Popular Science Monthly/Volume 71/October 1907/The Problem of Age, Growth and Death IV



IV. Differentiation and Rejuvenation

Ladies and Gentlemen: In order to present the subject of this evening, I will take a few brief moments at the beginning to review the results reached in the previous lecture. In the last lecture I spoke of the phenomena of growth, and endeavored then to make clear to you what I consider the fundamental conception of this study—that the decline in the growth power is extremely rapid at first and slow afterwards. This change in the rate of growth is of course due to things in the animal body itself. It is a logical conclusion for us to draw that if we are to study out the cause of the loss of growth power, we should do it rather at that period of development when the change in the rate of growth is most rapid, for then we should expect those modifications to exhibit themselves most clearly because the magnitude of cause is likely to be proportionate to the magnitude of result, and when the decline is most rapid, then we must expect to find the alterations which cause that decline in the organism to show themselves most conspicuously. You will remember, further, that we spoke of growing old as being a much more complicated question than one of growth alone, and that there occur, as the years advance, changes in the structure of the body. It is convenient to use one collective term for all these phenomena of becoming old, and that term, established by long usage, is senescence, the becoming old. What, therefore, we have to search for at present is a cause, a proximate cause at least, of senescence. In order to make the view I am to bring forward this evening quite clear to you, I must first of all take advantage of your kindness and recapitulate briefly what I said in regard to cells, for you will remember that the cell is the foundation and unit of organic structure. With your permission I should like to recall more exactly to your minds what I said of the cells by having thrown upon the screen the slide which we saw before and which we used as an illustration of the cell. Here is the picture. Above we see the typical cell from the oral epithelium of the salamander, and you remember in the center this more conspicuous body with a granular and reticulated structure which we called the nucleus, and surrounding it is this mass which we called the body of the cell, or the protoplasm. Here is other condition of a cell of the skin of the salamander in which the nucleus presents a slightly different appearance. Here also we have quite a body of protoplasm about the nucleus. Every cell consists of these two essential and fundamental parts, the nucleus and the protoplasm. Now the conclusion to which I shall gradually bring you by

PSM V71 D366 Cells from the oral epithelium of the salamander.png
Fig. 39. Cells from the Mouth (Oral) Epithelium of the Salamander.

the facts to be laid before you this evening is that the increase of the protoplasm is the thing which is to be regarded as the explanation of senescence. Though protoplasm is the physical basis of life, though it is the actual living substance of the body, its undue increase beyond the growth of the nucleus changes the proportions of the two. and that change of proportion seems to cause an alteration in the conditions of the living cell itself, and that alteration I interpret, as I shall explain more accurately later, as the cause of senescence, as the fundamental cause of old age. This slide also shows to us the early development of the cells through those phases which result in the multiplication of them. The nucleus changes in appearance and becomes a very different-looking structure. These changes I need not now go through again. Suffice it to say that after the complicated alterations have completed their cycle, we get in the place of a single cell, two, and

PSM V71 D367 Three sections through a rabbit embryo of seven and half days.png
Fig. 40. Three Sections through a Rabbit Embryo of Seven and One Half Days.

