The cell in development and inheritance (1900) 9

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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

Wilson EB. The Cell in Development and Inheritance. Second edition (1900) New York, 1900.

   Cell development and inheritance (1900): Introduction | List of Figures | Chapter I General Sketch of the Cell | Chapter II Cell-division | Chapter III The Germ-cells | Chapter IV Fertilization of the Ovum | Chapter V Reduction of the Chromosomes, Oogenesis and Spermatogenesis | Chapter VI Some Problems of Cell-organization | Chapter VII Some Aspects of Cell-chemistry and Cell-physiology | Chapter VIII Cell-division and Development | Chapter IX Theories of Inheritance and Development | Glossary
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Chapter IX Theories of Inheritance and Development

"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."

Huxley.

  • Evolution, Science and Culture, p. 291.


" Inheritance must be looked at as merely a form of growth."

Darwin.

  • Variation of Animals and Plants* II., p. 398.


"Ich mochte daher wohl den Versuch wagen, durch eine Darstellung des Beobachtetei Sie zu einer tiefern Einsicht in die Zeugungs- und Entwickelungsgeschichte der organischen Korpcr zu fuhren und zu zeigen, wie dieselben weder vorgebildet sind, noch auch, wie mu sich gewohnlich denkt, aus ungeformter Masse in einem bestimmten Momente plotzlkh ausschiessen."

Von Baejl*

  • Entwick. der Thierc, II., 1837, P- &


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 egg as a heritage from the parental 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 rapidity 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 fundamental 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 preformationists have been dispelled. 1 We know that the germ-cell contains no predelineated embryo ; that development is manifested, on the one hand, by the cleavage of the egg, 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 processes fall in the same category as those that take place in the tissuecells ; 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 nutrition and metabolism. The real problem of development is the orderly sequence and correlation of these phenontetia 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 ourselves to more recent discussions that have grown directly 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 Localisation 2 of Wilhelm His C74), 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 considered separately.

A. The Theory of Germinal Localization

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 advocated by some of the foremost students of development. It is maintained that, although the embryo is not preformed in the germ, it must nevertheless be predetermined in the sense that the egg contains definite areas or definite substances predestined for the formation of corresponding parts of the embryonic body. The first clear statement of this conception is found in the interesting and suggestive work of Wilhelm His ('74) entitled Unsere Korperform. 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 represent 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 germdisc. . . . 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 backwards 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 fertilized or even the unfertilized egg. 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." 1 His thus conceived the embryo, not as preformed, but as having all of its parts prelocalized in the egg-protoplasm (cytoplasm).


1 Cf. Introduction, p. 8.

3 I venture to suggest this term as an English equivalent for the awkward expression " Organbildende Keimbezirke " of His.


A great impulse to this conception was given during the following 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 established, and not visible." 8 The egg-cytoplasm has a definite molecular organization directly handed down from the parent; cleavage sunders the various " physiological molecules " and isolates them in particular cells. Whitman expresses a similar thought in the following year : " While we cannot say that the embryo is predelineated, we can say that it is predetermined. The ' histogenetic sundering ' of embryonic elements begins with the cleavage, and every step in the process bears a definite and invariable relation to antecedent 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." 4 It was, however, Flem 1 /. c. % p. 19.

2 It is clear from the context that by " substance " lankester had in mind the cytoplasm, though this is not specifically stated. 3 '77, p. 14. * '78, p. 49.


THE THEORY OF GERMINAL LOCALIZATION 399

ming who gave the first specific statement of the matter from the cytological point of view : " But if the substance of the egg-cell has a definite structure (Bau), and if this structure and the nature 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 egg differs from another, and it will be possible to look for it with the microscope. How far this search can be carried no one can say, but its ultimate aim is nothing less than a true morphology of inheritance l In the following year Van Beneden pointed out how nearly this conception 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 animal 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 egg, 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 ('88) on the frog's egg. On killing one of the blastomeres of the two-cell stage by means of a heated needle the uninjured 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 medulUry fold, one auditory pit, etc. 8 Analogous, 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 froggastrula and of the embryo formed from it is from the second cleavage onward a mosaic-work, consisting of at least four vertical independently developing pieces." 4 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 support through investigations on cell-lineage by Whitman, Rabl, and

1 Zellsuhstanz, '82, p. 70 : the italics are in the original.

2 '^ P- 571.

