The Cell in Development and Inheritance

tance, made independently and almost simultaneously in 18S4-85 by -Oscar Hertwig, Strasburger, Kolliker, and Wcismann/ while nearly at the same time (1883) the splendid researches of Van Beneden on the early history of the animal Qgg opened possibilities of research into the finer details of cell-phenomena of which the early workers could hardly have dreamed. We can only appreciate the full historical significance of the new period thus inaugurated by a glance at the earlier history of opinion regarding embryological development and inheritance. To the modern student the germ is, in Huxley's words, simply a detached living por- tion of the substance of a preexisting living body ^ carrying with it a definite structural organization characteristic of the species. By the earlier embryologists, however, the matter was very differently re- garded ; for their views in regard to inheritance were vitiated by their acceptance of the Greek doctrine of the equivocal or spontaneous generation of life ; and even Harvey did not escape this pitfall, near as he came to the modern point of view. " The Qgg,'' he savs, " is the mid-passage or transition stage between parents and offspring, between those who are, or were, and those who are about to be ; it is the hinge or pivot upon which the whole generation of the bird revolves. The Qgg is the terminus from which all fowls, male and female, have sprung, and to which all their lives tend — it is the result which nature has proposed to herself in their being. And thus it comes that individuals in procreating their like for the sake of their species, endure forever. The egg, I say, is a period or por- tion of this eternity." ^

This passage appears at first sight to be a close approximation to the modern doctrine of s^erminal continuitv about which all theories of heredity are revolving. In point of fact, however, Harvey's view is only superficially similar to this doctrine ; for, as Huxley pointed out, it was obscured by his belief that the germ might arise *' spontaneously," or through the influence of a mysterious '' calidiun innaUmi,'' out of not-living matter."* Neither could Harvey, great physiologist and embryologist as he w^as, have had any adequate con- ception of the real nature of the ^gg and its morphological relation to

1 It must not be forgotten that Haeckel expressed the same view in 1866 — only, how- ever, as a speculation, since the data necessary to an inductive conclusion were not obtained until long afterward. "The internal nucleus provides for the transmission of hereditary characters, the external plasma on the other hand for accommodation i)« adaptation to the external world" {Gen. MorpJi., pp. 2S7-289). ^ Whitman, too, in a brilliant essay, has shown how far Harvey was from any real grasp of the law of cenetic continuitv. which is well characterized as the central fact of modern biology. Evolution and Epigenesis, Wood's HoU Biological Lectures, 1894.

the body of which it forms a part, since the cclkilar structure of Uving things was not comprehended until nearly two centuries later, the spermatozoon was still undiscovered, and the nature of fertilization was a subject of fantastic and baseless speculation. For a hundred years after Harvey's time embryologists sought in vain t(^ penetrate the mysteries enveloping the beginning of the individual life, and despite their failure the controversial writings of this period form one of the most interesting chapters in the history of biology. By the extreme " evolutionists " or " prceformationists " the egg was believed to contain an embryo fully formed in miniature, as the bud contains the flower or the chrysalis the butterfly. Development was to them merely the unfolding of that which already existed ; inheritance, the handing down from parent to child of an infinitesimal re])roduction of its own body. It was the service of Bonnet to push this concep- tion to its logical consequence, the theory of eiJiboitciiicjit or encase- ment, and thus to demonstrate the absurdity of its grosser forms, pointing out that if the egg contains a complete embryo, this must itself contain eggs for the next generation, these other eggs in their turn, and so ad infinitum, like an infinite series of boxes, one within another — hence the term cniboitemcnt. Bonnet himself renounced this doctrine in his later writings, and Caspar Friedrich Wolff ( 1759) led the way in a return to the teachings of Harvey, showing by pre- cise actual observation that the egg does not at first contain any formed embryo whatever ; that its structure is wholly different from that of the adult; that development is not a mere process of unfolding, but involves the continual formation, one after an- other, of new parts, previously non-existent as such.

