The Cell in Development and Inheritance

Castle, W. E.— The early Embryology of Ciona. Bull. Mus. Conip. ZooL, XXVII. Conklin, E. G. — The Embryology of Crepidula : Journ. Morpli., XIII. 1 897. Driesch, H. — (See Literature. IX.) Errera, L. — Zellformen unci Seifenblaseii : Tivj^cbl. dcr 60 Vcrsatiuiilniij^ licutsilwr Nafnrforsc/ia'- imd Aerzte zii Wiesbaden. 1887. Hertwig, 0. — Das Problem der Befruchtung und der Isotropic des Eies, eine Theo- rie der Vererbung. Jena^ 1884. Hofmeister. — ^Die Lehre von der Pflanzenzelle. Leipzig., 1867. Jennings, H. S. — The Early Development of Asplanchna : BulL Miis. Coup. Zool.,

Lillie, F. R. — The Embryology of the Unionidae : Journ. Morph.., X. 1895. Id. — Adaptation in Cleavage : Wood'' s Hall Biol. Lectures. 1899. McMurrich, J. P. — Embryology of the Isopod Crustacea: Journ. Morpli., XI. i. Rhumbler, L. — Allgemeine Zellmechanik : Merkel u. Bonnet., Ergeb., WW. 1898. Sachs, J. — Pflanzenphysiologie. (See list VII.) Sedgwick, H. — On the Inadequacy of the Cellular Theory of Development, etc.: Quart. Journ . Mic . Sci. , XXXVII. i. 1894. Strasburger, E. — Uber die Wirkungssphare der Kerne und die Zellgrcisse : Histo-

logische Beitrdge, V. 1893. Zur Strassen, 0. — Embryonalentwickelung der Ascaris : Arch. Ento/n.. III. 1896. Watase, S. — Studies on Cephalopods ; I., Cleavage of the Ovum : Journ. Morph., Whitman, C. 0. — The Inadequacy of the Cell-theory of Development : Wood's Moll Id. — Considerations on Cell-lineage and Ancestral Reminiscence: Ann. X. ). " It is certain that the germ is not merely a body in which life is dormant or potential, but that it is itself simply a detached portion of the substance of a preexisting living body."

" Inheritance must be looked at as merely a form of growth." Darwin.2 " Ich mochte daher wohl den Versuch wagen, durch eine Darstellung des Beobachteten Sie zu einer tiefern Einsicht m die Zeugungs- und Entwickelungsgeschichte der organischen Korper zu fiihren und zu zeigen, wie dieselben weder vorgebildet sind, noch auch, wie man sich crewohnlich denkt, aus ungeformter Masse in einem bestimmten Momente plotzlich ausschiessen." Von Baer.3 Every discussion of inheritance and development must take as its point of departure the fact that the germ is a single cell similar in its essential nature to any one of the tissue-cells of which the body is composed. That a cell can carry with it the sum total of the heritage of the species, that it can in the course of a few days or weeks give rise to a mollusk or a man, is the greatest marvel of biological science. In attempting to analyze the problems that it involves, we must from the outset hold fast to the fact, on which Huxley insisted, that the wonderful formative energy of the germ is not impressed upon it from without, but is inherent in the &gg as a heritage from the paren- tal life of which it was originally a part. The development of the embryo is nothing new. It involves no breach of continuity, and is but a continuation of the vital processes going on in the parental . body. What gives development its marvellous character is the rapid- ity with which it proceeds and the diversity of the results attained in a span so brief.

But when we have grasped this cardinal fact, we have but focussed our instruments for a study of the real problem. How do the adult characteristics lie latent in the germ-cell ; and how do they become patent as development proceeds ? This is the final question that looms in the background of every investigation of the cell. In approaching it we may well make a frank confession of ignorance ; for in spite of all that the microscope has revealed, we have not yet penetrated the mystery, and inheritance and development still remain in their fun-

