Modern problems of biology

Front matter

22102045201

Med t

i

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 introduction which might render it easier for non-biologists to understand the following lectures. The researches quoted are chiefly American. This plan was adopted partly because the author was the American Exchange Professor and partly because 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.

VI 11

To their Royal Highnesses, the Grand-duke and the Grandduchess 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 Eischer in Jena. Professor von Bardeleben had the great kindness to revise the original manuscript. 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

II. Your Magnificence!

Gentlemen !

We endeavored in yesterday’s lecture to familiarize ourselves 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 characteristic 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 animals, 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.

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 development they become unlike or different; hence the designation differentiation. How these differentiations arise is an extremely interesting question about which we know very little, because as yet we have become acquainted almost

C

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 '' cytomorphosis.'' We are now to occupy ourselves with the laws of cytomorphosis so far as these have been determined. The development of simple cells into differentiated we call progressive 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 development 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 Harrison^ 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 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, neur oblasts , 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.”

Fig. 16. — 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. — After Ernst 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 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. i6. 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.

f H

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. — Ajler 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 extremities will be newly formed. It was formerly assumed that we had to do in such a case with a new regressive development. The investigation made by Emmel in my laboratory has rendered the real history clear. The cells of the outermost 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 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. 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

CYTOMORPIIOSIS 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 unlikeness 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. i8.

C'YTOMORPIIOSIS

The develoi)ing nerve librils 1 have already mentioned. In nerve cells there appear very hne fibers which develop gradually, 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 ^Hig-

Fig. 18. — Motor nerve cells from the spinal cord of a rabbit, ke, nucleus; den, dendrite; Ax, nerve fiber, and x, its origin; k, k', 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 intercellular spaces. It is not practicable to lay further illustrations before you.

Progressive development is closely connected with another

phenomenon. The embryonic tissues grow with immense rapidity, the differentiated tissues on the contrary grow slowly.

If we investigate the conditions more carefully we learn that the cells gradually lose the power of division as they are differentiated. If the differentiation progresses far, then probably the capacity of division is lost to the cells altogether. Formerly we had no exact conception 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 Problem 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 Bet he.

have discussed more fully the alterations of the rapidity of growth with age and its relation to the increase of differentiation. 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 fi — i

'ihy— X increase is not less than looo per cent. This holds true for man also.

Immediately after birth one finds the highest rapidity in the rabbit to be not quite i8 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- Fig. 20. — Cell from the pancreas of the

the time we are born and

same; fi, secretory fibrils; ke, the alterations which make nucleus; schs. I, closing pl&te— After K. C.

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.

CYTOMORPIIOSIS

Cytomorphosis includes more than differentiation proper. By continuing it leads to the degeneration of the cell. Degeneration appears in many cases to depend upon the transformation 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 cytomorphosis.

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 protoplasm. Next we observe that the protoplasm grows. Not until it has grown sufficiently does it acquire its characteristic color through the formation of hemoglobin, thus becoming a young red blood-corpuscle which may still grow a

CYTOMORPHOSrS 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 cartilaginous 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 developed 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,

p

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 indebted 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 embryonic cells which grow and differentiate themselves to

CYTOMORPlIOSrS form the leaf. Later the cells degenerate and die. The leaf becomes dead and falls. It would be easy to lay before

Fig. 21. — Cytomorphosis of the cartilage cells. From a section of the vertebral arch of a pig embryo, a-e, successive stages; in c, the letters kn refer to the limit of the cavity which is no longer filled by the degenerating cell.

Fig. 2 2. — 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 . Schneider.

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 d}'ing 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 remains 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 multiply 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 also the daily multiplication of cells which have remained embryonic.

We recognize that the embryonic cells are of great importance not only during the embryonic period, but also in the adult. How great this importance is is revealed in the investigation 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 undifferentiated tissues. There are worms which multiply by forming in the middle of their bodies the so-called budding zone. Karl Semper^^ 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. V/e 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 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 contrivance to make the postponement of a cytomorphosis possible, 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 between 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 each single organ there comes about, so to speak, an exaggeration 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 roundabout 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 separated spatially. When we state that the living substance has organization we base our view not only on the application 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 protoplasm of many cells, the so-called centrosome which we can only allude to here, although its occurrence again demonstrates 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 wonderful history of the chromosomes by his own observation, must be convinced that the nucleus has a very complicated organization.