each has its own nucleus, and each its own protoplasm. Notice here that the two cells which finally result are smaller than the original cells from which they sprang. These are by no means imaginary pictures, but accurate microscopic drawings from real cells of the salamander skin. The two cells which are thus produced from one parent cell are characterized by their smaller size, and this smaller size applies not only to the cell as a whole, but likewise to its nucleus. After having been thus reduced in size, the nuclei and the cells will both expand, and soon the daughter cells will return to the mother dimension and be as large as the parent cell from the division of which PSM V71 D368 Amoeba coli highly magnified.pngFig. 41. Amœba coli. highly magnified. Drawn from a cover-glass preparation from a twenty-four hour culture. they arose. There is thus, we learn, the constant fluctuation in the size of cells, a fluctuation in their dimensions accompanying the process of cell division. Presently we shall have more to say in regard to this matter of the change in the cell in size. The next picture (Fig. 40) which I want to recall to you is one which we also had in an earlier lecture. These represent slices through PSM V71 D368 Tertian malarial parasite and two human blood corpuscles.pngFig. 42. Tertian Malarial Parasite. Two human blood corpuscles alongside and drawn on the same scale. a very young rabbit before any of the organs of the rabbit have begun to develop. We can see here clearly the nuclei, as I pointed out to you before, nearly uniform in structure, and you notice that the protoplasm around each nucleus is quite small in amount. If you will recall the previous picture of the skin of the salamander, upon the screen a moment PSM V71 D368 Trypanosoma lewisi from the blood and human blood corpuscles.pngFig. 43. Trypanosoma Lewisi. from the rat's blood with two blood corpuscles alongside drawn on the same scale. ago, you will realize immediately, in comparing the two, that in these young cells the proportion of the protoplasm to the nucleus is very small. That is again one of the fundamental facts to which we shall recur in a moment. I wanted to show you this picture in order to revive in your minds the conception which I endeavored to give you before of the undifferentiated tissue, where the cells have nuclei pretty uniform in appearance and in size, each with its little mass of protoplasm about it, and this protoplasm appearing in all the cells under microscopic examination very much the same. We can not in this stage of development say of a given cell that it represents any special structure, by which, if we saw it isolated under the microscope, we could determine from what part of the young embryonic body it was derived. When we see a cell from the adult we can determine its origin with certainty by its microscopic appearance alone. As development progresses, the simple condition of the cells is gradually obliterated, but we find another condition arising which we call the differentiated one. Differentiation is a process which goes on in the body as a whole, but of course it is also a function of each individual cell. We can see something of the process of differentiation if we study the unicellular organisms, those creatures, each of which is complete in itself, although it consists of but a single cell, not of countless millions of cells as we do. The picture (Fig. 41) which I have chosen to throw upon the screen is one which I think might have an additional interest to you, for it is a photograph from the living cell known as the parasite producing dysentery. Its scientific name is amœba coli. It is a photograph from life. Here vaguely in the center, marked with a finer granulation, and some of the darker spots in it, we can distinguish the nucleus: here is the outline of the protoplasm of this cell. and in it are included some particles of food which this protoplasmic body has absorbed for purposes of digestion. This is a unicellular parasitic organism with scarcely any differentiation of its structure. The next of the slides shows us again another of these parasitic simple organisms. The figure here to the right of the field of view is the one which should especially attract your attention. The other two bodies near it are blood corpuscles, human blood corpuscles. The organism in this case is the one which causes malarial fever, and it is in a particular stage of its development; that which we distinguish as the tertian malarial parasite is the one here represented. You can see in this case also the outline of the nucleus, surrounded by the protoplasm—the whole thing only a little bigger than a single human blood corpuscle. Here also we note the absence of differentiation. Another stage of this same tertian malarial parasite is shown next. This I have projected upon the screen because it illustrates more clearly than the other the nucleus and the small amount of protoplasm about the nucleus. The malarial organism is one of great vitality, capable of enormously rapid multiplication, and it undoubtedly owes that faculty to its constitution, to the relation between the nucleus and the protoplasm. I will now show you another picture of parasites—one form of which, in a related species, occurs in man. This particular form is one which occurs in the rat and is called the Trypanosoma. You can see that the body, instead of being a small and simple structure, has elongated, acquired-a peculiar form, and here in the interior are lighter and darker spots. These do not show very clearly in the picture, because it is from a photograph of a living specimen under the microscope. The lighter and darker spots correspond to the details in the structure of the organism. Here is the tail of the organism, twisted, as you see, and in life capable of being bent. The movement of the animals in the natural fluid in which they are suspended is quite active. Alongside are some blood corpuscles, the figure, as you see, is magnified about the same as the one of the malarial parasite which I showed you a few moments ago. The next slide exhibits an

PSM V71 D370 Stentor coeruleus.png
Fig. 44. Stentor coeruleus A, cut into three Pieces; B, regeneration of the first piece; C, cf the middle piece; D, of the posterior piece. After Gruber.