8 The accuracy of this result was disputed by Oscar Hertwig ('93, 1), 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. I^ater observers, especially Schultze, Kndrcs, and Morgan, have, however, shown that both Hertwig and Roux were right, proving that the uninjured blastomere 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). 4 /.r., p. 30.


1


400


INHERITANCE AND DEVELOPMENT


many later observers, which have shown that in the cleavage of annelids, mollusks, 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. 1



Pig. 1B1. — Half-embryos of the frog (in transverse section) arising from a blastomere o two-cell stage after killing the other blastomere. [Roux.]

A. Hall-blastula (dead blastomere on the left). B. Later siage. C. Half-tadpole with one medullary fold and one mesoblast plate ; regeneration of the missing (right) half in process.

or. archentcric cavity; ex. cleavage-cavity; ch. notochord; m.f. medullary fold; nij. mesoblast-plate.


In an able series of later works Whitman has followed out the suggestion made in his paper of 1878, cited above, pointing out how essential a part is played in development by the cytoplasm and insisting 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 " Cf. p. 378


THE IDIOPLASM THEORY 401

difficulty and complexity of the problem. From his latest essay, indeed ('94), it is not easy to gather his precise position regarding the theory of cytoplasmic localization. Through all his writings, nevertheless, runs the leading idea that the germ is definitely organized before development begins, and that cleavage only reveals an organization that exists from the beginning. " That organization precedes cell-formation and regulates it, rather than the reverse, is a conclusion that forces itself upon us from many sides." l " The organism exists before cleavage sets in, and persists throughout every stage of cell-multiplication." 2

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 correspond 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 arrangement in the egg does not necessarily coincide with that of the adult parts, but only involves the latter as a necessary consequence — somewhat as a picture in the kaleidoscope gives rise to a succeeding picture composed of the same parts in a different arrangement. The germinal localization may, however, in a greater or less degree, foreshadow the arrangement of adult parts — for instance, in the egg of the tunicate or cephalopod, where the bilateral symmetry and anteroposterior differentiation of the adult is foreshadowed not only in the cleavage stages, but even in the unsegmented egg.

By another set of writers, such as Roux, De Vries, Hertwig, and Weismann, germinal localization is primarily sought not in the cytoplasm, but in the nucleus ; but these views can be best considered after a review of the idioplasm hypothesis, to which we now proceed.


B. The Idioplasm Theory

We owe to Nageli the first systematic attempt to discuss heredity regarded as inherent in a definite physical basis; 3 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 particular substance, the idioplastn, contained within a cell, and forming the physical basis of heredity. The idioplasm is to be sharply distinguished from the other constituents of the cell, which play no direct part in inheritance and form a "nutritive plasma" or tropho 1 '93, p. 115. 3 /,c, p. 112. 8 Theorie der Abstammungslehre, 1884.

2D


402 INHERITANCE AND DEVELOPMENT

plasm. Hereditary traits are the outcome of a definite molecular organization of the idioplasm. The hen's egg differs from the frog's because it contains a different idioplasm. The species is as completely contained in the one as in the other, and the hen's egg differs from a frog's egg 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 micella. 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 transformation of its substance, not through any material change, but through dynamic alterations of the conditions of tension and movement of the micellae. 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 throughout 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, Riickert, Herla, and Zoja establishes a strong probability that this equal distribution continues in the later divisions. Roux pointed out the telling fact that the entire complicated mechanism of mitosis seems designed to affect


UNION OF THE TWO THEORIES 403

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, Strasburger, Kolliker, and Weismann independently and almost simultaneously to the conclusion that the mule us contains the physical basis of inhetitance, and that chromatin^ its essential constituent, is the idioplasm postulated in NagclVs 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 prelocalization of the egg-cytoplasm as a fundamental character of the egg. 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 therefore be determined by the paternal chromatin in the germ-nucleus, and not by a predetermination of the egg-cytoplasm.

