BIOLOGY LIBRARY G MODERN PROBLEMS OF BIOLOGY MI N OT BY THE SAME AUTHOR Embryology. A Laboratory Text-book of Embryology. By CHARLES S. MINOT, S.D., LL.D., Professor of Comparative An- atomy, Harvard University Medical School. Second Edition, Revised. With 262 Illus. xiix402 pages. Cloth, 3.50. "Professor Minot is to be congratulated most warmly on the success, efficiency, thoroughness, and stimulating character of his work." — The Lan- cet, London. "The book is well written and well printed. The illustrations are numerous and well executed." — New York Medical Journal. P. BLAKISTON'S SON & CO. PHILADELPHIA MODERN PROBLEMS OF BIOLOGY LECTURES DELIVERED AT THE UNIVERSITY OF JENA, DECEMBER, 1912 BY CHARLES SEDGWICK MINOT LL. D., YALE TORONTO AND ST. ANDREWS; D. SC., OXFORD; DIRECTOR OF THE ANATOMICAL LABORATORIES, HARVARD MEDICAL SCHOOL; EXCHANGE PROFESSOR. AT THE UNIVER- SITIES OF BERLIN AND JENA, IQI2-I3. WITH FIFTY-THREE ILLUSTRATIONS PHILADELPHIA P. BLAKISTON'S SON & CO. 1012 WALNUT STREET 1913 BIOLOGY LIBRARY G COPYRIGHT, 1913, BY P. BLAKISTON'S SON & Co. THE. MAPLE. PRESS. YORK. PA TO THE UNIVERSITY OF JENA 281435 PREFACE His Royal Highness, the Grand-Duke of Saxe- Weimar, the Rector Magnificentissimus of the University, has graciously pleased, after Professor Eucken of Jena had been called to Harvard University as Exchange Professor, to express the wish that the Harvard Exchange Professor at Berlin this year should lecture also in Jena. This wish was communicated to Harvard University by the Prussian Ministry of Education. After further correspondence the formal invitation was sent me to deliver in Jena the six lectures which appear in printed form in the following pages. It is always a difficult problem to so present new biological discoveries that they will be comprehensible to a mixed public, and yet lose nothing of their scientific value. The reader therefore is requested to exercise a lenient judgment, when he finds that the performance leaves much to be desired. It seemed desirable to consider the first lecture as an introduc- tion which might render it easier for non-biologists to under- stand the following lectures. The researches quoted are chiefly American. This plan was adopted partly because the author was the American Exchange Professor and partly be- cause he was informed that his audience in Jena would like to hear especially about American discoveries. In the short time at command it was impossible to present the evidence for all that was said, and the reader must be begged to pardon the author if many assertions sound like obiter dicta. vii Vlll To their Royal Highnesses, the Grand-duke and the Grand- duchess of Saxony, the author expresses his thanks for their interest and for the great honor of their presence at one of the lectures. He has pleasure in thanking also their Excellencies, the Ministers of Education of Saxe-Weimar-Eisenach, Altenburg and Meiningen, his Magnificency the Prorektor, the Curator of the University and his Colleagues for their encouragement and hospitality, by which the visit to Jena was made very delightful. The lectures were written and have already been published in German by Gustav Fischer in Jena. Professor von Bar- deleben had the great kindness to revise the original manu- script. The translation has been prepared by the author and follows the original closely, though now and then a phrase has been rendered freely. CHARLES S. MINOT. BOSTON. CONTENTS PAGE PREFACE vii 1. THE NEW CELL DOCTRINE i 2. CYTOMORPHOSIS 24 3. THE DOCTRINE OF IMMORTALITY 43 4. THE DEVELOPMENT OF DEATH . . . 59 5. THE DETERMINATION OF SEX 82 6. THE NOTION OF LIFE IX PROBLEMS IN BIOLOGY THE NEW CELL DOCTRINE Your Magnificence! Gentlemen ! To his Royal Highness, the Grand Duke of Saxe- Weimar, I wish to express with the highest respect my sincere thanks for the interest which his Royal Highness has shown in the exchange of professors with America. It is a great honor to be the first Harvard professor to come to you, as the official representative of the American academical world. The University of Jena is as famous and as highly esteemed in America as in Germany. When I consider the reputation of the Jena professors I cannot venture to hope that the lectures I am to deliver will attain that degree of perfection to which you are accustomed. Therefore I request you to consider my lectures as an expression of my sense of obligation. Not merely Harvard University, but the whole United States are grateful to you that you have permitted Professor Eucken to come to us as exchange professor. I owe your Ministry of Education special thanks for the invitation sent me to appear here as the guest of your University. It is always a difficult task so to present scientific conclu- sions that they shall be comprehensible to the public and, at the same time, keep their precision and their scientific value; but when a branch of science has progressed so far that i 2 THE NEW CELL DOCTRINE conclusions of wide bearing can be drawn, it becomes desirable to communicate the results to wider circles. The new achievements of biology are significant and claim the interest of all thinkers, and therefore I have decided to attempt to make clear to you some of our fundamental conclusions. My fellow biologists are requested to excuse the mention of much already known to them. The general conclusions of biology are formed slowly. The phenomena of life are so complicated that they can be analyzed only by the most many-sided investigations. If one wishes completely to master the science one would have to be not only a biologist in the stricter sense, but also a chemist, a physicist and a geologist. It has become impossible for a single investigator of our time to acquire special knowledge in the whole field of biology, and you will certainly not expect from me that I attempt to make clear to you all the funda- mental conclusions of modern biology. Indeed for this, the time at our disposal would not suffice. Therefore I shall permit myself to treat only such questions as I have found occasion to consider often in the course of my special work. We may arrange the subjects to be discussed in the following order : 1. The New Cell Doctrine. 2. Cytomorphosis. 3. The Doctrine of Immortality. 4. The Development of Death. 5. The Determination of Sex. 6. The Notion of Life. You all know something of cells, which have been described often as the units of life. They are small masses of living sub- stance, in each of which lies a smaller body, which is desig- THE NEW CELL DOCTRINE nated as nucleus. The living sub- stance is commonly termed proto- plasm. Unfortunately with the progress of investigation we have become more and more uncertain what we can properly designate as protoplasm. The nucleus is also a living substance, but it is commonly not reckoned as protoplasm. Many authors apply the term protoplasm to the body of the cell, which often has a very complicated structure. Thus we see spaces which we name vacuoles, and which contain only fluid. Such a fluid is usually not considered part of the protoplasm. More frequently we find special en- closures, granules, etc., which reveal FIG. i. — Two blood cells from an embryo duck. The protoplasm has a uniform constitution and contains the centrosome. In the rounded nucleus the material is irregularly distributed and forms a larger mass of chromatine. The cells have been artificially colored. — After M, Heidenhain. FIG. 2. — Two plant cells from the vegetative point of a Phanerogam, a, younger; 6, older stage; k, nucleus; v, sap space; cy, protoplasm. — • After Strassburger. an entirely different constitution from the rest of the mass, which one is inclined to name protoplasm in the stricter 4 THE NEW CELL DOCTRINE sense. A further difficulty arises from the observation that in the nucleus also a substance can be found with peculiari- ties like the protoplasm. Thus it happens that with the enlargement of our knowledge we have become more and more uncertain what we can properly designate with this word "protoplasm." It corresponds better to the present condition of science if we say that a cell consists of nucleus and a cell body, because we thus restate clearly our direct observation. Nevertheless a biologist would hardly like to lay aside the word protoplasm, in part because it has such a great historic significance. As is known, cells were discovered by the botanists, and first by the Englishman Hook, and they received from botan- ists the name cell, which is completely suitable for the form first observed, for in many plants one sees the cells as small spaces, which are separated from one another by partitions. These spaces were designated simply as cells. Later it was rec- ognized that the essential thing was not the arrangement of the partitions, but the content of each cell. This content is protoplasm mixed with water and containing a nucleus. Two eminent German investigators have furnished us with a completely new conception of tne cell. Wilhelm Kiihne and Max Schulze have proven that the partitions are unessential and that we may have a complete cell without them. Thus a new conception arose, namely, that a cell consists of proto- plasm and nucleus. The great English biologist, Huxley, who appreciated the importance of the new views of Kiihne and Schulze, has presented them in a lecture to which he gave the title, "The Physical Basis of Life." Huxley's presentation is so clear and comprehensible that his readers cannot fail to ap- preciate the full significance of the views presented. Huxley's lecture occasioned great excitement among thinkers in Eng- THE NEW CELL DOCTRINE 5 land and America, and also on the European Continent. Everybody discussed at that time the question whether proto- plasm was really the physical basis of life or not. The solution of this problem we have not fully reached even yet. The de- scription of the cell which we owe to Max Schulze dominates everywhere and yet with the progress of science it has be- come insufficient. The size of the cell is of the greatest significance to biologists. Cells for the most part are rather small, and the size is ex- tremely variable. The cells of the human body, according to an estimate I have made, have an average diameter of perhaps 0.014 mm. Variations, however, are considerable; some cells, like the blood-corpuscles, are very small; certain nerve cells, on the other hand, attain a considerable size. The largest cells of all, known to us at present, are eggs. Those of certain animals appear as true giants in comparison with other cells. The largest eggs occur in birds. The entire yolk of the bird's egg corresponds to but a single cell. The albumen which surrounds the yolk and the shell do not belong to this cell, but are simply layers which are added by the oviduct to the egg proper, and which are secretions of the glands of the oviduct. Of all the animals now living the ostrich has the largest egg and the yolk of the ostrich egg is certainly the largest living cell known to us. These enormously enlarged eggs might be described as the monsters of the cellular world. They are ex- ceptions. By far the majority of cells are of such dimensions that they are visible with the microscope alone. The smallest organisms which we know are the vegetable germs, which may have a diameter of not more than one-tenth of a millimeter. As is well known to all, certain of these smallest organisms cause diseases which may be extremely dangerous to man. The investigators of infectious diseases have made the inter- THF NEW CELL DOCTRINE esting discovery that there are disease causers which are in- visible even with the microscope. In recent years we hear more and more of the so-called invisible organisms. In re- gard to this we must express our opinion with reservation, for it is by no means demonstrated that we have to deal in this case with actually living organisms. It is possible that we have to do only with chemical ferments. We have not time, how- ever, to enter upon this discussion. For the present at least we must hold to the opinion that vital phenomena can appear only when the amount of living substance is so great that it can be seen with the microscope. In other words, the minimum quantity of chem- ical substance which can serve as the basis of life is many times greater than the minimum quantity of substance which suffices for a chemical reaction. Here we encounter a fundamental char- acteristic of life To permit the activ- ities which are characteristic for life to go on we must bring together many sub- stances which stand in very special relations to one another. Hence the assertion that life is only possible when these conditions are fulfilled, and this requires that the total amount should be so much that we can see it with the microscope. Cells have been considered for a long time as independent bodies. Quite slowly this view has been changing. Many years ago botanists made the observation that vegetable cells may be united by fine threads of living substance. Similar relations have been observed