organism which swims free in the water, and is pretty well shown in this figure. It is called the Stentor. Here the chain of beads represents the nucleus. Upon the surface of the body there are fine lines indicating superficial structure. At this point there occurs what we call the mouth. Over the rest of this minute organism there is a thin cuticle, but at the mouth the cuticle is absent, and the protoplasm is naked or uncovered so that food can be taken in. There are bands of hairs showing coarse and stiff in the figure but capable of movement, and with the aid of those vibratile hairs, or cilia, the organism can swim about in water. Here is another internal structure, the vacuole; obviously in an animal like this we no longer have simple protoplasm alone, but the protoplasm in the interior of the cell has become in part changed into other things. Here then within the territory of a single cell we have differentiation. If now in these unicellular organisms we study both the protoplasm and the nucleus, we learn that most of these modifications which are so conspicuous upon microscopic observation are due to changes in the protoplasm. It is the protoplasm which acquires a new structure. In the nucleus, on the contrary, we find perhaps a change of form, minor details of arrangement by which one sort of nucleus, or one stage of the nucleus, can be distinguished from another, but always the nucleus consists of the same fundamental constants. There is the membrane bounding it; there is the sap or juice in the interior; the network of living threads stretching across it: and here and there imbedded in and connected with this network are the granules of special substance, which we call chromatin. These four things exist in the nuclei and are apparently always present, and there is usually not to be seen in the nucleus anything of change comparable, in extent at least, with the change which goes on in the protoplasm—on the other hand, the protoplasm acquires items of structure which were totally absent from it before. The nucleus rearranges its parts rather than changes them. This is a very important fact, and shows us, if we confine our attention even to these little organisms only, that the differentiation of the protoplasm is quantitatively the more important of the two—the differentiation of the nucleus the less important.

We can now turn from a consideration of these lowest organisms to the higher forms, among which we ourselves of course are counted, in which the body is formed by a very considerable number of cells. Again I should like to take advantage of your kindness and show you some of the pictures we have already reviewed, in order to utilize the features which they show as illustrations of the fundamental principle that the conspicuous change is in the protoplasm. Here we have nerve cells. In the first two photographs are represented two isolated nerve cells, to show their shape. They have been colored by a special process so dark that the nucleus which they contain in their interior is hidden from our view; it is of course none the less there. This dark staining enables us to trace out the shape of these cells very clearly, and you can see that instead of being round and simple in form they have their elongated processes stretching out to a very considerable distance; these processes serve to catch up from remote places nervous impulses and carry them into the body of the cell, and thus assist in the work of nervous transmission. The elongation of these threads is, as you see. adapted, like the elongation of a wire, to long-distance communication. Here are two other figures which represent nerve cells treated by a different process, and again artificially colored. But the color in this case has attacked certain spots in the protoplasm. consequently we see that the protoplasm around the nucleus in both of these figures is no longer simple and uniform, but contains these deposits of dark-colored material. Here are other nerve cells; the one

PSM V71 D372 Various kinds of human nerve cells.png
Fig. 45. Various kinds of Human Nerve Cells. After Sobotta.

in the center shows you the accumulation of pigmented matter in the protoplasm; again an index of a change and lack of the previous uniformity replaced by diversity in the composition of the various parts of the single cell. This figure shows us more clearly the principle of structure of a nerve cell, for here we have the central body of the cell composed of protoplasm with its nucleus in the middle and a small spot in the center of the nucleus, and these long branching processes running out in all directions which can take up nerve impulses from other similar or dissimilar cells, as the case may be, and carry them to the central body. To carry the message out there is typically but one process, which is different in appearance from the other processes which carry the impulses in. The latter are branching and are therefore called the tree-like or dendritic processes. Here is a single process like a long thread to carry the impulses away, and which

PSM V71 D373 Human muscle fiber and section of orbital gland.png
Fig. 46. Part of a human Muscle Fiber. Fig. 47. Section from an Orbital Gland.

is called the axon of the nerve cell. In this case the modification of the shape of the cell has adapted it to the better performance of its functions. Notice also in these cells the enormous increase in the amount of protoplasm as compared with the nucleus. In the young cell of the rabbit germ, of which I showed you several illustrations a few moments ago, we had very little protoplasm for each nucleus, but here the protoplasm has many, many times the volume of the nucleus, and this is a relatively old cell.