C. Union of the Two Theories

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 idioplasm. 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 gennnules y 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


404 INHERITANCE AND DEVELOPMENT

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 called pangens, 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 assumption that the pangefis 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. 2

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 egg, but as one secondarily determined through the influence of the nucleus. Both De Vries and Weismann assume, in fact, that the entire cytoplasm is a product of the nucleus, being composed of pangens that migrate out from the latter, and by their active growth and multiplication build up the cytoplasmic substance. 8


D. The Roux-Weismann Theory of Development

We now proceed to an examination of two sharply opposing hypotheses of development based on the theory of nuclear idioplasm. One of these originated with Roux ('83) and has been elaborated especially by Weismann. The other was clearly outlined by De Vries ('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-propagating units in the idioplasm. This hypothesis may therefore be laid

1 Cf. p. 290.

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.

8 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 emanation of specific substances from the nucleus, \vc need not, however, necessarily adopt the pangen-hypothesis.


THE ROUX-WEISMANN THEORY OF DEVELOPMENT 405

aside as an open question, 1 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 chromatingranules. 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 mothergranule is first doubled and then split into equivalent daughter-groups, the daughter-cells therefore receiving the same qualities and remaining 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 corresponding differentiation in the daughter-cells. By qualitative divisions, occurring 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 embryo. Every cell-nucleus, therefore, receives a specific form of chromatin which determines the nature of the cell at a given period and its later history. Every cell is thus endowed with a power of selfdcter?nination, 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 first essay ('85), published before De Vries's paper, he went no farther 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 subsequently produced 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." 2 In later writings Weismann advanced far beyond this, building up an elaborate artificial system, which appears in its final form in the remarkable

1 Cf. Chapter VI. 2 Essay IV., p. 193, 1885.


406 INHERITANCE AND DEVELOPMENT

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 Nagelfs conception. The pangens or biophores are conceived to be successively aggregated in larger and larger groups; namely, (1) 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 (" homoeokinesis," integral or quantitative division), the resulting nuclei remain precisely equivalent. In the second case (" heterokinesis" qualitative ox differential division), the daughter-cells receive different groups of chromatinelements, 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 remains in each cell, viz. that which has to control that particular cell or group of cells. ... In this cell it breaks up into its constituent biophores, and gives the cell its inherited specific character." 1 Development is, therefore, essentially evolutionary and not epigenetic ; 2 its point of departure is a substance in which all of the adult characters are represented by preformed, prearranged germs ; its course is the result of a predetermined harmony in the succession of the qualitative divisions by which the hereditary substance is progressively disintegrated. In order to account for heredity through successive generations, Weismann is obliged to assume that, by means of quantitative or integral division, a certain part of the original germ-plasm is carried on unchanged, and is finally delivered, with its original architecture unaltered, to the germ-nuclei. The power of regeneration is explained, in like manner, as the result of a transmission of unmodified or slightly modified germ-plasm to those parts capable of regeneration.