in animals. In the sev- FIG. 3. — Drawing to show the size of bacteria. Magnification 1000 (i mm. of the picture =0.001). Ay smallest bacilli (influ- enza); 5, streptococcus gracilis (round); C, largest cocci; D, pus cocci; E, ba- cillus megatherium; F, red blood corpuscle; G, splenic fever bacillus. — After H. Jaeger. THE NEW CELL DOCTRINE enties of the last century J. Heitz- mann, a Viennese physician who had emigrated to New York, affirmed that cells are not defi- nitely separated from one another. He advanced the statement that protoplasm is con- tinuous and has scattered nuclei. The opinions ex- pressed by Heitz- mann1 remained in their time almost without notice. Very gradually his view met with wider acceptance. The botanist Sachs has contributed much to develop our interpretation. For the zoologists the writings of the American Whit- man2 have been of the greatest im- portance. Whit- 17 « d •+* 3 O bO 11 8 THE NEW CELL DOCTRINE man and many others have greatly advanced the recogni- tion of the actual relations. We know now that when an ovum begins its development it must be regarded as a complete cell. This cell divides, the process being usually termed the segmentation of the ovum. When the ovum divides there arise two new cells which then divide again. If we investigate the relations of such cells in vertebrates we may observe without difficulty that the cells are com- pletely isolated from one another. They have no direct communication between themselves. They live alongside one another, but the living substance of one cell is nowise united with the living substances of the neighbor cell. In the course of the further development, however, the relation changes because the cells begin to unite with one another. This occurs chiefly in two ways. Consequently we obtain two kinds of tissues which we regard as the primitive tissues of the body, since from them all the tissues of the adult are slowly Ba FIG. 5. — Epithelium (epidermis) of a chicken embryo of the second day of incu- bation. The nuclei are mostly oval and lie scattered. The protoplasm forms a network. There are no intercellular partitions present. Eph, epitrichial layer; Ba, basal layer. differentiated. In one form we find the cells completely fused with one another and they build a continuous layer which we designate as epithelium. In such a primitive epithelium, Fig. 5, there are no limits between the single cells, but on the contrary one has a continuous layer of protoplasm in which the nuclei are scattered, though generally rather close to- THE NEW CELL DOCTRINE gether. When such an epithelium grows the nuclei multiply by division which is in itself a complicated process. The pro- FIG. 6. — Mesenchyma of a chicken embryo of the third day of incubation. Every nucleus is surrounded by a thin layer of protoplasm from which run out the strands that form the intercellular network. Cell boundaries are not present. toplasm also grows. We have in this case, therefore, a sub- stance which, though living, does not, strictly speaking, con- schl.y FIG. 7. — Adult epithelium. Epidermis of Lumbricus venetra. jchl. z, mucous cells; Cu, cuticula; J.z., cylinder cells; m.f, muscle fibers — below the epithelium. The single cells are separated by partition walls from one another. — After M. Heidenhain. sist of cells. The second form of tissue is called mesenchyma. In mesenchyma, Fig. 6, one observes nuclei which are found 10 THE NEW CELL DOCTRINE at more or less regular distances from one another, and also protoplasm which forms an open network. The meshes of this net contain a fluid, which is usually not interpreted as a part of the tissue proper, just as the fluids, for example, which we find in the articular cavities or in the body cavity of the adult are not reckoned as tissues of the body. In vertebrates, in which the protoplasm of the network of the-mesenchyma has been chiefly studied, we find that the network is at first extremely irregular; but early, as development progresses, the protoplasm accumulates in parts around the single nuclei and ; ',* S ft FIG. 8. — Hyaline cartilage of a human embryo. Between the cells the firm basal substance of the cartilage is developed in large quantities. — After J . Sabotta. X 280. forms, so to speak, a court of protoplasm around every nucleus. From each court radiate the threads of protoplasm, which establish the connection with the neighboring courts, and thus the mesenchyma remains a network still. These two forms of tissue, which are characteristic for the connection or fusion of cells, we call syncytium. On tracing the development THE NEW CELL DOCTRINE II further we learn that alterations occur so that we can observe the progressive separation of the single cells; thus, for example, in epithelium there arise partition walls, Fig. 7, separating the cells finally and completely from one another. In mesen- chyma the connections may become interrupted by which the protoplasmic masses around the single nuclei are joined together, Fig. 8. In this way the cells become completely isolated. When we encounter cells which have been separated in this way we have to do not with a primitive but with a secondary condition. The descriptions just given lead us to one of the chief con- clusions of the new cell doctrine. We have learned that the relations are much more complicated than was previously assumed. We turn to the discussion of protoplasm, or, as we have termed it before, of the cell body. It is necessary to direct at- tention to the fact that in the living world we know two chief types of cells; first, such cells as exist alone, the so-called uni- cellular organisms. Of such cells there are very many species which have been grouped into numerous genera. Each genus and each species has its special peculiarities which we learn chiefly through the microscope. When a cell of any of the just-mentioned species is observed for a longer period few al- terations in its structure can be observed. The chief changes we can observe are, first, an enlargement of the cell, and sec- ond, the inner alterations which are usually specially notice- able in the nucleus, which lead gradually to the division of the cell. The two new daughter cells remain extremely similar to the original mother cell in all peculiarities. Such an organism propagates itself in this manner endlessly and without essen- tially changing its structure. Very different are the conditions in the second type of 12 THE NEW CELL DOCTRINE cells, which we find only in the multicellular organisms, that is, in the higher plants and animals. In them we observe different cells which take over the function of propagation. In the case of animals such cells are called ova and spermatozoa. A sper- matozoon unites with an ovum, which we then designate as fertilized. A fertilized ovum is a complete cell which divides and continues dividing until the number of cells for the con- struction of an animal body has been produced. This number may be enormous. The ovum, or egg-cell, proliferates by division precisely as does the cell of a unicellular organism. The cells of the latter do not change, but the cells which arise from the ovum do change. The cells of the multicellular or- ganisms through several or many early generations retain a relatively similar structure, but later there follows a transform- ation which with the succeeding generations progresses, and at the same time becomes multifarious. In this manner the tis- sues of the adult arise gradually and in accordance with fixed laws. In consequence of these conditions it has come about that we have derived our conception of protoplasm and in part also of the nucleus chiefly from studies which investigators have made on the developing ova, for in the early generations of these cells we have relatively simple relations. Fortunately, however, there occur among the unicellular organisms species which are comparatively simple in structure, and which are therefore favorable for the study of protoplasm. If we wish to summarize the result of numerous investigations in brief form we may say that we have learned to recognize three conditions of protoplasm; that is, one condition of which we know as yet little, but which is of 'the greatest significance and which is characterized by the fact that the protoplasm appears to us under the miscroscope absolutely homogeneous. THE NEW CELL DOCTRINE 13 Homogeneous protoplasm is of the greatest rarity and as yet has been studied chiefly by the American, E. B. Wilson.3 It claims our highest interest because it represents apparently the simplest condition of the living substance which we know. In the second state we find the protoplasm consists of two fluids which exhibit a foam structure, that is to say, the two fluids are so mixed together that one, apparently the more fluid, forms droplets and the other holds these droplets apart and separates them from one another com- pletely. As is well known, Professor Butschli has specially studied protoplasm in this condition, and has founded the theory, which he has further defended, that we encounter in FIG. 9. — Striated muscle fibers of a rabbit, colored by Bielschowski's method and then teased so as to demonstrate the single muscle nbrillse. — After a preparation of Prof . Poll's. this foam structure the essential true fundamental structure of living substance. For this view much may be said. Whether, however, we may assume that protoplasm, which is apparently homogeneous, also really possesses a foam structure, although it escapes our present observation, must remain undecided. In its third condition protoplasm is no longer simple because new structures have arisen in it which are probably also living, but which differ from protoplasm THE NEW CELL DOCTRINE in appearance and behavior; thus, for example, if we study the development of muscles, we find at first cells with the usual so-called undifferentiated protoplasm. In this appear fine fibers which we name fibrillae, and which are no longer simple protoplasm, but really something new, Fig. 9. These fibrils effect the contraction of the muscle. They develop themselves, clearly in order to take over this special function of the muscle cells. Accordingly we designate the third condition of pro- toplasm as the differentiated. We must now turn to a consider- ation of the nucleus. It appears in the majority of cases as a body with definite limits, completely surrounded by protoplasm and with special sub- stances in its interior. Usually one can distinguish without difficulty a network, and in the meshes of this network the nuclear sap. The net- work varies extraordinarily in the single nucleus, but has one striking peculiarity, namely, that it may be easily artificially colored. On account of this peculiarity the substance has been named chromatin. Nucleus differs in one respect very noticeably from protoplasm, for the nuclei develop no new structures comparable to those which we may observe in protoplasm. A nucleus, to be sure, changes during the development of tissues more or less, but we cannot observe new structures in the nuclei. This fact is of special significance for the considerations which are to be presented in the next following lecture. For this reason attention is now directed to this peculiarity of the nucleus. FIG. 10. — A vesting nucleus after ordinary preservation and staining with iron hasma- toxyline. From a cell of the intestinal epithelium of a sala- mander.— After M. Heiden- hain. Magnified 2300. THE NEW CELL DOCTRINE 15 Although the nucleus changes comparatively little during the progressive division of the cell, yet during the division of the cell matters are very different, for during every cell H«sa*w "?M^W *-» ** ™- t&> • cleus passes through wonderful t r a nsf ormations. * During these transform ations the sharp limits of the nucleus disap- pear, and the nu- clear substance gathers together in small masses to which we apply the name chromo- somes. Each chromosome di- vides, and one piece of each chromosome tributes to formation of one of FIG. ii — Red blood cells of an embryo duck in vari- ous stages of division. The pictures show the origin, COn- division and migration of the chromosomes, the spindle, the t^ie reconst^tutlon °f tne daughter nuclei after the division of the cell bodies. — After M. Heidenhain. Magnified 2300. the new nuclei; the other piece to the formation of the other nucleus. This process may now be found exactly described in the text-books. Every * Reference to the so-called direct division of cells, or amitosis, is intentionally omitted. This form of division is rare, and the consideration of it is unessential for our present purposes. 