Next let us look again at the figure of the striated muscle fiber, which you may recall from the second lecture, so that it will suffice if your attention is again directed to the oval nuclei, and to the lines stretching crosswise on the muscle giving it a "striated" appearance. You remember, doubtless, that such fibers are the ones which enable us to make voluntary motions. Originally each fiber was a set of cells, and the cells had some protoplasm, but, gradually, as development progressed, there appeared in them longitudinal fibrils different from the protoplasm, and the fibrils also created ultimately the appearance of cross lines on the fiber. It is the fibrils which perform the muscular contractions. It is not the original unmodified protoplasm, but the modified or differentiated muscular cell which is capable of voluntary contraction.

The next picture (Fig. 47) shows us clearly and strikingly how much the differentiation may vary. We have here another type of differentiation. These are gland cells; we can see here, as I pointed out to you before, the material in the form of granules, which is to produce the secretion from these gland cells. This is an orbital gland, and here are the cells, which are very much smaller because they have discharged their secretion. Three of the cells are represented separately. The first shows us a cell full of the material which is to be discharged and is to form a part of the secretion of the saliva. The second is a cell which has partly lost its accumulated material, and the third is one which has discharged it almost completely, so that it has become very much reduced in size. We learn from such structures as these that the size of cells may vary also according to their functional condition. We have here a similar gland. This is sometimes called the salivary gland of the intestine, better termed the pancreas. Here we can see for each of these cells a nucleus and a body divided into two parts, a darker portion around the nucleus and a lighter part with little granules in it, which represents the accumulation of material which is to form the secretion. When the cells have discharged their secretion, they, like the cells in the salivary gland, are found to have diminished in size and become very much smaller indeed than they were in their earlier state when charged with the zymogen destined to be given out. In this case also we have an illustration of a functional variation in the size of the cells. This ends the series of pictures which I wanted especially to show to you as illustrating the changes of the cells as their differentiation progresses. We can see in the bodies of the cells the changes which have occurred.

Here is a picture which teaches us one thing more about these cells. Notice the scattered nuclei, each surrounded by protoplasm, completing the cell. The protoplasm of each of these cells is connected across with the protoplasm coming from another, so that the whole set of cells forms an irregular photoplasmic network. Now in the spaces between these cells are fine lines. These represent delicate structures which we call connective tissue fibrils, which have a mechanical function. By their tensile strength, their power to resist and pull, they give a certain supporting power to the tissues. Our picture represents one of the tissues which support and connect other portions of the body. Now the fibrils apparently lie entirely disconnected from the cells, but a more careful study of the history of the connective tissue has revealed the very interesting and instructive fact that the fibrils, now separate from the cells, arose by a metamorphosis of the protoplasm of the cells—that they are first formed out of some of the protoplasm of these cells, then split off from them, and come to lie in the intercellular regions, so that here we have another type of cell differentiation brought to our notice, one in which the product is separated from the parent body to which it owes its origin. Now you will perceive immediately, if you recall the series of pictures which have just passed before us on the screen, very great differences in the types of differentiation which occur in the body, and had we time we might find a very much larger range easily to be represented before us.

PSM V71 D375 Embryonic synctium from the human umbilical cord.png
Fig. 48. Embryonic Syncytium from the Umbilical Cord of Man; c, c, cells; F, fibrils.