This is some- what as Harvey, himself following Aristotle, had conceived it — a process of cpigcncsis as opposed to evolution. Later researches established this conclusion as the very foundation of embryological science. But although the external nature of development was thus deter- mined, the actual structure of the egg and the mechanism of inheri- tance remained for nearly a century in the dark. It was reserved for Schwann (1839) and his immediate followers to recognize the fact, conclusively demonstrated by all later researches, that tJic egg is a cell having the same essential structure as other cells of the body. And thus the wonderful truth became manifest that a single cell may contain within its microscopic compass the sum-total of the heritage of the species. This conclusion first reached in the case of the female sex was soon afterward extended to the male as well. Since the time of Leeuwenhoek (1677) it had been known that the sperm or fertilizing fluid contained innumerable minute bodies endowed in nearly all cases with the power of active move-

ment, and therefore regarded by the early observers as parasitic animalcules or infusoria, a view which gave rise to the name sperma- tozoa (sperm-animals) by which they are still generally known. ^ As long ago as 1786, however, it was shown by Spallanzani that the fertilizing power must lie in the spermatozoa, not in the liquid in which they swim, because the spermatic fluid loses its power when filtered. Two years after the appearance of Schwann's epoch-mak- ino- work Kolliker demonstrated (1841) that the spermatozoa arise directly from cells in the testis, and hence cannot be regarded as parasites, but are, like the ovum, derived from the parent-body. Not until 1865, however, was the final proof attained by Schweigger- Seidel and La Valette St. George that the spermatozoon contains not only a nucleus, as Kolliker believed, but also cytoplasm. It was thus shown to be, like the ^^,g, a single cell, peculiarly modified in structure, it is true, and of extraordinary minuteness, yet on the whole morphologically equivalent to other cells. A final step was taken ten years later (1875), when Oscar Hertwig established the all-important fact that fertilization of the egg is accomplished by its union with one spermatozoon, and one only. In sexual repro- duction, therefore, each sex contributes a single cell of its own body to the formation of the offspring, a fact which beautifully tallies with the conclusion of Darwin and Galton that the sexes play, on the whole, equal, though not identical parts in hereditary trans- mission. The ultimate problems of sex, fertilization, inheritance, and development were thus shown to be cell-problems.

Meanwhile, during the years immediately following the announce- ment of the cell-theory, the attention of investigators was especially focussed upon the question : How do the cells of the body arise .? The origin of cells by the division of preexisting cells was clearly recognized by Hugo von Mohl in 1835, though the full significance of this epoch-making discovery was so obscured by the errrors of Schleiden and Schwann that its full significance was only perceived long afterward. The founders of the cell-theory were unfortunately led'to the conclusion that cells might arise in two different ways, viz. either by division or fission of a preexisting mother-cell, or by "Iree cell-formation," new cells arising in the latter case not from pre- existing ones, but by crystallizing, as it were, out of a formative or nutritive substance, termed the " cytoblastema " ; and they even beheved the latter process to be the usual and typical one. It was only after many years of painstaking research that " free cell- formation " was absolutely proved to be a myth, though many of

iThe discovery of the spermatozoa is generally accredited to Ludwig Hamm. a pupil of Leeuwenhoek (1677). though Ilartsoeker afterward claimed the ment of havmg seen them as early as 1674 (Dr. Allen Thomson). Schwann's immediate followers threw doubts upon it,^ and as early as 1855 Virchow positively maintained the universality of cell-divi- sion, contending that ever}- cell is the offs})ring of a preexisting parent-cell, and summing up in the since famous aphorism, " oniuis celliila c Cillula.^''^ At the ]:)resent day this conclusion rests upon a foundation so firm that we arc justified in regarding it as a universal law of development.