1 6 THE NEW CELL DOCTRINE student of medicine, or of biology, has opportunity in his prac- tical laboratory work to see for himself the formations of the dividing nucleus, and I may therefore allow myself to omit a detailed description of this phenomenon. But there is something else I should like to say to you concerning the nuclei. It is now established that the nucleus has an entirely different chemical composition from the protoplasm. In protoplasm and in nucleus we have to do cheifly with proteids, for they are the chief components of both structures. The proteids in the nucleus are, however, in certain respects simpler than those in protoplasm. For this and other reasons, it is believed that the nutritive material must first reach the nucleus in order to be worked over in the nucleus and to be later returned from the nucleus to the protoplasm. The chemical relations between the nucleus and the protoplasm are of the greatest significance. I must ask you to consider that I am not a competent biological chemist. In recent years chemical biology has made many beautiful and im- portant discoveries. It is understood that we must seek the explanation of most vital phenomena in the chemical altera- tions which occur in the body. If we should ever get so far as completely to understand life it will be only when chemists are in a position to explain vital phenomena chemically. We incline to the belief that the nucleus is absolutely necessary to the functions of life. It is besides instructive to learn that in certain lower organisms, in which we can distinguish no definite nucleus, such as we usually observe, nevertheless nuclear substance occurs scattered in the protoplasm. From such observations we draw the conclusion that for the main- tenance of life it is necessary to have not only the complicated protoplasm, but also the presence of the differently compli- cated nuclear substance. We cannot hope to reach a basis THE NEW CELL DOCTRINE for the explanation of life until we shall know how the chemical alterations go on in the living substance, which is a highly complicated mixture of many organic combina- tions of various sorts, all carried by great quantities of water. A good example of the complica- tion of the phe- nomena is offered us by the condition of the nucleus in certain unicellular organisms. In the cells of the highest plants and ani- mals the nucleus is always a simple unit, but there are many species of protozoa known in which the nucleus is double, so that there appear to be two nuclei of un- equal size, Fig. 12. FIG. 12. — A unicellular animal, an infusorium (Nassala T, i r _j. elegans). Natural length o.i mm. 9, Macronucleus; 10, micro nucleus. — After Schewiakojf, from Lang's Covered that the VergUichende Anatomic. larger nucleus plays a role in the nutrition and growth of the cell while the smaller nucleus has assumed exclusively the functions which lead to the division of the cell. Nature makes here for us an experiment in that she has separated in space the 7 1 8 THE NEW CELL DOCTRINE two functions of the nucleus, which are usually carried out by a single unitary nucleus. Vital phenomena rest on chemical processes by which energy is set free to show itself through the activities of the living being. The first thing which the beginner learns is that chemical change, or metabolism, plays the chief role in all biological phenomena. The biologists describe the intake and excre- tion of the nutritive material, and attempt to trace the change to which this material is subjected in the cell. Cells possess, of course, no mouth. They can absorb material only through their surfaces. Therefore the surface of every cell is of the utmost importance for the continuation of its life, and the investigation of this surface and its tension has been eagerly pursued of late. Important results have already been pro- duced; as, for example, it has been discovered that the surface tension during the impregnation of the ovum must be changed if the spermatozoon is to enter, and after the spermatozoon is in the interior of the egg the surface tension is again changed. The gifted German- American investigator, Jacques Loeb,4 has advanced the hypothesis that the egg has a superficial layer of lipoid substance which at the time of impregnation passes into a soluble condition. This hypothesis has since been confirmed by the experiments of Ralph L. Lillie.5 The egg of the sea urchin, after remaining some time in sea water, becomes more resistant so that the spermatozoon cannot penetrate the eggs as easily as when they were fresh. If such resistant eggs are treated with sea water, to which one has added 0.3 per cent, of ether, by which supposedly lipoid substances are dissolved, it is found that the eggs are more easily fertilized. But even if Loeb's hypothesis is not ab- solutely correct, the phenomenon itself remains extremely THE NEW CELL DOCTRINE significant because we must assume that almost incredibly small quantities of material occasion alterations in the ovum. -Ky ,.'••> FIG. 13. — Muscle nuclei of the giant salamander (Necturus) in various stages. A, nucleus of 7 mm. larva before differentiation; B, from a 26 mm. larva at the beginning of differentiation; C, from the adult animal, 23 mm. long, after comple- tion of the differentiation. — After A. C. Eycleshymer. So soon as one spermatozoon penetrates the ovum, as is found in the case of most eggs, no spermatozoa can follow. If we 20 THE NEW CELL DOCTRINE study the phenomena with the microscope we are unable to observe that anything is given off from the spermatozoon to the ovum. The changes in the ovum, therefore, by which other spermatozoa are excluded depend upon minimum quantities. Teleology, or the adaptation to an end, rules all living bodies. Accordingly we must assume a priori that the limited size of cells is a purposeful adaptation. It is probable that the size of the cells is favorable to the metabolism which occurs chiefly in protoplasm. It depends on the one side upon the surface of the cell, and on the other upon the nucleus, which must itself be nourished and also supply material to the cell body. Therefore it is important that the distances remain small. As an example of the relation of the nucleus to the differentiation of protoplasm, I wish to cite the investi- gations of Eycleshymer6 on the development of muscle fibers. The work was done in my laboratory. He observed that the mass of chromatin increases in the nucleus of very young muscle fibers, and that thereafter the formation of the muscle fibrils begins. As the development of the fibrils progresses, the amount of chromatin in the nuclei diminishes, Fig. 13. It is clear that the chemical combinations are distrib- uted through the protoplasm chiefly by diffusion, a slow process. Hence the great importance of the small dis- tances. A more exact conception of this we may gain from the investigations on the early development of pigeons, which have been carried out at the University of Chicago, at the suggestions of Professor Whitman.7 The egg of the pigeon, like most other eggs, is fertilized by a single spermato- zoon. The influence of this does not at first stretch very far in the ovum, so that the territory which we may designate as saturated is small. All around this territory we have, so to THE NEW CELL DOCTRINE 21 speak, non-saturated protoplasm, into which a number of spermatozoa make their way and maintain themselves for some time, disappearing, however, in a few hours, and ap- parently in the same measure as the influence of the fertiliza- tion proper expands. In animals, which have relatively small eggs, the whole becomes more rapidly saturated by fertilization, so that only one spermatozoon can go in. We spoke before of the great influence of small quan- tities upon the protoplasm. It is certainly the greatest ad- vance of modern physiology that we have become better acquainted with the significance of this phenomenon. We have here to consider especially a new kind of action at a distance which takes place constantly in our own bodies. When, forty years ago, I made my first physiological experi- ments, the nervous system was the only means known to us to effect action at a distance within the animal body. We studied industriously nerve fibers, sensations in the brain, and the stimuli which passed from the central nervous system to the various organs of the body. Since then we have dis- covered the phenomenon of so-called internal secretion. The glands form secretions which are further used in the body. The majority of glands have a duct which carries off the se- cretion; thus, for example, in the case of the liver we have the ductus hepaticus which conducts the secretion of the liver to the intestinal canal. It is known now, however, that there are glands which have no duct, Fig. 14. Nevertheless, these form secretions which are delivered immediately to the blood and then are distributed by means of the circulation through the entire body. It has been learned that each internal secre- tion, which is formed in very small quantities, exerts a sur- prisingly great influence on other parts of the body which may be quite remote from the gland. I may mention as in- 22 THE NEW CELL DOCTRINE ternal secretions the products of the thyroid gland, the hy- pophysis and the suprarenal bodies. The thyroid gland in- fluences the condition of the muscles; the hypophysis, the growth of bones; and the suprarenal capsule the activity of nerves. In passing it should be remarked that the phenom- ena are not simple, but complicated. In all cases, however we see that many cells of one kind depend as to their structure jr. FIG. 14. — Section of a thyroid gland. The organ consists of closed cavities, each of which is bordered by a layer of epithelial cells. Since the gland has no duct, the secretion can be carried off only by the blood. — After Koelliker. and their activity upon the influence of these internal secre- tions. It is not the case of a single cell, but always of many which have the same constitution. The brilliant investigations of Ehrlich and others have founded the new doctrine of immunity. In this case we have to do with the phenomenon similar to that of in- ternal secretion. An animal becomes poisoned by patho- genic organisms, and then forms itself a contra-poison, or so- THE NEW CELL DOCTRINE 23 called antitoxin. That the toxins and antitoxins occur has been demonstrated with certainty, but the quantities are so small that we have not yet succeeded in isolating them. From these and other similar phenomena we learn that the condition, composition and structure of the living substance is of fundamental significance, and is, strictly speaking, more important for the comprehension of vital phenomena than the fact that the physical basis of life shows a strong tendency to form cells. We may now put into words the deduction which we may draw from to-day's lecture. Our conclusions may be ex- pressed as follows: The new cell doctrine still recognizes the importance and significance of cells. Cells remain the units of morphology, but from the physiological standpoint they appear as adap- tations which, especially by their size and proportions, create favorable conditions for metabolism. The living substance is more important to biologists than its tendency to form cells. Hence we consider the chief problem of biology to be the investigation of the structure and chemical composition not of cells, but of the living substance. The new conception has won its way gradually. It corresponds to so fundamental a change of our views that we are justified in .describing the new conception as the new cell doctrine. II. CYTOMORPHOSIS.