1 Germ-plasm y pp. 76, 77. 2 /.<-., p. 15.


CRITIQUE OF THE ROUX-WEISMANN THEORY


E. Critique of the Roux-Weismann Theory

It is impossible not to admire the thoroughness, candour, and logical skill with which Weismann has developed his theory, or to deny that, in its 6nal form, it does afford up to a certain point a formal solution of the problems with which it deals. Its fundamental weakness is its ^«<«;-metaphysical character, which, indeed, almost places it outside



C

Fig. 183. — Hair and who

A. Normal sixtecn-cell stage, showing th B. Half sixteen-ccll stage developed from 01 by shaking (Driesch). C. Half blaslula r IX Half-sized six!een-cell Mage of Toiopnti iown>. This embryo, developed from


like at




cleavage in the eggs of sea-urchins.

four micromeres above (from Driesch, after Selenka).

blaslomere of Ihe two-cell stage after killing the oilier <ulling. [he dead blaslomere at the right (I)riesch). Its, viewed from Ihe micromere-potc (the eighl lower n isolated blaslomere of the two-cell stage, segmented


the sphere of legitimate scientific hypothesis. Save in the maturation of the germ-cells ("reducing divisions"), none of the visible phenomena of cell-division give even a remote suggestion of qualitative division. All the facts of ordinary mitosis, on the contrary, indicate that the division of the chromatin is carried out with the most exact equality,


408


INHERITANCE AND DEVELOPMENT


The hypothesis mainly rests upon a quite different order of phenomena, namely, on facts indicating that isolated blastomeres, or other cells, have a certain power of self-determination, or " self-differentiation" (Roux), peculiar to themselves, and which is assumed to be primarily due to the specific quality of the nuclei. This assumption, which may or may not be true, 1 is itself based upon the further assumption of qualitative nuclear division of which we actually know nothing whatever. The fundamental hypothesis is thus of purely a priori character; and every fact opposed to it has been met by subsidi


Fig. 184. — Normal and dwarf gastrulas of Amphioxus.

A. Normal gastrula. B. Half-sized dwarf, from an isolated blastomere of the two-cell stage. C. Quarter-sized dwarf, from an isolated blastomere of the four-cell stage.


ary hypotheses, which, like their principal, relate to matters beyond the reach of observation.

Such an hypothesis cannot be actually overturned by a direct appeal to fact. We can, however, make an indirect appeal, the results of which show that the hypothesis of qualitative division is not only so improbable as to lose all semblance of reality, but is in fact quite superfluous. It is rather remarkable that Roux himself led the way in this direction. In the course of his observations on the development of a half-embryo from one of the blastomeres of the two-cell stage of the frog's egg f he determined the significant fact that the half-embryo in the end restores more or less completely

1 Cf.p. 426.


CRITIQUE OF THE ROUX-WEISMANN THEORY


409


the missing half by a peculiar process, related to regeneration, which Roux designated as post-generation. Later studies showed that an isolated blastomere is able to give rise to a complete embryo in many other animals, sometimes developing in its earlier stages as though



Fig. 185,.- Dwarf and


ouble embryos of


A. Isolated blaslomere of [he two-cell si


ige segmenting like


B. Twin gaBtrulas from a single egg. O'Do



by shaking, of the blastomeres of the two-eel


stage. D.EJ-'. D


forms as the last



still forming part of a complete embryo ("partial development"), but in other cases developing directly into a complete dwarf embryo, as if it were an egg of diminished size. In 1891 Driesch was able to follow out the development of isolated blastomeres of sea-urchin


4IO INHERITANCE AND DEVELOPMENT

eggs separated by shaking to pieces the two-cell and four-cell stages. Blastomeres thus isolated segment as if still forming part of an entire larva, and give rise to a half- (or quarter-) blastula (Fig. 183). The opening soon closes, however, to form a small complete blastula, and the resulting gastrula and Pluteus larva is a perfectly formed dwarf of only half (or quarter) the normal size. Incompletely separated blastomeres give rise to double embryos like the Siamese twins. Shortly afterward the writer obtained similar results in the case of Amphioxus, but here the isolated bias tome re behaves from the beginning like a complete ovum of half the usual size, and gives rise to a complete blastula, gastrula, and larva. Complete embryos have also been obtained from a single blastomere in the teleost Fundulus (Morgan, '95, 2), in Triton (Herlitzka, '95), and in a number of hydromedusae (Zoja, '95, Bunting, '99); and nearly complete embryos in the tunicates Ascidiella (Chabry, '87), Phallusia (Driesch, '94), and Molgula (Crampton, '98). 1 Perhaps the most striking of these cases is that of the hydroid Clytia y in which Zoja was able to obtain perfect embryos, not only from the blastomeres of the twoceil and four-cell stages, but from eight-cell and even from sixteenceli stages, the dwarfs in the last case being but one-sixteenth the normal size.