In the second lecture a picture was projected upon the screen, which showed motor nerve cells of various animals. You will recall that I directed your attention to the fact that the largest animal, the elephant, has the largest cells, and the smallest animals, the rat, the mouse and the little bat, have the smallest ones. But let me point out to you that the question of the size of cells is exceeding complex, and that in studying it we have to exercise a great deal of caution. We know that, with the exception of the nerve cells and to a minor degree with the exception of the muscle fibers, the cells in each animal are more or less uniform constants in size. The cells of different organs differ somewhat from one another. A single organ may have in its different parts typical sizes of cells, but each of these kinds of cells has its definite dimensions. When one animal is larger than another, it has more cells. Now it is a very important fact for us that animals have a more or less constant size of their cells. They do not differ from one another by a difference in the size of their cells; the bigness of an animal does not depend upon the size, but upon the number, of its cells. We can, therefore, in studying the changes of size, to which I shall next direct your attention, omit altogether these details, and speak of the cells in a general way safely as having a certain uniform or standard size. This will save us a great deal of time, for we learn, as we study cells, that their size increases with the age of the animal. The animal, when it is young, has cells with a small amount of protoplasm. And that, you will perceive from the pictures which have been thrown upon the screen, is an absolutely necessary corollary of the discovery that differentiation is mainly a function of the protoplasm. If there is to be a large degree of differentiation it is necessary that the quantity of protoplasm in the single cells should be increased, so that there may be the raw material on hand out of which the differentiated product can be manufactured. If there is not such a preliminary increase of the protoplasm, then the differentiation can not occur. In order that perfection of the adult structure should be attained, it is necessary that the mere undifferentiated cells, each with a small body of protoplasm, should acquire first an increased amount of protoplasm, and that then from the increased protoplasm should be taken the material to result in differentiation, in specialization.

An undifferentiated cell performs all the fundamental functions of life. An amœba, or any unicellular organism such as I have presented to you upon the screen, does everything which is indispensable to life. It takes food; it forms secretions and excretions; its activity depends upon chemical alterations going on in the food in the interior of its body: it is capable of sensation and of locomotion. It is probable that every living cell has all of these fundamental properties of protoplasm. When a cell becomes differentiated, however, though it does not necessarily give up any of its vital properties, it becomes different from other cells because one of its properties is made conspicuous. And in order to acquire that conspicuousness, that excess of development of one function of the cell, a modification in the structure is necessary. The apparatus in the interior of a cell to produce the exaggeration of the function must be developed, so that to effect the complex physiological machinery of the adult body, this differentiation, of which I have so often spoken, is indispensable. A nerve cell carries on all the vital functions, but it has in addition a special series of modifications of its protoplasm which enable it to accomplish the transmission of the nervous impulses with greater efficiency than ordinary protoplasm can do, probably at a higher speed and with a more perfect adjustment of communication between the various parts of the body than is possible with any machinery of pure protoplasm. So too, the glands have cells which are especially capable of elaborating chemical substances which, when the}-are poured out, accomplish the work of digestion, for instance. But these cells are likewise alive in all their parts. They have all the fundamental vital properties, but there is this tremendous exaggeration of the one faculty, and that involves an alteration so great in the protoplasm that we can see it with the microscope; the microscope affords us a perfect demonstration of differentiation, which we can correlate with the function.

The primary object, therefore, of all differentiation is physiological. The higher organism, with its complex physiological relations, is something really higher in structure than the lower organism. The term "higher" in biology implies a much more complex interrelation of the parts, a much more complex relation of the organism to the outside world; and above all it implies in the highest animals a complex intelligence of which only a rudimentary prophecy exists in the lowest forms of life, possibly scarcely more than a mere sensation. We owe then to differentiation our faculties, which we prize. It is the result of differentiation that I am able to address you and present before you the thoughts which have been accumulated as the result of the studies of many years. It is a result of differentiation that you have such, parts that you not only hear the actual sound of my voice, but interpret—at least I hope so—the meaning of my words and can understand the ideas which I am endeavoring to present to you. If you carry away something from these lectures, and recall it at some future time, that also will be a result of the differentiation of structure; for every one of you started as a minute germ, consisting of protoplasm with a nucleus, and entirely without any differentiation; and by a process so complex that the mystery of it escapes entirely all our powers of analysis, those parts which you have have been slowly and secretly fashioned. We have approached one of the fundamental problems of existence. When we talk of differentiation, we talk of the endowments which bring us into relation with the external world—into relations with our kind, and which make our internal life so complex, a complexity which in itself is a great problem. We touch here the fundamental mysteries of existence; we are hovering upon the outskirts of our human conceptions. We are not yet able to press beyond. But perhaps the time may come when the limit to which I can now bring you will be moved farther back, and some of the things which are at the present time utterly mysterious and incomprehensible to us will be comprehended and be explicable to you.