Now, if the cells of the body always arise by the division of pre- existing cells, all must be traceable back to the fertilized egg-cell as their common ancestor. Such is, in fact, the case in every plant and animal whose development is accurately known. The first step in development consists in the division of the <:.^^ into two j^arts, each of which is a cell, like the <t^^^ itself. The two then divide in turn to form four, eight, sixteen, and so on in more or less regular progres- sion (Fig. 4.) until step by step the f^gg has split up into the multitude of cells which build the body of the embryo, and finally of the adult. This process, known as the cleavage or segmentatioji of the ^^^^ was observed long before its meaning was understood. It seems to have been first definitely described in the case of the frog's ^^,^,, by Prevost and Dumas ( 1824), though earlier observers had seen it; but at this time neither the ^g'g nor its descendants were known to be cells, and its true meaning was first clearly perceived by Bergmann, Kolliker, Reichert, Von Baer, and Remak, some twenty years later. The interpretation of cleavage as a process of cell-division was fol- lowed by the demonstration that cell-division does not begin with cleavage, but can be traced back into the foregoing goieration ; for the egg-cell, as well as the sperm-cell, arises by the division of a cell pre- existing in the parent-body. // is therefore derived by direct descent from an egg-cell of the foregoing generation, and so on ad infinitnni. Embryologists thus arrived at the conception so vividly set forth by Virchow in 1858-'^ of an uninterrupted series of cell-divisions extend- ing backward from existing plants and animals to that remote and unknown period when vital organization assumed its present form. Life is a continuous stream.

The death of the individual involves no breach of continuitv in the series of cell-divisions bv which the life of the race flows onwards. The individual body dies, it is true, but the germ-cells live on, carrying with them, as it were, the traditions of the race from which they have sprung, and handing them on to their descendants. 1 Among these may be especially mentiDncd Mohl, L'ngcr, Nageli, Martin liarry, Goodsir, and Remak. 3 See the quotation from the original edition of the Celhdarpathologie at the head of Chapter II., p. 63.

We have thus arrived at the form in which the problems of heredity and development confront the investigator of the present day. It remains to point out more clearly how they are related to the general problems of evolution and to those post-Darwinian discussions in which Weismann has taken so active a part. All theories of evolu- Fig 4. — Cleavage of the ovum of the sea-urchin Toxopncustes, X 33°. ^''oni ''f^^- ^''^^ suc- cessive divisions up to the i6-cell stage (//) occupy about two hours. / is a section of the embryo (blastula) of three hours, consisting of approximately 128 cells surrounding a central cavity or blastocoel.

tion take the facts of variation and heredity as fundamental postulates, for it is by variation that new characters arise and by heredity that they are perpetuated. Darwin recognized two kinds of variation, both of which, being inherited and maintained through the conserving action of natural selection, might give rise to a permanent transfor- mation of species. The first of these includes congenital or mborn variations, i.e. such as appear at birth or are developed "spontane- ously," without discoverable connection with the activities of the organism itself or the direct effect of the environment upon it, though Darwin clearly recognized the fact that even such variations must indirectly be due to changed conditions acting upon the parental organism or on the germ. In a second class of variations were placed the so-called acquired characters, i.e. definite effects directly produced in the course of the individual life as the result of use and disuse, or of food, climate, and the like. The inheritance of congen- ital characters is now universally admitted, but it is otherwise with acquired characters. The inheritance of the latter, now the most debated question of biology, had been taken for granted by Lamarck a half-century before Darwin ; but he made no attempt to show how such transmission is possible. Darwin, on the other hand, squarely faced the physiological requirements of the problem, recognizing that the transmission of acquired characters can only be possible under the assumption that the germ-cell definitely reacts to all other cells of the body in such wise as to register the changes taking place in them. In his ingenious and carefully elaborated theory of pangenesis,^ Darwin framed a provisional physiological hypothesis of inheritance in ac- cordance with this assumption, suggesting that the germ-cells are reservoirs of minute germs or gemmules derived from every part of the body ; and on this basis he endeavoured to explain the trans- mission both of acquired and of congenital variations, reviewing the facts of variation and inheritance with wonderful skill, and buildinc: up a theory which, although it forms the most speculative and hypo- thetical portion of his writings, must always be reckoned one of his most interesting contributions to science.