These experiments render highly improbable the hypothesis of qualitative division in its strict form, for they demonstrate that the earlier cleavages, at least, do not in these cases sunder fundamentally different materials, either nuclear or cytoplasmic, but only split the egg up into a number of parts, each of which is capable of producing an entire body of diminished size, and hence must contain all of the material essential to complete development. Both Roux and Weismann endeavour to meet this adverse evidence with the assumption of a " reserve idioplasm," containing all of the elements of the germplasm which is in these cases distributed equally to all the cells in addition to the specific chromatin conveyed to them by qualitative division. This subsidiary hypothesis renders the principal one {i.e. that of qualitative division) superfluous, and brings us back to the same problems that arise when the assumption of qualitative division is discarded.

The theory of qualitative nuclear division has been practically disproved in another way by Driesch, through the pressure-experiments already mentioned at page 375. Following the earlier experiments of Pfliiger ('84) and Roux ('85) on the frog's cgg f Driesch subjected segmenting eggs of the sea-urchin to pressure, and thus obtained flat plates of cells in which the arrangement of the nuclei differed totally

1 The " partial " development in the earlier stages of some of these forms is considered at page 419.


CRITIQUE OF THE ROUX-WEISMANN THEORY


411


from the normal (Fig. 186); yet such eggs when released from pressure continue to segment, without rearrangement of the nuclei, and give rise to perfectly normal larvae. I have repeated these experiments not only with sea-urchin eggs, but also with those of an annelid {Nereis), which yield a very convincing result, since in this case the histological differentiation of the cells appears very early. In the normal development of this animal the archenteron arises from four large cells or macromeres (entomeres), which remain after the successive formation of three quartets of micromeres (ectomeres) and the parent-cell of the mesoblast. After the primary differentiation of the germ-layers the four entomeres do not divide again until a very late period (free-swimming trochophore), and their substance always retains a characteristic appearance, differing from that of the other



Pig. 186. — Modification of cleavage in sea-urchin eggs by pressure.

A. Normal eight-cell stage of Toxopruustes. B. Eight-cell stage of Echinus segmenting under pressure. Both forms produce normal Flutei.


blastomeres in its pale non-granular character and in the presence of large oil-drops. If unsegmented eggs be subjected to pressure, as in Driesch's echinoderm experiments, they segment in a flat plate, all of the cleavages being vertical. In this way are formed eight-celled plates in which all of the cells contain oil-drops (Fig. 187, D). If they are now released from the pressure, each of the cells divides in a plane approximately horizontal, a smaller granular micromere being formed above, leaving below a larger clear macromere in which the oil-drops remain. The sixteen-cell stage, therefore, consists of eight deutoplasm-laden macromeres and eight protoplasmic micromeres (instead of four macromeres and twelve micromeres, as in the usual development). These embryos developed into free-swimming trochophores containing eight instead of four macromeres, which have the typical clear protoplasm containing oil-drops. In this case there can


412


INHERITANCE AND DEVELOPMENT


be no doubt whatever that four of the entoblastic nuclei were normally destined for the first quartet of micromeres (Fig. 187, B), from which arise the apical ganglia and the prototroch. Under the conditions of the experiment, however, they have given rise to the nuclei of cells which differ in no wise from the other entoderm-cells. Even




Fig. 187. — Modifications of cleavage by pressure in Nereis.

