chromosome was connected with the determination of sex. Considering the importance of this idea we may render it in his own words:

A most significant fact, and one upon wliich almost all investigators 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, 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 offspring two sorts of individuals in approximately equal numbers, imder normal conditions, that exhibit marked differences in structure. A careful consideration will suggest that notliing but sexual characters thus divides the members of a species into two well-defined groups, and we are logically forced to the conclusion that the peculiar chromosome 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."