damental aspects as great a riddle as they were to the Greeks. What we have gained is a tolerably precise acquaintance with the external aspects of development. The gross errors of the early preformation- ists have been dispelled. ^ We know that the germ-cell contains no predelineated embryo ; that development is manifested, on the one hand, by the cleavage of the ^gg, on the other hand, by a process of differentiation, through which the products of cleavage gradually assume diverse forms and functions, and so accomplish a physiological division of labour. We can clearly recognize the fact that these pro- cesses fall in the same category as those that take place in the tissue- cells ; for the cleavage of the ovum is a form of mitotic cell-division, while, as many eminent naturalists have perceived, differentiation is nearly related to growth and has its root in the phenomena of nutri- tion and metabolism. The real problem of development is the orderly sequence and congelation of these pJic7iomena toward a typical result. We cannot escape the conclusion that this is the outcome of the organization of the germ-cells ; but the nature of that which, for lack of a better term, we call ''organization," is and doubtless long will remain almost wholly in the dark.

In the following discussion, which is necessarily compressed within narrow limits, we shall disregard the earlier baseless speculations, such as those of the seventeenth and eighteenth centuries, which attempted a merely formal solution of the problem, confining our- selves to more recent discussions that have grown directlv out of modern research. An introduction to the general subject may be given by a preliminary examination of two central hypotheses about which most recent discussions have revolved. These are, first, the theory of Germinal Localization'^ of Wilhelm His ('74), and, second, the Idioplasm Hypothesis of Nageli ('84). The relation between these two conceptions, close as it is, is not at first sight very apparent ; and for the purpose of a preliminary sketch they may best be con- sidered separately.

Although the naive early theory of preformation and evolution was long since abandoned, yet we find an after-image of it in the theory of germinal localization which in one form or another has been advo- cated by some of the foremost students of development. It is main- tained that, although the embryo is not \)X^formed in the germ, it must nevertheless be Y>^Qdetermined in the sense that the <tgg contains 2 I venture to suggest this term as an English equivalent for the awkward expression " Organbildende Keimbezirke " of His.

definite areas or definite substances predestined for the formation of corresponding parts of the embryonic body. The first clear state- ment of this conception is found in the interesting and suggestive work of Wilhehii His ('74) entitled Unsere Kdrperform. Considering the development of the chick, he says : '* It is clear, on the one hand, that every point in the embryonic region of the blastoderm must rep- resent a later organ or part of an organ, and, on the other hand, that every organ developed from the blastoderm has its preformed germ (vorgebildete Anlage) in a definitely located region of the flat germ- ^ disc. . . . The material of the germ is already present in the flat germ-disc, but is not yet morphologically marked off and hence not directly recognizable. But by following the development back- wards we may determine the location of every such germ, even at a period when the morphological differentiation is incomplete or before it occurs ; logically, indeed, we must extend this process back to the fertihzed or even the unfertilized Q,gg. According to this principle, the germ-disc contains the organ-germs spread out in a flat plate, and, conversely, every point of the germ-disc reappears in a later organ ; I call this the principle of organ- forming germ-regions T ^ His thus conceived the embryo, not as ^x^formed, but as having all of its parts // ^x^localized'm the egg-protoplasm (cytoplasm).

A great impulse to this conception was given during the follow- ing decade by discoveries relating, on the one hand, to protoplasmic structure, on the other hand, to the promorphological relations of the ovum. Ray Lankester writes, in 1877: ''Though the substance of a cell 2 may appear homogeneous under the most powerful microscope, it is quite possible, indeed certain, that it may contain, already formed and individualized, various kinds of physiological molecules. The visible process of segregation is only the sequel of a differentiation already estabUshed, and not visible."^ The egg-cytoplasm has a defi- nite molecular organization directly handed down from the parent; cleavage sunders the various " physiological molecules " and iso- lates them in particular cells. Whitman expresses a similar thought in the following year : " While we cannot say that the embryo is pre- delineated, we can say that it is predetermined. The ' histogenetic sundering ' of embryonic elenients begins with the cleavage, and every step in the process bears a definite and invariable relation to antece- dent and subsequent steps. ... It is, therefore, not surprising to find certain important histological differentiations and fundamental structural relations anticipated in the early phases of cleavage, and foreshadowed even before cleavage begins." ^ It was, however, Flem-