* Your Magnificence! Gentlemen! We endeavored in yesterday's lecture to familiarize our- selves with the new cell doctrine, according to which a much greater importance is attributed to the composition of the living substance than to the fact that this substance has a strong tendency to form cells; all the same, cells remain the most convenient units of biological research, although they can by no means be found always completely separated from one another. But even if the cells are not separated, it is practical and convenient to designate each nucleus, together with its surrounding protoplasm, as a cell. Every fully formed tissue of the animal body has at least one character- istic kind of cells, or in other words the cells of a tissue exhibit among themselves similar relations and similar structure. Hence we can direct our attention to the single cell which we value as the paradigma. In man, as in the great majority of multicellular ani- mals, development begins with simple cells which arise by the segmentation of the ovum. From the simple cells the tissues of the adult develop gradually. As I told you yesterday, we * The term cytomorphosis was proposed by me in 1901. The corresponding conception- was first definitely propounded in the Middleton Goldsmith Lecture, published in 1901. This lecture has recently appeared in the German translation in my book "Die Methode der Wissenschaft" (Gustav Fischer, Jena). My book "The Problem of Age, Growth and Death" (New York, Putnam's, 1908), treats of cytomorphosis in some detail, although in somewhat popular form. 24 CYTOMORPHOSIS 25 observe no similar developmental processes in unicellular organisms. The transformation of cells which leads to the formation of tissues is designated, as differentiation. In the earliest stages of the embryo the cells are remarkably like one another, Fig. 15, but in the course of their further develop- ment they become unlike or different; hence the designation differentiation. How these differentiations arise is an ex- tremely interesting question about which we know very little, because as yet we have become acquainted almost FIG. 15. — Section through the posterior part of a rabbit embryo of seven and a half days, to show the three germ layers, each of which consists of undifferentiated cells. Magnification 250. exclusively only with such alterations as are visible with the microscope. The visible alterations, however, we must assume, are the consequence of chemical processes which we still have to discover. The visible alterations have been studied with the utmost care by many eminent biologists, and we are able to say that they follow strict laws. It is convenient to have for the complete transformation of cells a short, scientific term. As such I propose "cytomofphvsis" We are now to occupy ourselves with the laws of cytomor- phosis so far as these have been determined. The develop- ment of simple cells into differentiated we call progressive 26 CYTOMORPHOSIS development. The first question which we have to answer is: Does a regressive development also occur? The pro- gressive is well known to us and we know much about it. I incline strongly to the opinion that it is the only kind of development, but there are not lacking investigators who have come to the belief that under certain conditions develop- ment may be reversed. My point of view is determined in part by the fact that it has been possible in cases where a regressive development had been assumed to make sure by careful investigation that opinion had been misled by appearances and that in reality the development was progressive in these cases also. I may mention three examples; first, the nerve fibers. If one cuts through a nerve, the fibers in its peripheral part degenerate quite rapidly. After several days, however, under favorable conditions, newly formed nerve fibers appear in the peripheral part. Many investigators have eagerly advanced the view that these nerve fibers rise in their place and that they have been newly formed in the degenerating nerve. More careful research has made it certain that the newly formed fibers have simply grown out upon the ends of the healthy fibers, left in the central part of the nerve. If one cuts off the roots of a tree, the roots which are separated from the trunk decay; but if the tree is left one can find later in their place living roots, which, however, have not arisen from the dying roots, but have grown out from the central healthy parts. The fundamental experiments of Harrison8 make it sure that nerve fibers in all cases are formed only in the way mentioned. About the origin of nerve fibers there has been a long controversy. My countryman, Harrison, has occupied himself for several years with this question, and has supported his conclusion by the most varied investigations. Four CYTOMORPHOSIS 27 years ago he invented a new method to keep isolated cells and pieces of tissue living in vitro. Utilizing the new method, he subjected young nerve cells, neuroblasts, to observation and was able to see under the microscope nerve fibers grow out from the living cell. Cultures in vitro are now made frequently, and we expect from the application of Harrison's ingenious method many valuable discoveries. From time to time we find the paradox justified which says: "New methods are more important for science than new thoughts.77 > 7 ^ I ] Jt / 1 ' 'fi FIG. 1 6. — Degenerating muscle fibers after experimental injury, a, b, after 3 days; c, after 8 days; d, 26 days; e, 10 days; /, 21 days; g, 43 days.— 4f/er Erws* Ziegler. The second example we get from muscles. If the fibers of a skeletal muscle are mechanically injured they degenerate quickly; later, however, we find new formed muscles. Here the processes are of quite a peculiar sort. Every muscle fiber consists chiefly of muscular substance which we can easily demonstrate by the contractile fibrils. It is the 28 CYTOMORPHOSIS muscular substance which breaks down after the injury. The muscle fibers, however, contain also the so-called muscle corpuscles, which are nothing more than little accumulations of undifferentiated protoplasm, containing the nucleus, Fig. 16. After the injury these corpuscles do not degenerate, so that undifferentiated protoplasm remains from which the new formation starts. The differentiated part of the muscle disappears and there is in this case no question of a regressive development. FIG. 17. — ^Longitudinal section of the regenerating extremity of a young lobster one day after amputation. There is formed at first a blood clot (bd) under which the epithelial cells e, e', grow across to form the commencement of the new part. Magnification 240. — After V. E. Emmel. The third example we will take from the lobster. If the extremities of the larvae of this animal are* cut off, the ex- tremities will be newly formed. It was formerly assumed that we had to do in such a case with a new regressive develop- ment. The investigation made by Emmel 10 in my laboratory has rendered the real history clear. The cells of the outer- most layer of the skin in these larvae are undifferentiated cells, which after the injury grow and spread over the wounded surface. They then multiply and by their steady growth CYTOMORPHOSIS 29 create the new extremity. Afterward they differentiate themselves in part in order to form the various tissues which are characteristic for the limbs of Crustacia, Fig. 17. The nerves and probably the blood-vessels penetrate subsequently into the newly formed extremity. To conclude: Until it is shown in at least one case with absolute certainty that regressive development occurs it must remain very improbable in the minds of earnest biologists that such a development occurs at all, or can occur. Cytomorphosis defines comprehensively all structural relations which cells or successive generations of cells undergo. It includes the entire period from the undifferentiated stage to the death of the cell. The differentiations which occur in the body are very different among themselves, and as is well known these differences are much greater in the higher than in the lower animals. Hence it is by no means easy to recognize at once what is common to these changes, but some important results have already been won. First of all it is to be stated that the differentiation in all cases shows itself by visible new functioning structures in the protoplasm. There exists here between the protoplasm and the nucleus a marked contrast, for, as you have learned, the nucleus acquires, strictly speaking, no new structures, although it also changes with the progressive development. We know that the visible alterations in protoplasm are initiated by invisible ones. Various experiments afford the proof of this. The first rudiment of the fore-leg of the larva of an amphibian may be cut off and then grafted into another part of the body, where the rudiment will develop further.11 The rudiment, or anlage, at the stage which is specially suited to this experiment, is a little bud on the surface of the larva. Microscopic examination shows that 3O CYTOMORPHOSIS its cells are simple and more or less similar to one another.^ Tissues in the stricter sense are not present. In spite of the fact that these cells attain their further development under unnatural conditions they in themselves form muscle fibers, connective tissue and bone. In spite of the fact that the microscope shows us nothing in these cells by which we can recognize their future development, we must assume that the specification already exists. Professor Harrison, as I have already mentioned, devised a method to cultivate tissues in vitro. One can cut out from an embryo chick little pieces at will and cultivate them artifically in vitro and bring them to further development. In this manner W. H. Lewis has succeeded in studying the specific cell formation. The cells of the mesenchyma grow in the manner of mesenchyma ; the cells of epithelium as epithelium. Neither in the nucleus nor in the protoplasm in these cells can we demonstrate peculiarities which we can regard as the causes of the unlike- ness of their growth, but surely there exist in these cells peculiarities which are not visible to us and which determine the performances of the cells. It is not going too far to assume that in all cases the invisible alterations of protoplasm precede the visible. The young cells in an undifferentiated vertebrate embryo have little protoplasm. The first thing that must happen is that the protoplasm grows, a phenomenon which one may easily observe with the microscope. After the protoplasm has grown, differentiation proper may begin. It is always gradual and consists essentially in this, that something new becomes visible in the protoplasm. In part, especially in the so-called epithelium, we have to do with the formation of superficial membranes around each cell. More important probably are the new formations in the protoplasm, Fig. 18. CYTOMORPHOSIS 31 The developing nerve fibrils I have already mentioned. In nerve cells there appear very fine fibers which develop grad- ually, making a network in the cell, Fig. 19. There also appear deposits of a substance which reacts to stains differently from the protoplasm and the fibrils, Fig. 18, k, k'. The deposits in question have received the somewhat fantastic name of "tig- Ax FIG. 18. — Motor nerve cells from the spinal cord of a rabbit, ke, nucleus; den, dendrite; Ax, nerve fiber, and x, its origin; k, kf, Nissl bodies. — After K. C. Schneider. roid substance." We notice also peculiar cavities which form a net- work in the protoplasm of the cell, and are filled with fluid. In the gland cells one sees the material distributed in the protoplasm which is utilized later for the execution of the specific activities of the gland cells, Fig. 20 This material is not the secretion proper, but a primary stage. In quite another wise do the intervening supporting tissues develop, for in them the cells show a strong tendency to separate from one another and to produce special structures in the inter- 32 CYTOMORPHOSIS cellular spaces. It is not practicable to lay further illustra- tions before you. Progressive development is closely connected with another phenomenon. The embryonic tissues grow with immense rapidity, the differen- tiated tissues on the contrary grow slowly. If we investigate the conditions more care- fully we learn that the cells gradually lose the power of division as they are differen- tiated. If the differ- entiation progresses far, then probably the capacity of division is lost to the cells alto- gether. Formerly we had no exact concep- tion of the rapidity of growth in embryos. This is a question about which I have been greatly interested for many years. In the book " The Prob- lem of Age, Growth and Death," which I published in 1908, I FIG. 19. — Nerve cell from the spinal cord of man. The Nissl bodies have been dissolved out and the cell so colored that the neurofibrils are brought out. fi, fibrils; x, fibrils in a dendrite; ax, nerve fiber; lu, space left by the dissolving of the Nissl bodies; ke, nucleus. — From K. C. Schneider, after Bethe. CYTOMORPHOSIS 33 have discussed more fully the alterations of the rapidity of growth with age and its relation to the increase of differentia- tion. The development of a mammal begins with an extra power of growth. How gradual the increase is it is not yet possible to determine exactly, but certainly the original daily increase is not less than 1000 per cent. This holds true for man also. Immediately after birth one finds the highest rapidity in the rabbit to be not quite 18 per cent, per day; in the chick not quite 9, and in the schs.i guinea pig about 51/2 per cent. The relations for man are similar. It is therefore clear that the animals mentioned and man also have lost at the time of their birth 99 per cent, of their original growth capacity. In fact, from the biological stand- point we are really old by the time we are born and the alterations which make us old have for the most part already occurred. The further losses which we suffer from birth to old age are comparatively small, and we live long only because these losses take place slowly. If the progress of alteration after birth should be even only approximately as swift as before birth we should live only a very short time. And in fact the microscope shows us that the multiplication of cells after birth is by no means so great as before, and that it goes on slowly. ke FIG. 20. — Cell from the pancreas of the larva of Salamandra maculosa. sec. k, sec. kf, secretory granules; x, formative focus of the same; fi, secretory fibrils; ke, nucleus; schs. i, closing plate. — After K. C. Schneider. 