The increase of the protoplasm is then, as we have clearly seen from the pictures, the mark both of advancing organization and of advancing age. It is certainly somewhat paradoxical to assert that the increase of the protoplasm is a sign of old age, a sign of senescence, since protoplasm is the physical basis of life. It undoubtedly is such, and we should hardly anticipate that its increase would have a deleterious effect. But such is, it seems to me, clearly the case. But it is not merely, of course, a question of the increase of protoplasm which we must bear in mind in estimating the cause and effect, but also the question of differentiation, in consequence of which protoplasm becomes something else and different from what it was before. This alteration, then, together with the increase of the protoplasm, is the change which in all parts of the body marks the passage from youth to old age.

It seems to me not going at all too far to say that the increase of protoplasm is a fundamental phenomenon. I wish to give you a more precise notion of this increase; and I am glad to be able to do so in consequence of a research carried on by Professor Eycleshymer in my laboratory and completed by him afterwards in his own laboratory at the University of St. Louis. He studied the development of the muscle fibers in the great salamander, known scientifically by the name of Necturus. These muscle fibers are somewhat cylindrical in shape. Their ends can be accurately determined so that the precise length of a fiber can be measured, and its diameter also. Hence the total volume of a fiber may be calculated. It is possible also to measure the nuclei and to count the number of nuclei in a fiber. Thus by measuring the diameter and length of the fiber, and then estimating the number and the diameters of the nuclei, we can calculate the proportions. As a matter of fact, the nuclei remain nearly constant in volume, not really quite so, but sufficiently constant to serve as a basis of measurement. Dr. Eycleshymer found that when a Necturus had a length of eight millimeters, it possessed, for each nucleus in its muscle fiber, 2,737 units of protoplasm, but when it was seventeen millimeters, it possessed for each nucleus 4,318 units per nucleus; at twenty-six millimeters, 8,473 units; and in the adult, which measures approximately 230 millimeters, it has 22,379 units per nucleus. In other words, as a salamander passes from the eight-millimeter condition, when the development of its muscle fibers is just fairly begun, up to the adult state, when the differentiation of the muscle fibers has been completed, it increases the proportion of protoplasmic substance and protoplasmic derivatives from 2,700 to 22,300 per nucleus. I give round numbers. The increase is approximately sevenfold. There is in the adult in the muscle fiber seven times as much protoplasmic substance in proportion to the nucleus as there was at the start of development when the muscle fiber could first be clearly recognized as such. This is an accurate measure and gives us a good idea of the general law of protoplasmic increase. It is the only instance, I yet know of, in which we have an accurate measure and can give quantitative values, though we do know that there is a more or less similar increase occurring in perhaps every tissue of the body.

While the increase of the protoplasm is going on, we find that there is an advance in the structure, in the differentiation. Now you may recall what I have mentioned earlier in this lecture, the further fundamental fact that the loss in the rate of growth is greatest in the young, least in the old, and that as we go back from old age towards youth, and then into the embryonic period, we find an ever-increasing power of growth, but that it is during the embryonic period that the loss of the power of growth is greatest. It is to the embryonic period, therefore, that I have turned in order to ascertain whether the rate of differentiation shows a similar relation in the development of the organism.