2 It is clear from the context that by " substance " Lankester had m nimd the:_Qto;^iasiii, though this is not specifically stated. ^ '77, p. 14. , * '785 P- 49- ming who gave the first specific statement of the matter from the cyto- logical point of view : '^ But if the substance of the egg-cell has a definite structure (Bau), and if this structure and the nTture of the network varies in different regions of the cell-body, we may seek in it a basis for the predetermination of development wherein one ec'-o- differs from another, and it will be possible to look for it luitk 7he microscope. How far this search can be carried no one can say, but its ultimate aim is nothing less than a true morphology of inhcritancer 1 In the following year Van Beneden pointed out how nearly this con- ception approaches to a theory of preformation: "If this were the case {i.e. if the egg-axis coincided with the principal axis of the adult body), the old theory of evolution would not be as baseless as we think to-day. The fact that in the ascidians, and probably in other bilateral animals, the median plane of the body of the future ani- mal is marked out from the beginning of cleavage, fully justifies the hypothesis that the materials destined to form the right side of the body are situated in one of the lateral hemispheres of the ^gg, while the left hemisphere gives rise to all of the organs of the left half." 2 The hypothesis thus suggested seemed, for a time, to be placed on a secure basis of fact through a remarkable experiment subsequently performed by Roux i^'^^) on the frog's ^gg. On killing one of the blastomeres of the two-cell stage by means of a heated needle the un- injured half developed in some cases into a well-formed half-larva (Fig. 182), representing approximately the right or left half of the body, containing one medullary fold, one auditory pit, etc.^ Analo- gous, though less complete, results were obtained by operating with the four-cell stage.

Roux was thus led to the declaration (made with certain subsequent reservations) that " the development of the fro^-- gastrula and of the embryo formed from it is from the second cleavage onward a mosaic-work, consisting of at least four vertical indepen- dently developing pieces."* This conclusion seemed to form a very strong support to His's theory of germinal localization, though, as will appear beyond, Roux transferred this theory to the nucleus, and thus developed it in a very different direction from Lankester or Van Beneden. His's theory also received very strong apparent sup- port through investigations on cell-lineage by Whitman. Rabl, and

^ The accuracy of this result was disputed by Oscar Hertwig ('93, i), who found that the uninjured blastomere gave rise to a defective larva, in which certain parts were missing, but not to a true half-body. Later observers, especially Schultze, Endres, and Morgan, have, however, shown that both Hertwig and Roux were right, proving that the uninjured blasto- mere may give rise to a true half-larva, to a larva with irregular defects, or to a whole larva of half-size, according to circumstances (p. 422). * l-C; P- 30-

many later observers, which have shown that in the cleavage of anne- lids, moUusks, platodes, tunicates, and many other animals, every cell has a definite origin and fate, and plays a definite part in the building of the body.^ Fig. 182. — Half-embryos of the frog (in transverse section) arising from a blastomere of the two-cell stage after killing the other blastomere, [Roux.] A. Half-blastula (dead blastomere on the left). B. Later stage. C. Half-tadpole with one medullary fold and one mesoblast plate ; regeneration of the missing (right) half in process.

ar. archenteric cavity ; c.c. cleavage-cavity ; ch. notochord ; ?«./. medullary fold ; m.s. meso- blast-plate. In an able series of later works Whitman has followed out the sug- gestion made in his paper of 1878, cited above, pointing out how essential a part is played in development by the cytoplasm and insist- ing that cytoplasmic preorganization must be regarded as a leading factor in the ontogeny. Whitman's interesting and suggestive views are expressed with great caution and with a full recognition of the

difficulty and complexity of the problem. From his latest essay, in- deed ('94), it is not easy to gather his precise position regarding the theory of cytoplasmic localization. Through all his writings, never- theless, runs the leading idea that the germ is definitely organized before development begins, and that cleavage only reveals an organi- zation that exists from the beginning. '' That organization precedes cell-formation and regulates it, rather than the reverse, is a conclu- sion that forces itself upon us from many sides." ^ " The organism exists before cleavage sets in, and persists throughout every stage of cell-multiplication." ^