In the form advocated by Darwin the theory of pangenesis has been generally abandoned in spite of the ingenious attempt to remodel it made by Brooks in 1883.- In the same year the whole aspect of the problem was changed, and a new'period of discussion inaugurated by Weismann, who put forth a bold challenge of the entire Lamarckian principle.'^ " I do not propose to treat of the whole problem of hered- ity, but only of a certain aspect of it, — the transmission of acquired characters, which has been hitherto assumed to occur. In taking this course I may say that it was impossible to avoid going back to the foundation of all phenomena of heredity, and to determine the sub- stance with which they must be connected. In my opinion this can only be the substance of the germ-cells ; and this substance trans-

fers its hereditary tendencies from generation to generation, at first unchanged, and always uninfluenced in any corresponding manner, by that which happens during the life of the individual which bears it. If these views be correct, all our ideas upon the transformation of species require thorough modification, for the whole princij^le of evolution by means of exercise (use and disuse) as professed by La- marck, and accepted in some cases by Darwin, entirely collapses" {I.e., p. 69).

It is impossible, he continues, that acquired traits should be trans- mitted, for it is inconceivable that definite changes in the body, or "soma," should so affect the protoplasm of the germ-cells as to cause corresponding changes to appear in the offspring. How, he asks, can the increased dexterity and power in the hand of a trained piano- player so affect the molecular structure of the germ-cells as to pro- duce a corresponding development in the hand of the child 'i It is a physiological impossibility. If we turn to the facts, we find, W'eis- mann affirms, that not one of the asserted cases of transmission of acquired characters will stand the test of rigid scientific scrutiny. It is a reversal of the true point of view to regard inheritance as taking place from the body of the parent to that of the child. The child inherits from the parent germ-cell, not from the parent-body, and the germ-cell owes its characteristics not to the body which bears it, but to its descent from a preexisting germ-cell of the same kind. Thus the body is, as it were, an offshoot from the germ-cell (Fig. 5). As

Fig. 5. — Diagram illustrating Weismann's theory of inheritance. G. The germ-cell, which by division gives rise to the body or soma (5) and to new germ-cells (G) which separate from the soma and repeat the process in each successive generation. far as inheritance is concerned, the body is merely the carrier of the germ-cells, which are held in trust for coming generations. Weismann's subsequent theories, built on this foundation, have given rise to the most eagerly contested controversies of the post- Darwinian period, and, whether they are to stand or fall, have played a most important part in the progress of science. For^aside_fromjhe truth or error of his special theories, it has been Weismann's great service to place the keystone between the work of the evolutionists and that of the cytologists, and thus to bring the cell-theory and the

evolution-theory into organic connection. It is from the point of view thus suggested that the present volume has been written. It has accordingly not been my primary object to dwell on the Diiuiitice of histology, still less to undertake an exhaustive description of all the modifications of cell-structure and cell-action ; and for these the stu- dent must refer to other and more extended treatises. Yet the broader questions with which we have to deal cannot ])rofitably be discussed apart from the concrete phenomena by which they are suggested, and hence a considerable part of the text is necessarily given over to descriptive detail ; but I hope that the reader will not lose sight of the relation of the part to the whole, or forget the primary intention of the work.

We shall follow a convenient, rather than a strictly logical, order of treatment, beginning in the first two chapters with a general sketch of cell-structure and cell-division. The following three chapters deal with the germ-cells, — the third with their structure and mode* of origin, the fourth with their union in fertilization, the fifth with the phenomena of maturation by which they are prepared for their union. The sixth chapter contains a critical discussion of cell-organization, completing the morphological analysis of the cell. In the seventh cha])ter the cell is considered with reference to its more fundamental chemical and physiological properties as a prelude to the examination of development which follows. The succeeding chapter approaches the objective point of the book by considering the cleavage of the ovum and the general laws of cell-division of which it is an expression. The ninth chapter, finally, deals with the elementary operations of development considered as cell-functions and with the theories of inheritance and development based upon them.