34 CYTOMORPHOSIS Cytomorphosis includes more than differentiation proper. By continuing it leads to the degeneration of the cell. De- generation appears in many cases to depend upon the trans- formation of the entire protoplasm so that no more true protoplasm remains in the cell. Under such conditions the cells do not remain viable. A good example of this process is afforded by the epidermis, the outer skin, the lowest layer of which consists of undifferentiated cells, which can grow and multiply, Fig. 22. Some of these cells liberate themselves from their parent layer and migrate toward the surface. During their migration their protoplasm is gradually changed into horny substance, and when this change is complete the cells have completed their cytomorphosis and are dead. The surface of our body is covered by dead cells. In this case as in all similar cases degeneration leads to the death of the cell. We can accordingly distinguish four chief stages of cytomor- phosis. 1. Undifferentiated or embryonic condition. 2. Differentiation. 3. Degeneration. 4. Death. Only in this succession can alterations of cytomorphosis occur, but it must be added that if regressive development should occur it would form an exception to this rule. The red blood- corpuscles afford us an excellent example of a complete cytomorphosis. They begin their development as simple cells, with a well formed nucleus but little proto- plasm. Next we observe that the protoplasm grows. Not until it has grown sufficiently does it acquire its character- istic color through the formation of hemoglobin, thus be- coming a young red blood-corpuscle which may still grow a CYTOMORPHOSIS 35 little, although the nucleus at the same time begins to grow smaller. After the nucleus has become considerably smaller it is separated from the body of the corpuscle. As to how this separation occurs authorities are still disputing. The part left without the nucleus is the so-called mature blood- corpuscle which, however, is not able to maintain its own but soon breaks down. Every day in each of us numberless millions of blood-corpuscles are disappearing. The car- tilaginous cells also pass through a complete cytomorphosis which, when the cartilage is replaced by bone, terminates with the dramatic disappearance of the cells. Cartilage is devel- oped from embryonic mesenchymal cells. The cells enlarge and there appears between them the basal substance which imparts to cartilage its characteristic physical consistency, Fig. 8. The so-called ossification of cartilage begins with the completion of the chondral cytomorphosis, during which the cells pass through rapid degenerative hypertrophy, Fig. 21, which involves the destruction of the basal substance, and which closes with the disintegration, or autolysis, of the cell. Thereupon bone is; formed in the place of the cartilage which has disappeared. ( The nerve cells, at least in vertebrates, pass through their cytomorphosis in a special tempo. Their differentiation advances quite early to the high point at which the cells long remain. The degenerative alterations follow very slowly, so that we usually do not encounter mental weakness in man until advanced age, the weakness being caused by senile atrophy of the brain cells. We are in- debted to the peculiar course of the cytomorphosis of the brain for the extraordinarily long-lasting functional capacity of this organ. The leaves of plants offer us an excellent example of cytomorphosis. The leaf bud consists of em- bryonic cells which grow and differentiate themselves to CYTOMORPHOSIS form the leaf. Later the cells degenerate and die. The leaf becomes dead and falls. It would be easy to lay before -Pa FIG. 21. FIG. 22. FIG. 21. — Cytomorphosis of the cartilage cells. From a section of the vertebral arch of a pig embryo, a-e, successive stages; in e, the letters kn refer to the limit of the cavity which is no longer filled by the degenerating cell. FIG. 22. — Epidermis from the sole of the domestic cat. Ba. Schi, basal germ layer; Hor. La, hor. z, hor. z', horny layer formed by dead cells; ker. k, layer of cells which are being cornified; Ml. La, middle layer of cells which are migrating upward and at the same time enlarging; Pa, site of a hypodermic papilla. — After K. A . Schnei- der. CYTOMORPHOSIS 37 you many other examples of completed cytomorphosis of the most various cells. It might, however, be better to pass over to other considerations. Death and subsequent removal of cells play a great role in our lives. Even in early developmental stages we find cells dying and even whole organs which maintain themselves only for a certain period and then disappear almost or completely. Thus there is an embryonic kidney in which only small re- mains can be found in the adult. It is therefore clear that there must be some arrangement provided to make good the loss of cells. Nature accomplishes this by not bringing all cells to further development and by preserving a stock of less differentiated cells in the body. Of these I have already mentioned an example, the epidermis, the cells of the under layer of which preserve an embryonic character. Only by the presence of these cells which keep the essential embryonic type is the continual renewal of the epidermis made possible. For every hair there remains a special group of embryonic cells upon the hair papilla, which provide for the growth of the hair. Since these cells are not differentiated, they can multi- ply and thus furnish cells for the formation of the hair. The cells of the hair complete their cytomorphosis, but their sister cells remain on the papillae undifferentiated. We thus see that while cytomorphosis can go on only in the one direction, it remains true that the cytomorphosis can be arrested and that it may go on in the different tissues with unequal rapidity. Thus it comes that we encounter cells in the adult animal in every possible stage of cytomorphosis. There arise in every one of us every day cells which complete their cytomorphosis, and there are others which have hardly begun it. We cannot understand the relations in the adult animal if we do not consider at once both the daily dying off of old cells and 38 CYTOMORPHOSIS also the daily multiplication of cells which have remained embryonic. We recognize that the embryonic cells are of great impor- tance not only during the embryonic period, but also in the adult. How great this importance is is revealed in the inves- tigation of regeneration. Very many animals, if parts of their body are removed, will form the missing parts anew. If, for example, we break off the tip of the tail of a lizard, there will arise a new tip which is formed by the growth of undif- ferentiated tissues. There are worms which multiply by forming in the middle of their bodies the so-called budding zone. Karl Semper13 has studied the process in annelids, and discovered that in them the budding zone consists of cells of the embryonic type. Gradually these cells advance in their cytomorphosis, and so there arises a new tail for the anterior part of the worm and a new head for the posterior part, and thereupon the two parts separate and two complete works have arisen from the single animal. We are accustomed to designate those animals which have a more complicated structure as the higher. Now it is clear that if an animal is composed of relatively few cells great complexity of structure is impossible. Further we observe that when a highly formed animal is to be produced, nature takes care that a large number of embryonic cells is produced. In the lower animals development is of the so-called larval type. From the little ovum there arises quickly a young animal which lives free and must take care of itself. Such a larva must possess, even if only in simple form, all the principal organs, and since the cells must be so far differentiated that they can take over the various functions, they necessarily lose in part their capacity to multiply, and, what is still more important, the capacity to produce other kinds of cells. We see always that when CYTOMORPHOSIS 39 a cell has begun to develop in one direction it cannot start out to develop in any other direction. In the higher animals, on the contrary, we find a relatively large egg which has become large through the storing up in it of yolk or nutritive material. The developing ovum can nourish itself for a long time from this yolk. In this type of development we encounter not larvae but embryos which are characterized thereby that they contain many cells of the embryonic or undifferentiated type. These cells assume definite groupings to form the rudiments or anlages of the various organs. So that we may say that the anatomical development progresses without there being a corresponding alteration in the structure of the single cells. Thus we observe in the human embryo the stomach, or other organ, which shows the essential characteristics of its total form and of its relations to other parts of the body, and yet consists of cells not differentiated. In my opinion we are justified in regarding the embryonic development as a con- trivance to make the postponement of a cytomorphosis pos- sible, in order that the total number of cells available for differentiation shall be larger. Of the great importance of the number of cells we can get some notion by considering the cortex of the brain. The number of pyramidal cells in the cerebral cortex of man is over 4,000,000,000. This number is not astonishing; a cubic millimeter of blood contains be- tween four and five million corpuscles. The purpose of differentiation is known. Every living cell certainly carries on all the essential functions of life. In the higher organisms we encounter a division of labor. Each organ takes over as its special task one or another function, which the organ performs to the advantage of the whole. These functions are not new; they are always such as are common to the living substance in general, and in 40 CYTOMORPHOSIS each single organ there comes about, so to speak, an exaggera- tion of a single function. Protoplasm is sensitive and irritable. In our case, our sense organs take care of the sensations to the advantage of the whole body. Protoplasm has contractility, and this function is assumed by the muscles again to the advantage of the whole body. Similarly, the glands take over the formation of secretions — the excretory organs, the elimination of urea, etc. Now we know that the various structures which we can see in protoplasm, and which are characteristic for the sense organs, muscles, gland cells, etc., determine in each case the special performances of their respective cells. Briefly expressed, the whole meaning of differentiation is physiological. The peculiarities which we can recognize with the microscope in differentiated cells exist in order to render it possible for the cells to accomplish their special activity. It would be superfluous to linger over this conception, to amplify, or even to justify it by a rounda- bout demonstration. I wish, however, to specially emphasize the fact that the entire doctrine of cytomorphosis renders it clear that structure in living substance is the essential thing. This has become clear to us from the phenomenon of differentiation. We may probably go still further and say that even in those cases in which we as yet cannot recognize any microscopic structure, structure is still present. The conception of the significance of structure — of organization, which we win from the investigation of differentiated cells, applies also to protoplasm. It is well known, as I have already mentioned, that protoplasm is chemically extremely complicated, but the chemical combinations are not simply mixed together as in a simple solution, but are in part sepa- rated spatially. When we state that the living substance has organization we base our view not only on the application CYTOMORPHOSIS 41 of that notion of structure which we derive from the study of differentiated cells, but also on direct observation. Such investigation has not yet brought us very far. It teaches us that protoplasm is not completely uniform, but usually contains fine granules which are unlike among themselves. Micro-chemistry is a nascent science from which we may expect much, although she has presented us yet with but little. It is the science which investigates the chemical substances and processes in cells with the help of the microscope. We have already succeeded in proving that granules, chromidia, fatty substances, lipoids and various proteids exist in protoplasm in a visible form. We have also learned through micro-chemistry something of the distribution of iron and phosphorus in the cell. We have not yet got very far, but far enough to be justified in saying that the organization of living substance is known in part by direct observation. We are acquainted with another structure in the proto- plasm of many cells, the so-called centrosome which we can only allude to here, although its occurrence again demon- strates the importance of organization. A word more concerning the nucleus. In the nucleus organization can be observed easily and without exception, and