All of these views, excepting those of Roux, lean more or less distinctly toward the conclusion that the cytoplasm of the egg-cell is from the first mapped out, as it were, into regions which corre- spond with the parts of the future embryonic body. The cleavage of the ovum does not create these regions, but only reveals them to view by marking off their boundaries. Their topographical arrange- ment in the ^gg does not necessarily coincide with that of the adult parts, but only involves the latter as a necessary consequence — some- what as a picture in the kaleidoscope gives rise to a succeeding pic- ture composed of the same parts in a different arrangement. The germinal localization may, however, in a greater or less degree, fore- shadow the arrangement of adult parts — for instance, in the ^gg of the tunicate or cephalopod, where the bilateral symmetry and antero- posterior differentiation of the adult is foreshadowed not only in the cleavage stages, but even in the un segmented ^gg.

By another set of wTiters, such as Roux, De Vries, Hertwig, and Weismann, germinal localization is primarily sought not in the cyto- plasm, but in the nucleus; but these views can be best considered after a review of the idioplasm hypothesis, to which we now proceed. We owe to Nageli the first systematic attempt to discuss heredity regarded as inherent in a definite physical basis ; ^ but it is hardly necessary to point out his great debt to earlier writers, foremost among them Darwin, Herbert Spencer, and Hackel. The essence of Nageli's hypothesis was the assumption that inheritance is effected by the transmission not of a cell, considered as a whole, but of a par- ticukr sub'stance, the idioplasm, contained within a cell, and forming "the physical basis of heredity. The idioplasm is to be sharply dis- tinguished from the other constituents of the cell, which play no direct part in inheritance and form a "nutritive plasma" or tropJio-

plasm. ■■ Hereditary traits are the outcome of a definite molecular organization of the idioplasm. The hen's ^gg differs from the frog's because it contains a different idioplasm. The species is as com- pletely contained in the one as in the other, and the hen's ^gg differs from a frog's ^gg as widely as a hen from a frog. The idioplasm was conceived as an extremely complex substance, consisting of elementary complexes of molecules known as micellce. These are variously grouped to form units of higher orders, which, as development proceeds, determine the development of the adult cells, tissues, and organs. The specific peculiarities of the idioplasm are therefore due to the arrangement of the micellae ; and this, in its turn, is owing to dynamic properties of the micellae themselves. During development the idioplasm undergoes a progressive trans- formation of its substance, not through any material change, but through dynamic alterations of the conditions of tension and move- ment of the micella. These changes in the idioplasm cause reactions on the part of surrounding structures leading to definite chemical and plastic changes, i.e. to differentiation and development.

Nageli made no attempt to locate the idioplasm precisely or to identify it with any of the known morphological constituents of the cell, it was somewhat vaguely conceived as a network extending through both nucleus and cytoplasm, and from cell to cell through- out the entire organism. Almost immediately after the publication of his theory, however, several of the foremost leaders of biological investigation were led to locate the idioplasm in the nucleus, and concluded that it is to be identified with chromatin. The grounds for this conclusion, which have already been stated in Chapter VII., may be here again briefly reviewed. The beautiful experiments of Nussbaum, Gruber, and Verworn proved that the regeneration of differentiated cytoplasmic structures in the Protozoa can only take place when nuclear matter is present {cf. p. 342). The study of fertilization by Hertwig, Strasburger, and Van Beneden proved that in the sexual reproduction of both plants and animals the nucleus of the germ is equally derived from both sexes, while the cytoplasm is derived almost entirely from the female. The two germ-nuclei, which by their union give rise to. that of the germ, were shown by Van Beneden to be of exactly the same morphological nature, since each gives rise to chromosomes of the same number, form, and size. Van Beneden and Boveri proved (p. 182) that the paternal and maternal nuclear substances are equally distributed to each of the first two cells, and the more recent work of Hacker, Rlickert, Herla, and Zoja estabhshes a strong probability that this equal distribution con- tinues in the later divisions. Roux pointed out the telling fact that the entire complicated mechanism of mitosis seems designed to affect

the most accurate division of the entire nuclear substance in all of its parts, while fission of the cytoplasmic cell-body is in the main a mass-division, and not a meristic division of the individual parts. Again, the complicated processes of maturation show the significant fact that while the greatest pains is taken to prepare the germ-nuclei for their coming union, by rendering them exactly equivalent, the cytoplasm becomes widely different in the two germ-cells and is devoted to entirely different functions.