Bergh. R. S. — Vork'sungen liber die Zelle und die einfachen Gewebe : Wiesbaden^ Delage, Yves. — La Structure du Protoplasma et les Theories sur THdrdditd et les grands Problemes de la Biologie Gcnerale : Paris, 1895. Hacker. V. — Pra.xis und Theorie der Zellen- und Befruchtungslehre : Jena. 1899. Huxley. T. H. — Review of the Cell-theory: British and Foreign Medico-Chiriirgical Review, XIL, 1853. Remak. R. — Untersuchimgen iiber die Entwicklung der Wirbelthiere : Berlin,

Schleiden, M. J. — Beitrage zur Phytogenesis : M'uller's Arclih\ 1838. Translation in Sydenham Soc, XII. Loudon, 1847. Schwann. Th. — Mikroscopische Untersuchungen liber die Uebereinstimmung in der Structur und dem Wachsthum der Thiere und Pflanzen : Berlin, 1839. Trans- lation in Sydenham Soc. XII. London, 1847. Virchow, R. — Die Cellularpathologie in ihrer Begriindung auf physiologische und pathologische Gewebelehre : Berlin^ 1858. " Wir haben gesehen, dass alle Organismen aus wesentlich gleichen Thcilen, namlich aus Zellen zusammengesetzt sind, dass diese Zellen nach wesentlich densellKTi Cieset/en sich bilden und wachsen, dass also diese Prozesse iiberall auch durch dieselben Krafte hervorge- bracht warden miissen." Schwann.^

In the passage quoted above Schwann expressed a truth which subsequent research has estabhshed on an ever widening basis ; and we have now^ more than ever reason to believe that despite unending diversity of form and function all cells may be brought into definite relation to a common morphological and physiological type. We are, it is true, still unable to specify all its essential features, and hence can give no adequate brief definition of the cell. For practical pur- poses, however, no such definition is needed, and we may be content with the simple type that has been familiar to histologists since the time of Leydig and Max Schultze.

It should from the outset be clearly recognized that the term "cell" is a biological misnomer; for cells only rarely assume the form implied by the word of hollow chambers surrounded by solid walls. The term is merely an historical survival of a word casually employed by the botanists of the seventeenth century to designate the cells of certain plant-tissues which, when viewed in section, give somewhat the appearance of a honeycomb.^ The cells of these tis- sues are, in fact, separated by conspicuous solid walls which were mistaken by Schleiden, followed by Schwann, for their essential i)art. The living substance contained within the walls, to which Hugo von Mohl gave the r\2imQ protoplasm^ (1846), was at first overlooked or was regarded as a waste-product, a view based upon the fact that m many important plant-tissues such as cork ox wood it may wholly disappear, leaving only the lifeless walls. The researches of Herg- mann, Kolliker, Bischoff, Cohn, Max Schultze, and many others

2 The word seems to have been first employed by Robert Hooke, in 1665, to designate the minute cavities observed in cork, a tissue which he describcl as made up of •' httle boxes or cells distinct from one another " and separated In- solid walls. 3 The same word had been used by Purkinje some years before (1840) to designate the formative material of young animal embryos. showed, however, that most living cells are nut hollow but solid bodies, and that in many cases — for example, the colourless corpuscles of blood and lymph — they are naked masses of protoplasm not sur- rounded by definite walls. Thus it was proved that neither the vesicular form nor the presence of surrounding walls is an essential character, and that the cell-contents, i.e. the protoplasui, must be the seat of vital activity.

Within the protoplasm (Figs. 6 <S) lies a body, usually of definite rounded form, known as the nnclcus,'^ and this in turn often contains Passive bodies fmeta- plasm or paraplasm) suspended in the cy- toplasmic mesh work Fig. 6. — Diagram of a cell. Its basis consists of a meshwork containing numerous minute granules {microsomes) and traversing a transparent ground-substance. a one or more smaller bodies or nucleoli. By some of the earlier workers the nucleus was supposed to be, like the cell-wall, of sec- ondary im])ortance, and many forms of cells were described as being devoid of a nucleus ("cytodes" of Haeckel). Nearly all later re- searches have indicated, however, that the characteristic nuclear material, whether forming a single body or scattered in smaller masses, is always present, and that it plays an essential part in the life of the cell.