since the nucleus also belongs with the living substance, its peculiarities also serve to strengthen us in the belief in the importance of organization. Whoever knows the won- derful history of the chromosomes by his own observation, must be convinced that the nucleus has a very complicated organization. Now to the conclusion. Cytomorphosis is the funda- mental conception of the entire development of all multi- cellular organisms, and is the foundation at once of morph- ology and physiology. It explains to us many processes 42 CYTOMORPHOSIS which we otherwise could not understand. It includes the whole doctrine of the normal and pathological differentiation of cells. The principal conclusion which we may deduce from this doctrine is that all living substance possesses an organiza- tion, and that probably without organization life is impossible. III. THE DOCTRINE OF IMMORTALITY. Your Royal Highnesses! To your Royal Highnesses I wish to express my profound and respectful thanks for the honor of your presence, which has for me a great and unforgetable significance. The partici- pation of your Royal Highnesses in to-day's lecture is a high distinction not only for me but for my university, which we gratefully acknowledge. Everything living arises only from the living. The phe- nomenon of propagation of animals and of plants has always excited the interest of mankind. The ancients recognized that only living parents could have a living progeny, and it was said "Omne vivum ex vivo." But for a long time the opinion prevailed that life might continually arise anew. We know now, however, with certainty that a new generation of this kind does not occur, and assume that under present con- ditions a new generation of life is improbable, perhaps impos- sible. We know too little to venture a positive opinion. Schaefer,15 the gifted physiologist of Edinburgh, has expressed a supposition that new generation still occurs upon our earth and escapes our observation because we do not know the con- ditions which render such generation possible. This is an interesting speculation, but with this possible exception we must attribute to the saying, " omne vimim ex vivo" absolute validity. With the progress of our knowledge we have made interest- ing discoveries concerning the manner in which the uninter- 43 44 THE DOCTRINE OF IMMORTALITY rupted continuation of the living substance is assured in various organisms. The simplest cases occur in the lower organisms, in bacteria, etc., in unicellular plants and ani- mals. In these the single individual, or the single cell, grows up to a certain size and then divides. In this man- ner the two daughter cells come to have part of the same substance as the mother cell, and so it goes on. This substance, so far as we can observe, does not change essentially with time. In the higher plants and animals we have in each case to do with many cells and we observe that the functions are unequally distributed among these cells. For the execution of various functions the cells become unlike among themselves. This is the phenomenon of dif- ferentiation of which we have already spoken. The majority of the cells are destined for the care of the whole, and perform their special functions. Some of the cells, however, are not utilized in this manner, but serve for propagation. When a flower unfolds in our garden we find in it certain special cells which have to do with the propagation. These do not show such differentiation as we may find in other cells of the plant, but remain at first relatively simple in their structure. The propagating cells mentioned separate themselves from the mother plant and form the seed. As essential in this case it appears that two cells are necessary for the process, one of which we designate as the egg cell and the other as the seminal cell. Two such cells unite and form a new cell, with which the further development begins. The mother plant may then die. We note in this case that the fate of the cells is extremely unlike, in that some of them are given over to death, while others remain permanently alive and serve for the propagation of the species. In the next lecture, in which we shall investigate the development of death, we shall occupy THE DOCTRINE OF IMMORTALITY 45 ourselves with the consideration of the phenomenon of death. At present we shall devote our attention to the propagating of cells. The kind of propagation which we find in the plant is called sexual and occurs also in animals. It is, however, by no means necessary that the propagation should occur by sexual means. Of the methods which nature applies for the multiplication of living individuals, I should like to mention a few to you briefly. Many methods of asexual reproduction are known to us. The art of increasing plants in this way is practiced by every gardener, and nature also makes use of the possibilities. Among animals we often find a multiplication of individuals effected by simple division. The zoologist describes to us the column-like growth of certain jelly-fish and the following transverse division of the column, so that a number of discs arise, each of which becomes a jelly-fish. Asexual reproduc- tion occurs among invertebrates in various forms. The pecu- liar division of certain annelids has been already mentioned. The budding zone is formed, and produces a new head and a new tail. In a parasitic tapeworm we have discovered a vesicular stage in the life cycle. At certain spots upon the wall of the vesicle arise new heads, each of which initiates the formation of a new tapeworm. Specially interesting are the cases of precocious division, which we have learned about recently, and in which we encounter the division of an egg before the embryo proper has developed. Thus Kleinenberg16 observed in certain earthworms that two individuals develop regularly from one egg, an observation which has been con- firmed by the American investigator, E. B. Wilson.17 Still more remarkable are the occurrences in certain parasitic hymenoptera, in which not merely two but many individuals 46 THE DOCTRINE OF IMMORTALITY are created from a single egg. This phenomenon is termed polyembryony. It was surprising to discover recently that polyembryony occurs in a mammal. In the year 1885 Von Jhering observed that the armadillo regularly produces four embryos in one sac, and he expressed the supposition that they arise from a single ovum. Professor Patterson18 of the Uni- versity of Texas has studied the phenomenon carefully in a species which occurs in Texas. The development of ordinary mammals begins with the formation of a small vesicle. At one pole of this vesicle there accumulate a small number of cells in which no differentiation is recognizable. The accumula- tion is termed the germinal disc and produces the embryo. Patterson obtained eggs of the armadillo in- the vesicular stage, and found upon each vesicle four distinct germs discs. Each disc forms an embryo. Thus it becomes certain that in this mammal four embryos, always of the same sex, arise from one ovum. It is also possible to cause artificial polyembryony with certain eggs. When an egg begins its development, it divides and when the egg is small the division usually produces two cells alike in size. Driesch19 was the first to make the interest- ing experiment so to shake an egg in the two-called stage that the two cells were separated from one another. Under favorable conditions each of the separated cells forms an embryo. The original experiment was made with the eggs of sea urchins. The artificial polyembryos do not attain a nor- mal size, and therefore do not develop quite as do the natural embryos. The experiments of Driesch have been repeated by many Americans and much extended, and indeed with such eagerness that for a certain period we ^termed our embryolo- gists " egg Shakers. " You know probably that the Shakers are a Quaker sect, dedicated to celibacy. THE DOCTRINE OF IMMORTALITY 47 A special form of division is budding, which plays an im- portant role, especially among the hydroids. The process is described in all text-books, and need therefore be mentioned merely. A little superficial group of cells begins to grow and forms finally a new polyp. In the cases considered thus far, a number of cells partici- pate in the propagation. In the case of the so-called partheno- genesis the creation of a new individual starts from a single cell. This cell is an egg, which develops without being fer- tilized. Great interest was excited by the discovery of artifi- cial parthenogenesis by A. C. Mead.20 In the artificial devel- opment we utilize chemical action which replaces fertiliza- tion proper, and so excites the ovum that it develops further. In all these cases the propagation is effected by the separa- tion of living material from the body of a living individual. The separated substance remains continuously alive. The substance may be comprised of many, several, or only one cell. The number of cells is unessential; essential is only that the substance is alive and remains alive. The separated substance inherits the primitive organiza- tion, or, more exactly expressed, has the parental organiza- tion, because it is unaltered parental substance. We come up against a question which we unfortunately cannot yet answer : How is the organization regulated ? It seems a matter of indifference how the asexual propagation is accomplished. Each time the development proceeds, until the original organization is completed. When the budding zone of anne- lids forms a new tail in the anterior part of the animal and a new head for the posterior part, we can only say that a regula- tion of the organization is shown. There is no means for determining more exactly the process. It seems to be clear that this regulation is not to be sought only in the developing cells 48 THE DOCTRINE OF IMMORTALITY themselves but also in part at least in an influence exerted by the rest of the body. In the case of polyembryony, the rudi- ment, or anlage, possesses the capacity of forming all tissues and organs. During regeneration also, which in many animals may go very far, we see that the complete structure is produced anew and we recognize here again the phenomenon which we call regulation. The physiological explanation of regulation we do not yet possess, although we have learned already a little concerning it. The sexual propagation plays a greater role than the asexual, and is often the exclusive method of progagation, especially in the higher plants and animals. We learned in yesterday's lecture that the cells of the animal body dif- ferentiate themselves, that is to say, that their protoplasm acquires new qualities and that their power of division diminishes. Differentiated cells are not suited for propaga- tion. If it should occur that all the cells of an animal or a plant should pass through a complete cytomorphosis, they would all die off, the organism would reach its end, and could produce no progeny. As a matter of fact, however, all the cells do not become differentiated. Of the undifferentiated cells, the necessary number in each species is reserved for the formation of the sexual cells. In phanerogams we find undifferentiated cells in the buds. When the bud forms a flower and sexual cells are developed in connection with it, we learn that some of these undifferentiated cells are made use of. It is entirely unknown to us how the transformation of undifferentiated cells into sexual cells is caused. We can observe with the microscope alterations in the structures of the cells, but the cause of these alterations remains hidden from us. In lower animals we find relations which to a certain extent resemble those prevailing in the phanerogams, THE DOCTRINE OF IMMORTALITY 49 since in them also there occur slightly differentiated cells which are applied for the formation of sexual cells. The other cells, which constitute by far the majority, we name the somatic cells, and therefore say that every animal body consists of many somatic and a few sexual cells. It we pass from the lower to the higher animals, we find that the separa- Germ-CellG FIG. 23. — Section through the posterior part of an embryo of the dog-fish, Squalus acanthias. Germ cells designates the group of sexual cells which have united in one group, which still lies far from the position of the future sexual gland. Ect, ectoderm ; Md, spinal cord; Nch, axis of the body (notochord); Mes, mesoderm; Ent, entoderm; Yolk, yolk-mass. tion of the two classes of cells, the names of which we have just heard, becomes sharper. We have succeeded recently in observing the precocious separation, or isolation, of the sex cells in vertebrates. Their number is very small in propor- tion to the number of somatic cells. In the young embryo 5o THE DOCTRINE OF IMMORTALITY of the dog-fish there lies at either side in the neighborhood of the developing intestinal canala group of cells, Fig. 23, germ Chrysemys Arch FIG. 24. cells, which resemble one another closely, and which may be easily distinguished from the other cells of the body. They THE DOCTRINE OF IMMORTALITY Lepidosteus Lepidosteus Roof End. Sub- germ Cav Sub-cerm. End., \Periph. End. \Vit End. FIG. 24. — Diagrams to show the migration of sexual cells in four different verte- brates. Arch, primitive intestine (archenteron) ; Int, intestine; Lat. Mes, lateral mesoderm; Mes, mesothelium, or wall of the body cavity; Meson, embryonic kidney (mesonephros); Myo, anlage of the muscle (myotome); Noto, primitive axis (noto- chord); S.C, sexual cells in migration; W.D, renal, or Wolffian duct. — After Bennett M. Allen. 