It was in the main these considerations that led Hertwig, Stras- burger, Kolliker, and Weismann independently and almost simultane- ously to the conclusion that the nucleus contains the physical basis of inJietitance, and that chromatin, its essential constituent, is the idio- plasm postulated in NdgelVs theory. This conclusion is now widely accepted and rests upon a basis so firm that it must be regarded as a working hypothesis of high value. To accept it is, however, to reject the theory of germinal localization in so far as it assumes a prelocali- zation of the egg-cytoplasm as a fundamental character of the tgg. For if the specific character of the organism be determined by an idioplasm contained in the chromatin, then every characteristic of the cytoplasm must in the long run be determined from the same source. A striking illustration of this point is given by the phenomena of colour-inheritance in plant-hybrids, as De Vries has pointed out. Pigment is developed in the embryonic cytoplasm, which is derived from the mother-cell ; yet in hybrids it may be inherited from the male through the nucleus of the germ-cell. The specific form of cytoplasmic metabolism by which the pigment is formed must there- fore be determined by the paternal chromatin in the germ-nucleus, and not by a predetermination of the egg-cytoplasm.

We have now to consider the attempts that have been made to transfer the localization-theory from the cytoplasm to the nucleus, and thus to bring it into harmony with the theory of nuclear idio- plasm. These attempts are especially associated with the names of Roux, De Vries, Weismann, and Hertwig ; but all of them may be traced back to Darwin's celebrated hypothesis of pangenesis as a prototype. This hypothesis is so well known as to require but a brief review. Its fundamental postulate assumes that the germ-cells contain innumerable ultra-microscopic organized bodies or geinjnules, each of which is the germ of a cell and determines the development of a similar cell during the ontogeny. The germ-cell is, therefore, in Darwin's words, a microcosm formed of a host of inconceivably minute self-propagating organisms, every one of which predetermines

the formation of one of the adult cells. De Vries ('89) brought this conception into relation with the theory of nuclear idioplasm by assuming that the gemmules of Darwin, which he z?^^^ pangcns, are contained in the nucleus, migrating thence into the cytoplasm step by step during ontogeny, and thus determining the successive stages of development. The hypothesis is further modified by the assump- tion that the pangens are not cell-germs, as Darwin assumed, but ultimate protoplasmic units of which cells are built, and which are the bearers of particular hereditary qualities. The same view was afterward accepted by Hertwig and Weismann.^

The theory of germinal localization is thus transferred from the cytoplasm to the nucleus. It is not denied that the egg-cytoplasm may be more or less distinctly differentiated into regions that have a constant relation to the parts of the embryo. This differentiation is, however, conceived, not as a primordial characteristic of the ^gg, but as one secondarily determined through the influence of the nucleus. Both De Vries and Weismann assume, in fact, that the entire cyto- plasm is a product of the nucleus, being composed of pangens that migrate out from the latter, and by their active growth and multipli- cation build up the cytoplasmic substance.^

We now proceed to an examination of two sharply opposing hy- potheses of development based on the theory of nuclear idioplasm. One of these originated with Roux i^^^ and has been elaborated especially by Weismann. The other was clearly outlined by De Varies ('89), and has been developed in various directions by Oscar Hertwig, Driesch, and other writers. In discussing them, it should be borne in mind that, although both have been especially developed by the advocates of the pangen-hypothesis, neither necessarily involves that hypothesis in its strict form, i.e. the postulate of discrete self-propa- gating units in the idioplasm. This hypothesis may therefore be laid

2 The neo-pangenesis of De Vries differs from Darwin's hypothesis in one very important respect. Darwin assumed that the gemmules arose in the body, being thrown off as germs by the individual tissue-cells, transported to the germ-cells, and there accumulated as in a reservoir; and he thus endeavoured to explain the transmission of acquired characters. De Vries, on the other hand, denies such a transportal from cell to cell, maintaining that the pangens arise or preexist in the germ-cell, and those of the tissue-cells are derived from this source by cell-division.