Besides the presence of protoplasm and nucleus, no other struc- tural features of the cell are yet known to be of universal occurrence. 1 First described by Fontana in 1 781, and recognized as a normal element of the cell by Robert Brown in 1S33. We may therefore still accept as valid the definition ^qven more than thirty years ago by Leydig and Max Schultzc, that a cell is a mass of protoplasm containing a nucleus} to which we may add Schultze's statement that both nucleus and protoplasm arise tlirougli the division of the corresponding elements of a preexisting cell. Nothing could be less appropriate than to call such a body a ''cell " ; yet the word has become so firmly established that every effort to replace it bv a better has failed, and it probably must be accepted as part of the established nomenclature of science.^

The cell is a rounded mass of protoplasm which in its simplest form is approximately spherical. The form is, however, seldom realized save in isolated cells such as the unicellular plants and ani- mals or the egg-cells of the higher forms. In vastly the greater number of cases the typical spherical form is modified by unequal growth and differentiation, by active movements of the cell-substance, or by the mechanical pressure of surrounding structures, but true angular forms are rarely if ever assumed save by cells surrounded by hard walls. The protoplasm which forms its active basis is a viscid, translucent substance, sometimes apparently homogeneous, more fre- quently finely granular, and as a rule giving the appearance of a meshwork, which is often described as a spongelike or netlike '' reticu- lum." ^ Besides the active substance or protoplasm proper the cell almost invariably contains various lifeless bodies suspended in the meshes of the network; examples of these are food-granules, pig- ment-bodies, drops of oil or w^ater, and excretory matters. These bodies play a relatively passive part in the activities of the cell, being either reserve food-matters destined to be absorbed and built up into the living substance, or by-products formed from the proto- plasm as waste-matters or in order to play some role subsidiary to the actions of the protoplasm itself. The passive inclusions in the protoplasm maybe collectively designated as metaplasm (Hanstein) ox paraplasm {Yi\\^^^^x\ in contradistinction to the -AQixxc protoplas}n.

2 Sachs has proposed the convenient word cnergid {flora, '92, p. 57) to dcsijjnatc the essential living part of the cell, i.e. the nucleus with that portion of the active cyt«»plasm that falls within its sphere of influence, the two forming an organic unit both in a morpho- logical and in a physiological sense. It is to be regretted that this convenient and appro- priate term has not come into general use. (See also Flora, '95, p. 405. and cf. Kupfter ('96), Meyer ('96), and Kolliker ('97).)

3 Such meshworks are sometimes plainly visible in the living protoplasm (p. 44). It is always more or less an open question how far the appearances seen in hxed (coagulated) material correspond with the conditions existing in life. See pp. 42-46. It is often difficult to distinguish between such metaplasmic bodies and the granules commonly supposed to be elements of the active protoplasm; indeed, as will appear beyond (p. 29), there is reason to believe that "protoplasmic" and "metaplasmic" granules cannot be separated by any definite limit, but are connected by various gradations. Among the lifeless products of the protoplasm must be reckoned also the ctli-ica!/ or lucnibrauc bv which the cell-body may

Fig. 7 — Spermatogonia of the salamander. [Meves.] Above, two cells showing large nuclei, with linin-threads and scattered chromatin-graniiles ; in each cell an attraction-sphere with two centrosomes. Below, three contiguous spermatogonia, showing chromatin-reticulum, centrosomes and spheres, and sphere-bridges. be surrounded ; but it must be remembered that the cell-wall in some cases arises by a direct transformation of the protoplasmic substance, and that it often retains the power of growth by intussusception like living matter.

It is unfortunate that some confusion has arisen in the use of the word protoplasm. When Leydig, Schultze, Brlicke, De Bary, and other earlier writers spoke of "protoplasm," they had in mind only the substance of the cell-body, not that of the nucleus. Strasburger, Kolliker, etc.) in its orif^inul narrower sense (equivalent to Stras-. burger's cytoplasm), while perhaps the majority of writers have accepted the terminology of Strasburger and Flemming. On the whole, the terms cytoplasin and kiDyop/asui seem too useful to be rejected, and, without attaching too much importance to them, they will be employed throughout the present work. It must not, how- ever, be supposed that either of the words denotes a single homo- geneous substance; for, as will soon appear, both cytoplasm and karyoplasm consist of several distinct elements.