52 THE DOCTRINE OF IMMORTALITY are the sexual cells and they accomplish during the later development a wonderful migration, for they move through the wall of the digestive canal and then through the mesen- tery until they reach the spot where the sexual gland arises. We known this interesting history through the investigations of F. A. Woods,21 which were made in my laboratory. For- merly one assumed that the sexual cells arose in the gland, but this is probably not the case in any vertebrate. Another American, B. M. Allen,22 has greatly enlarged our knowledge of the history of the sex cells in vertebrates. By the re- searches of this investigator, we now know that also in the turtle, the frog, and in two fishes, Amia and Lepidosteus, the sexual cells may be recognized very early. They lie at first far from the sexual gland into which they later migrate. The paths which these cells take during their migration differ for the species mentioned, Fig. 24. Several European investigators have also occupied themselves with the history of the sexual cells in vertebrates. In spite of the fact that much remains to be cleared up, we may nevertheless assert that vertebrates have special germinal paths, as they are called. In other words sexual cells are held apart. They pass through their development by themselves and have nothing in common with the somatic cells. They do not participate in the structure of the body, but remain almost like guests which are cared for by the other cells. When the proper time comes the sexual cells change themselves, as the case may be, into male or female elements. Since we know the history of these cells exactly in several cases, we are able to assert that in sexual as in asexual propagation the living substance continues uninterruptedly. This continuation up to the origin of the sexual elements we have actually ob- served. THE DOCTRINE OF IMMORTALITY 53 In insects also a special germinal path has been discovered. The small egg of these animals is usually oval in form. The French anatomist, Charles Robin, reported in 1862 that a special group of cells appears soon after the conclusion of the segmentation of the ovum. Balbiani showed twenty years later that these pole cells, which are not to be confused with the so-called polar globules or directive corpuscles, afterward Bl, FIG. 25. — Preparations from the egg of a beetle, Leptinotarsa. A, the whole egg after completion of segmentation, at the posterior end one sees the accumulation of the superficial sexual cells, p.z, Xso; B, two cells, X85o; bl.c, ordinary somatic or blastodermiccell; p.c, sexual cell (pole cell). C, section through an egg, Xio5',Bl, blastodermic layer of somatic cells; p.c, sexual cells which migrate into the interior of the egg, in order to enter the sexual gland proper. — After R. W . Hegner. pass into the sexual gland. The investigation of R. W. Heg- ner23 of Wisconsin University offers us the most exact account of the history of these cells which we possess as yet. From his paper the pictures in Fig. 25 have been taken. The pole cells of Robin are sexual cells which separate precociously from the somatic cells, and after they have completed their migration, change in the sexual gland into sexual elements. We know for animals as for plants a physiological cause 54 THE DOCTRINE OF IMMORTALITY for the remarkable alterations which produce from a sexual cell, as the case may be, an ovum or a spermatozoon. In the fifth lecture we shall return to the consideration of the visible alterations during this transformation. Let us now assume that we have eggs and spermatozoa, and occupy ourselves with their further history. Science has acquired correct notions of ' these elements very gradually. A hundred years have not yet passed since the publication of the discovery of the eggs of mammals by Carl Ernst von Baer. Eighty years ago one considered the spermatozoa as parasites, although they had been known since 1628. The investiga- tions of Koelliker first demonstrated the true significance of spermatoza. That the semen acted to fertilize ova has been long known, but so long as one did not know the male and female sexual elements of the higher animals one could have no clear conception of reproduction. During the period of ignorance all sorts of wonderful theories arose, which, how- ever, had no value because precisely that which they should explain was, in its essentials, unknown. We must express a warning against theories of this sort, because even to-day we are much inclined to make up for lacking knowledge by theories. It was not until the seventies of the previous century that it became possible to understand the role of sexual elements in reproduction through the epoch-making investigations of the gifted Oskar Hertwig. Hertwig was at that time Privatdozent in Jena, and I rejoice that it is permitted me to express here the admiration which all biolo- gists bestowed on his discovery. Hertwig showed that fertilization consists essentially in the union of one spermato- zoon with one ovum. Since the ovum is very large in pro- portion to the male element we are accustomed to describe this union as the penetration of the spermatozoon into the THE DOCTRINE OF IMMORTALITY 55 ovum. Her twig investigated various species of eggs and observed the same fundamental phenomena in them all. Out of the head of the entering spermatozoon there arises a nucleus-like structure or pronucleus. Before or during impregnation the nucleus of the ovum loses a portion of its contents by a process which we call the phenomenon of maturation. The part of the nucleus of the ovum which remains forms the female pronucleus. The two pronuclei unite and form a new complete nucleus. The fertilization is now accomplished, and further development begins. The fertilized ovum divides, and so does also the so-called segmen- tation nucleus, which owes its origin to the fusion of the two pronuclei. We see therefore that substances from the mater- nal side and from the paternal side are employed for the act of propagation. A new individual obtains its life from both parents. In this case also the history is uninterrupted. W. H. Moenkhaus,25 Professor in the University of Indiana, has furnished us the most brilliant proof of the accuracy of the assertion just made. He reared the hybrids of two fishes, Menidia and Fundulus. The chromosomes of Menidiaare noticeably smaller than those of Fundulus. In the hybrids Moenkhaus discovered both forms of chromo- somes appearing clearly at the time of cell division. This extremely interesting case teaches us by direct observation that living substance from both parents propagates itself in the progeny in visible form. At the beginning of today's lecture we cited the Latin saying " omne vivum ex vivo." It required the prolonged researches of many investigators to reveal to us the ways which living substance adopts in order to continue without a break. The relations may be easily recognized in asexual reproduction, but in the case of sexual reproduction we must 56 THE DOCTRINE OF IMMORTALITY ascertain the history of the sexual cells, the occurrence of sexual elements in all animals, and the internal processes during fertilization, in order to establish the necessary founda- tion for the modern doctrine of immortality. From the numerous researches made, we draw the safe conclusion that living beings consist of protoplasm and nucleus which have arisen from earlier living protoplasm and earlier living nuclei. The animals and plants of today exist only because protoplasm in itself is immortal. Only when protoplasm changes itself or is destroyed by external influences does it die. To us the verse "omne vivum ex vivo" means the immortality of protoplasm. This fact procures us a better insight into heredity. It is well known to us all that every living species maintains itself with slight alteration. This phenomenon signifies to us that protoplasm possesses the capacity, when supplied with food material, to produce more protoplasm of the same constitution as itself. We can offer no further explanation of this wonderful capacity. For us it is merely a fact which; however, offers us a theory of heredity, namely that the progeny are similar to their parents because they are developed from the same protoplasm. The creation of a new generation appears to us merely as the continuation of the activity and growth of the previous generations. There has been no lack of theories of heredity. The best of the older theories in my opinion is that of Darwin, which he termed "pangenesis." He assumed that the cells give off little granules or atoms which circulate freely through the whole body and which, when they are supplied with the proper nutrition, multiply themselves by division and then may later develop into cells. Darwin for the sake of clearness has named these granules "cell gemmules," or THE DOCTRINE OF IMMORTALITY 57 simply "gemmules." He assumed that they pass over from the parents to the descendants, and usually develop themselves in the first generation. Darwin's pangenesis explains heredity. It is the hypothesis of a master, and as a succinct and comprehensive explanation of the facts of heredity must always command admiration. Since Darwin's time many modifications of the doctrine of pangenesis have been proposed. These modifications, however, possess for us merely historical interest, for with the progress of science they have become superfluous. The new doctrine of heredity is due to Professor Moritz Nussbaum, who laid special stress upon the discovery of the germinal paths in animals, for he recognized in these an arrangement to separate special germinal cells from the somatic cells. He concluded that a portion of the germ-plasm is withheld from the developing ovum, kept comparatively unaltered, and employed for the formation of sexual elements, so as to become directly the germ-plasm of a new generation. It is clearly superfluous to still employ the expression germ- plasm which corresponds to speculative needs, and which we may now leave out of consideration. It is simpler to speak merely of living substance. Nussbaum's theory has in the course of time become, strictly speaking, the only theory of heredity which we value. If the time at our disposal permitted, it would be interesting to analyze carefully some of the theories of heredity which have arisen in association with Nussbaum's doctrine. The majority of these theories search for a special germ-plasm, to use Weissmann's expression. Nageli speaks of idioplasm. Some authorities have sought to bring heredity into relation with visible parts of the protoplasm or of the nucleus. Oskar Hertwig was the first to interpret the nucleus as the organ of 58 THE DOCTRINE OF IMMORTALITY heredity, a view which many eminent investigators have since defended. We must today admit that the nucleus plays a part in heredity, but not an exclusive role. The investiga- tions of two Americans, Conklin26 and Lillie,27 furnish the proof that in certain cases distinct regions can be distin- guished in the protoplasm of the undeveloped ovum. When the development proceeds each of these regions plays a special role in the formation of the body. It is possible to alter the normal distribution of the substances, which are character- istic for the regions, without killing the ovum. This is accomplished by the centrifuge. Conklin has succeeded in observing in centrifuged eggs that the substances, which have acquired a new position in the ovum, nevertheless form the same structures as before. From these observations he draws the just conclusion that organ-forming substances are present in these ova from the beginning. That which arises in the course of the development of the new individual is, in these cases, certainly determined at least in part by the protoplasm of the ovum. Hence we must admit that the protoplasm also participates in heredity. I do not see how we can accept the theory that the nucleus is exclusively the organ of heredity. On the contrary we must say that the essence of reproduction is the continuation of the growth of immortal protoplasm. The history of protoplasm is uninterrupted, and therefore we say : the immortality of the protoplasm and of the nucleus is also the explanation of heredity. IV. THE EVOLUTION OF DEATH. Mortality was formerly regarded as the necessary end- phenomenon of life. It was not until our own times that it appeared probable to us that so-called natural death does not occur with all organisms. The