We have a large series of microscopic preparations of rabbit embryos in the embryological laboratory of the Harvard Medical School. Utilizing these, I found that at seven or eight days of development there is scarcely a trace of differentiation. The cells are in the condition of those which I showed to you earlier in the lecture upon the screen. At sixteen and a half days, a stage of development of which I have some good preparations, I found that a great deal had been accomplished. At seven days there was no brain, there was no spinal cord, nothing that could possibly be called skin or muscle, or intestine or heart. None of those things were yet produced. But at sixteen and one half—in other words, after a very brief period indeed—only nine days of the whole life of the animal—there have arisen from this inchoate beginning all the principal organs of the body. The brain is there, divided up into its principal fundamental parts; the spinal cord has its nerves in connection with the various parts of the body; there is a trace of the skeletal element: the stomach, the liver, the pancreas, the intestines, are all present and well defined; the heart is a large and beating organ, amply supplied with blood, connected with vessels, which carry out and bring back the blood and are all far along in their development. Equally instructive is the microscopic examination, for we can see that the cells themselves have been changed. Not only have the great organs been mapped out in this brief period, but the cells which belong to them have for each organ acquired a characteristic quality. In the brain there are nerve cells with their long processes to carry the impulse in; the single process (axon) to carry it out. The glands in the stomach have the cells which are to build them already there. The muscles which are to move the stomach are beginning to appear as cells of a special form. Nerve fibers extend down into the gastric region and to the various distant organs of the body. Muscle fibers can l)e recognized along the back and in the limbs, and so in every part of the body we can detect cells already far advanced in their development. It is not certainly too much to say that in the brief period of these nine days fully as much differentiation has been accomplished as is accomplished during the entire remainder of the life of the animal. We do not, at present at least, possess any method of measuring differentiation, which enables us to state it numerically, but no one who is familiar with these matters and observes the structure, as I have myself observed it, would hesitate for a moment, it seems to me, to decide that my assertion is perfectly within the bounds of truth, that within a period of nine days, half of the entire differentiation which is to occur in the whole life of the rabbit has been completed. We must from this conclude that the rate of differentiation is very rapid at first and afterwards declines, and as we compare the different stages of development we can see readily that this is the case. The progress in the additional development in the rabbit from sixteen and one half days up to the time of its birth is far greater than the progress which occurs after birth. We find, moreover, in the study of these embryonic conditions, some instructive things, for in certain parts of the body the process of differentiation hurries along, and as the cells are differentiated their power of growth, to a large extent, is stopped. On the other hand, there are various provisions in the developing animal for keeping back certain cells, allowing them to remain in the young state. Such cells may afterward differentiate.

From all that has been said it seems to me legitimate to conclude that there is an intimate correlation between the rate of differentiation and the rate of growth. I am inclined to go the one step farther, and bring them into the relation of cause and effect; and I present to you as the main general conclusion of this first part of our series of lectures, the conception that the growth and differentiation of the protoplasm are the cause of the loss of the power of growth. Now if cells become old as their protoplasm increases and becomes differentiated, we should expect to find that there would be a provision for the production of young cells. It is rather mortifying to reflect that the simple conception which I have now to express to you, although it lay close at hand, failed to combine itself in my mind for many years with the conception of the process of senescence as I have just described it to you. It is somewhat, it seems to me, like two acquaintances of mine who lived long side by side, seeing one another frequently until they were fairly past the period of youth, when their attachment became very close and by a sacrament they were permanently joined together. So in the minds of men often two ideas lie side by side which ought to be married to one another, and there is no one ready, so dull is the owner of the mind, to pronounce the sacramental words which shall join them, and the rite long remains unperformed, and when at last such neighbor ideas, which naturally should be united in close companionship, are brought together and made, as it w-ere, into one, we are astonished that the inevitableness of the union had not obtained our notice before, it is so very obvious. And so in regard to the conception of what constitutes the restoration of the young state, I have only this excuse to offer, which I have indicated to you, that even the natural thought fails to occur to us. We are very dull even if we are scientific.

The pictures now before you represent certain early stages in the progress of development of a mammal by the name of Tarsius, a creature related to the lemurs. The various figures illustrate the multiplication

PSM V71 D381 Tarsius spectabile sections of three ova in early stages.png
Fig. 49. Tarsius spectabile. Sections of Three Ova in very Early stages. 1, before cleavage: 2, cleavage into four cells; 3, multicellular stage.

of the cells. That which I wish to call your attention to can be well demonstrated by the comparison of the first figure, in which there is a single nucleus, with the figure at this point having a number of nuclei. Both figures represent the very earliest stages of development and show the full size of the whole germ, which is about the same in the two stages. The total amount of living material has not changed essentially, but evidently there has occurred a marked increase of the nuclear substance. The nuclei have in the right-hand figure multiplied in number and their combined volume is much greater than the total volume of the single nucleus in the left-hand figure.