^ This conception obviously harmonizes with the role of the nucleus in the synthetic process. In accepting the view that the nuclear control of the cell is effected by an emana- tion of specific substances from the nucleus, we need not, however, necessarily adopt the pangen-hypothesis. aside as an open question,^ and will be considered only in so far as it is necessary to a presentation of the views of individual writers. The Roux-Weismann hypothesis has already been touched on at page 245. Roux conceived the idioplasm {i.e. the chromatin) not as a single chemical compound or a homogeneous mass of molecules, but as a highly complex mixture of different substances, representing different qualities, and having their seat in the individual chromatin- granules. In mitosis these become arranged in a linear series to form the spireme-thread, and hence may be precisely divided by the splitting of the thread. Roux assumes, as a fundamental postulate, that division of the granules may be either quantitative or qualitative. In the first mode the group of qualities represented in the mother- granule is first doubled and then split into equivalent daughter-groups, the daughter-cells therefore receiving the same qualities and remain- ing of the same nature. In ''qualitative division," on the other hand, the mother-group of qualities is split into dissimilar groups, which, passing into the respective daughter-nuclei, lead to a eorrespondin^^ diff'erentiation in the daughter-cells. By qualitative di\isions, occur- ring in a fixed and predetermined order, the idioplasm is thus split up during ontogeny into its constituent qualities, which are, as it were, sifted apart and distributed to the various nuclei of the embrvo. Every cell-nucleus, therefore, receives a specific form of chromatin which determines the nature of the cell at a given period and its later his- tory. Every cell is thus endowed with a power of self determination, which lies in the specific structure of its nucleus, and its course of development is only in a minor degree capable of modification through the relation of the cell to its fellows ("correlative differentiation ").

Roux's hypothesis, be it observed, does not commit him to the theory of pangenesis. It was reserved for Weismann to develop the hypothesis of qualitative division in terms of the pangen-hypothesis, and to elaborate it as a complete theory of development. In his Hrst essay ('85), published before De Vries's paper, he went no far- ther than Roux. " I believe that we must accept the hypothesis that in indirect nuclear division, the formation of non-equivalent halves may take place quite as readily as the formation of equivalent halves, and that the equivalence or non-equivalence of the subsecjuently i)ro- duced daughter-cells must depend upon that of the nuclei. Thus, during ontogeny a gradual transformation of the nuclear substance takes place, necessarily imposed upon it, according to certain laws, by its own nature, and such transformation is accompanied by a gradual change in the character of the cell-bodies." - In later writ- ings Weismann advanced far beyond this, building up an elaborate artificial system, which appears in its final form in the remarkable

book on the germ-plasm ('92). Accepting De Vries's conception of the pangens, he assumes a definite grouping of these bodies in the germ-plasm or idioplasm (chromatin), somewhat as in Nageli's concep- tion. The pangens or biopJiores are conceived to be successively ag- gregated in larger and larger groups; namely, (i ) determinants, which are still beyond the limits of microscopical vision ; (2) ids, which are identified with the visible chromatin-granules ; and (3) idants, or chromosomes. The chromatin has, therefore, a highly complex fixed architecture, which is transmitted from generation to generation, and determines the development of the embryo in a definite and specific manner. Mitotic division is conceived as an apparatus which may distribute the elements of the chromatin to the daughter-nuclei either equally or unequally. In the former case {'' Jiomoeokinesis,'' integral or quantitative division), the resulting nuclei remain precisely equiva- lent. In the second c^lSQ {'' /ietej'oki7iesis,'' qualitative ox dijferential division), the daughter-cells receive different groups of chromatin- elements, and hence become differently modified. During ontogeny, through successive qualitative divisions, the elements of the idioplasm or germ-plasm (chromatin) are gradually sifted apart, and distributed in a definite and predetermined manner to the various parts of the body. " Ontogeny depends on a gradual process of disintegration of the id of germ-plasm, which splits into smaller and smaller groups of determinants in the development of each individual. . . . Finally, if we neglect possible complications, only one kind of determinant re- mains in each cell, viz. that which has to control that particular cell or group of cells. ...