development of the higher plants and animals begins with the fertilized ovum. By continued division such an egg produces the cells which form the plant or animal, as the case may be. Many years ago Huxley defended the thesis that all cells which arise from a single ovum belong together and constitute a cycle. He further proposed to regard all the cells of a single cycle as constituting the individual proper. The problem of individuality, however, which formerly often occupied thinkers, has lost much in interest and signifi- cance, owing to the progress of biology. In the higher animals as in the unicellular, we encounter real individuals , but in the lower multi cellular animals we recognize on the contrary no dis- tinct individualities. Thus, for example, in the case of corals and sponges, we cannot speak of individuals. Under these conditions Huxley's conception of the cycle was very seduc- tive to biologists. It could apparently be very well applied to the unicellular organisms because in many of them con- jugation had been observed. Conjugation is a phenomenon closely related with sexual reproduction. It was assumed that conjugation served to excite the cell division of unicellu- lar organisms. If conjugation and the fertilization of the ovum are homologous phenomena, then we are justified in regard- s' 60 THE EVOLUTION OF-BEATH ing in both cases the exciting of cell division as the immediate consequence alike of conjugation and fertilization. In both cases there would arise homologous cycles of cell generations. Thus we should have to deal in both types of organisms with individuals in Huxley's sense. The only difference between the two types, which from our present point of view must be regarded as important, is that the cells in the lower type sepa- rate from one another, while in the higher, on the contrary, they unite to form a plant or animal. Death, as we ordinarily observe it, is the breakdown of a multicellular organism, and natural death is a consequence of old age. This considera- tion leads us directly to the question: Do old age and natural death occur in unicellular organisms? Weissmann, who has written several times concerning death, has not conceived the problem rightly, so that his discussions of death go astray in several essential respects. The first serious experiments to determine by direct ob- servation whether old age occurs in unicellular animals were carried out by the French investigator, Maupas.28 " He reared Protozoa through many generations. Of each genera- tion he took a few individuals, allowed them to propagate themselves and noted the rapidity with which the divisions followed upon one another. He found that the rapidity diminished until a new conjugation occurred, whereupon the animals recovered. Later tests of these results have shown that the experiments of Maupas were open to criticism, in part because at that time the great influence of external con- ditions upon Infusoria was unknown, so that the possibility remains that the retardation of the division he observed was conditioned not by internal but by external causes. Further, in order to bring about conjugation, he introduced into his cul- tures newly captured, wild individuals. His cultures, there- THE EVOLUTION OF DEATH 6 1 fore, were not kept strictly pure. In America a long series of researches on the rapidity of division in Protozoa has been made, largely upon the instigation of G. N. Calkins,29 who discovered that Infusoria may suffer "depression" a result which has been confirmed by further investigations of his own, of his pupils, and of other American investigators. The de- pression arises gradually, the animals become inert, nourish themselves poorly, and divide slowly or even not at all. If the depression lasts too long the animals may die off Calkins considered the depression" to be senescence, or a growing old. (He has since himself questioned the justness of this interpretation.) Our conception of senescence is based on the observation of the higher animals and plants, and comprises not merely the increasing weakness, but also alterations in structure which go far and are very striking. The Infusoria during their depression show no corresponding alterations of their organization; hence in my belief we cannot homologize this phenomenon in the Protozoa with the senescence of higher animals. In this belief we are confirmed by the fact that the newer investigations of conjugation make it improbable that it serves to renew and hasten the growth and division of unicellular organisms. Indeed, it is possible that conjuga- tion does not have this function at all. Significant here are the studies of the very talented investigator, H. S. Jennings,30 which demonstrate that conjugation serves to increase varia- bility. Jennings observed that Paramaecium exhibit consid- erable variability. During ordinary division the individuals remain more alike, but after conjugation their variation in- creases. His careful statistics leave no doubt as to his results. It is probable that sexual reproduction also has the purpose of maintaining the variability of the forms. The interperta- 62 THE EVOLUTION OF DEATH tion that impregnation has the purpose of increasing varia- bility in order to offer room for the play of natural selection originated with Weissmann. (It is said that Treviranus had previously expressed this view, but I have as yet been unable to personally confirm this statement.) Impregnation has also certainly to care both for heredity and for the initiation of further development. We know now that these functions may be separated experimentally. If sexual reproduction be conceived as a modification of conjugation, then we may assume that the function of initiating development was acquired later. Returning to the Infusoria we encounter in them, so far as the available observations go, a so-called depression indeed, but no senescence in the strict sense. (Quite conclusive as to the absence of senescence are the experiments of L. L. Wood- ruff,* who has maintained a pedigreed race of Paramecium for five years without conjugation. If all the possible indi- viduals had survived, they would have made a volume of protoplasm many million times the volume of the earth). Calkins, as said above, originally interpreted depression as a true senescence and declared the diminution of metab- olism to be the essential characteristic of old age. This view has been adopted by C. M. Child31 and E. G. Conklin.32 Pro- fessor Child has made experiments with a simple worm, Plana- ria. He treated these animals with alcohol by placing them in water to which i per cent, of alcohol had been added. The results he obtained are interesting and valuable. He seems to me, however, to go too far when he asserts that if the metabolism diminishes the animals become old. It is true that in the higher animals, when they become old, met- * L. L. Woodruff: Biologisches Centralblatt, XXXIII, p. 34, 1913. Professor Woodruff has informed me that on April 3, 1913, he had the 36501!! generation of the mentioned Paramecium colony. THE EVOLUTION OF DEATH 63 abolism becomes slower, but certainly one cannot therefore assert that every lessening of the metabolism implies a becom- ing old. According to the sum total of our knowledge we must regard organization as the cause of function. This is the only interpretation which a physiologist may admit. When therefore an organization is so altered that the metabo- lism diminishes, this diminution has to be considered a conse- quence and not a cause. Metabolism, however, is influenced by many factors, as every practicing physician experiences daily. If we accept Child's opinion we are led logically to the conclusion that each one of us may become alternately young and old according as his metabolism increases or diminishes. We should have to say, for example, that a man who performs strenuous and muscular work was reju- venated, while on the contrary one carrying on mental work, during which the metabolism is less, might become older. It seems to me clear that we cannot interpret the diminution of the metabolism as a characteristic of age in the sense of Cal- kins. In other words, we cannot view the depression in proto- zoa as senescence. Thus we reach the conclusion that natural death, so far as we know at present, does not occur in unicellu- lar organisms, and as a consequence of this we mention the corollary that natural death first appeared in the world as the higher multicellular plants and animals were evolved. We pass now to the examination of senescence in the higher animals, a theme which has claimed my active interest for many years. If we consider the phenomena as they are ./ known to us all, we recognize at once that a diminution of the rapidity of growth is characteristic of age, and thus we are induced to investigate growth. Obviously we must determine how the rapidity of growth alters with advancing age. For such an investigation it is important to exclude the influence 64 THE EVOLUTION OF_J1EATH of temperature, which is known to have a great influence upon growth. Nature makes this exclusion for us in the case of warm-blooded animals. I selected for my own experiments on warm-blooded animals guinea pigs for various practical reason and I maintained a colony of these animals for many years. Every animal of the colony was weighed at definite intervals of age. After many thousands of determinations of the weight ~JfLSi{Jvir( wnj/i 1fff 50- FIG. 26. — Graphic representation of the increase of weight in children of the Boston schools.— After H. P. Bowditch. (Knaben, boys. Madchen, girls. Jahre, years.) had been collected, they were worked over statistically.33 My first problem was to invent a method which permitted the representation of the rate of growth. Formerly investi- gators were satisfied to represent growth graphically in a very simple way. Curves were constructed in which the abscissae corresponded to the age, and the ordinates to the weight, Fig. 26. Such a curve, however, although it represents the THE EVOLUTION OF DEATH 65 increase of weight, does not show the rate of growth. The real rate can be represented in the following manner with approximate accuracy. From the weight which an animal has on a given day and that which is found at the next weigh- ing, I reckoned the average daily increase during the period between the two weighings, and then changed these increases into the per cent, value of the weight at the beginning of the period. This method may be modified by calculating instead of the daily, the monthly or yearly percentage increases. 51117232935 ^5 60 75 90 105 120 135 150 165 180 195 210 days FIG. 27. — Curve of the daily percentage increase in weight of male guinea-pigs. The method is of course mathematically not exact, since the weight is constantly changing. It suffices, however, for our purposes. It is easy after one has calculated a series of per- centage increments in weight to construct a curve. The results obtained in this way I wish to lay before you. When guinea-pigs are born, they suffer in consequence of the great sudden disturbance of their conditions of living a temporary inhibition of their development. They recover within two or three days, and thereupon we observe that they may increase their weight over 5 per cent, in one day, Fig. 27. 66 THE EVOLUTION OF DEATH By the time they are seventeen days old, they grow only only about 4 per cent, and at forty-five days only a little more than i per cent, and from this age on the rate of growth sinks slowly until at the end of the first year it becomes almost zero. The general process is the same in females, Fig. 28, as in males, although certain inequalities occur. It is obvious that if we consider the curves, Figs. 27 and 28, carefully, we can distinguish in them two chief periods, which, however, pass into one another without definite boundaries. 5 Tl 17 23 2933 tf 60 75 90 105 120 135 150 165 180 195 210 days FIG. 28. — Curve of the daily percentage increase in weight in female guinea-pigs. In the first, shorter period, the rate diminishes rapidly. This period lasts about one and a half months. The second period exhibits a much slower decrease and lasts perhaps ten months. The result was unexpected. If we accept the rate of growth as the measure of senescence, we must say that young animals grow old enormously faster than old animals. Since alter- ations in the rate of growth of guinea-pigs progress as de- scribed, it was to be expected that in still younger stages of development the rate of growth would be found still greater. Now chickens when they enter the world are not so far THE EVOLUTION OF DEATH 67 developed as guinea-pigs, and much less far developed are newborn rabbits. I have determined the rate of growth in both of these animals, and found that chickens, as soon as 3>2 13 U 33 r.» nrl o 'sob DFP 1 1 1Q5H ULU JL A JJJOU CLiyv%1. f,!/ ^ 1 JUHw fiA95&*->n k HV[AY£ 91955^ MAY 2 2 1967 MAY 1 ^ 1967 ^ LD 21-100m-8,'34 KOV 3 193 3 Binder r' r 281435 , .vCrf U UNIVERSITY OF CALIFORNIA IvIBRARY