We can get a further notion of the nuclear increase by studying the very early development of a salamander. Here upon the screen is the egg of a salamander. It represents really but a single cell. It then divides into two cells; each of those cells has a nucleus which we can not see because these pictures are taken from the living egg, and the living egg is not transparent. Here it is dividing into four, here the upper portion of the four cells has been split off, and we have seven cells showing in the figure, and an eighth on the back. Here the number of cells has increased very much, and as you view these figures you will notice that they look very much indeed like oranges divided into segments. It seems, in fact, as if this egg, which was spherical in form, were being divided up into a certain number of segments. The process was first observed in the eggs of some of the amphibia, frogs, toads and salamanders, and it was therefore called segmentation, because it was not known at that time what the process really meant. We have then before us an ovum and a series of stages of the segmentation of the ovum, and the result of that segmentation is to produce an ever-increasing number of cells which, in the last. of the figures upon the screen, have become so numerous that we are no longer able to readily count them. Every one of these cells has its own nucleus. When the process of segmentation is complete and reaches its final limit, we then see, if we examine that stage of development, cells of the young type, such as I have described to you, in which there is a nucleus with a small amount of protoplasm about each nucleus. It seems to me, therefore—and this is a new interpretation which I present

PSM V71 D382 Ambystomum punctatum progressive segmentation of the ovum.png
Fig. 50. Amblystomum punctatum. Progressive Segmentation of the Ovum. 1, unsegmented ovum; 9, advanced segmentation.

to you—that the process of segmentation of the ovum, with which the development of all the animals of the higher type invariably begins, is really the process of producing young cells. It is the process of rejuvenation. There is not any considerable growth of the living protoplasmic material of these eggs, and at the final stage the total volume of the egg is scarcely bigger than before; and such increased volume as has occurred has been due to the absorption of some of the surrounding water. In many animals not even this increase by the absorption of water takes place. During the segmentation of the ovum the condition of things has been reversed so far as the proportions of nucleus and protoplasm are concerned. "We have nucleus produced, so to speak, to excess. The nuclear substance is increased during this first phase of development.

Naturally, as we embryologists looked upon these things in earlier days and thought of the progress of development, we conceived of the earlier stage as younger, and of the ovum as being the youngest stage of; all, a conception which in terms, of time is obviously correct, but as regards the nature of the development, it seems to me clearly, is not correct. The ovum is a cell derived from the parent body, fertilized by the male element, and presenting the old state to us, the state in which there is an excessive amount of protoplasm in proportion to the nucleus; and in order to get anything which is young, a process of rejuvenation is necessary, and that rejuvenation is the first thing to be done in development. The nuclei multiply; they multiply at the expense of the protoplasm. They take food from the material which is stored up in the ovum, nourish themselves by it, grow and multiply until they become the dominant part in the structure. Then begins the? other change; the protoplasm slowly proceeds to grow, and as it grows, differentiation follows, and so the cycle is completed. Whether other naturalists will be inclined to accept this conception that the process of the segmentation of the ovum is that which we must call rejuvenation or not, I can not say, for the matter has as yet been very little discussed, but you will see that it hangs as a theory well-together. We have first an explanation of the process of the production of the young material, and out of that young material the fashioning of the embryo. The cycle of life has two phases, an early brief one, during which the young material is produced, then the later and prolonged one, in which the process of differentiation goes on, and that which was young, through a prolonged senescence, becomes old. I believe these are the alternating phases of life, and that as we define senescence as an increase and differentiation of the protoplasm, so we must define rejuvenation as an increase of the nuclear material. The alternation of phases is due to the alternation in the proportions of nucleus and protoplasm.

In the next lecture I shall be able to convince you, I hope, that this conception of the relation of the power of growth to the proportion of nucleus and protoplasm enables us to understand various problems of development, certain possibilities of regeneration and reconstruction of lost parts, and that it also leads us naturally forward to the consideration of the problem of death as it is now viewed by biologists, so that our next lecture will be upon the subject of regeneration and death, the natural topics to follow after to-night's discussion.