A. B. Normal four- and eight-cell stages. C. Normal trochophore larva resulting, with four entoderm-cells. D. Eight-cell stage arising from an egg flattened by pressure ; such eggs give rise to trochophores with eight instead of four entoderm-cells. Numerals designate the successive cleavages.


in a highly differentiated type of cleavage, therefore, the nuclei of the segmenting egg are not specifically different, as the Roux-Weismann hypothesis demands, but contain the same materials even in the cells that undergo the most diverse subsequent fate. But there is, furthermore, very strong reason for believing that this may be true in later


NATURE AND CAUSES OF DIFFERENTIATION 413

stages as well, as Kolliker insisted in opposition to Weismann as early as 1886, and as has been urged by many subsequent writers. The strongest evidence in this direction is afforded by the facts of regeneration ; and many cases are known — for instance, among the hydroids and the plants — in which even a small fragment of the body is able to reproduce the whole. It is true that the power of regeneration is always limited to a greater or less extent according to the species. But there is no evidence whatever that such limitation arises through specification of the nuclei by qualitative division, and, as will appear beyond, its explanation is probably to be sought in a very different direction.

F. On the Nature and Causes of Differentiation

We have now cleared the ground for a restatement of the problem of development and an examination of the views opposed to the Roux-Weismann theory. After discarding the hypothesis of qualitative division the problem confronts us in the following form. If chromatin be the idioplasm in which inheres the sum total of hereditary forces, and if it be equally distributed at every cell-division, how can its mode of action so vary in different cells as to cause diversity of structure, i.e. differentiation ? It is perfectly certain that differentiation is an actual progressive transformation of the egg-substance involving both physical and chemical changes, occurring in a definite order, and showing a definite distribution in the regions of the egg. These changes are sooner or later accompanied by the cleavage of the egg into cells whose boundaries may sharply mark the areas of differentiation. What gives these cells their specific character? Why, in the four-cell stage of an annelid egg, should the four cells contribute equally to the formation of the alimentary canal and the cephalic nervous system, while only one of them (the lefthand posterior) gives rise to the nervous system of the trunk-region and to the muscles, connective tissues, and the germ-cells? (Figs. 171, 188, £.) There cannot be a fixed relation between the various regions of the egg which these blastomeres represent and the adult parts arising from them ; for in some eggs these relations may be artificially changed. A portion of the egg which under normal conditions would give rise to only a fragment of the body will, if split off from the rest, give rise to an entire body of diminished size. What then determines the history of such a portion? What influence moulds it now into an entire body, now into a part of a body ?

De Vries, in his remarkable essay on Intracellular Pangenesis ('89), endeavoured to cut this Gordian knot by assuming that the character of each cell is determined by pangens that migrate from


414


INHERITANCE AND DEVELOPMENT


the nucleus into the cytoplasm, and, there becoming active, set up specific changes and determine the character of the cell, this way or that, according to their nature. But what influence guides the migrations of the pangens, and so correlates the operations of development? Both Driesch and Oscar Hertwig have attempted to



Fig. i88. - Diagrams il


lustratingthe value of the quanclE in a polyclade {I*ptoplana\ . n [amel

[[branch (Unto), and a gs


slrronod {Crepidula). A. Lrfhflama, showing mesoblast-formatioii


in the second qu;ir!ct. //.



lom«5oh].is[ ifmm tjuiidrjiH


1 11). (.'. Uhio, eclomesoblasl formed only from j".


Inallthi' figures Ihesucc


essIyb quarleis are numbered with Arabic figures ; ecloblasl unshaded.


mesoblest dolled, enloblasr


vertically lined.


answer this question, though the first-named author does not commit himself to the pangen-hypothesis. These writers have maintained that the particular mode of development in a given region or blastomere of the egg is a result of its relation to the remainder of the mass, i.e. a product of what may be called the intra-em'bryonic environ


NATURE AND CAUSES OF DIFFERENTIATION 41 5

ment. Hertwig insisted that the organism develops as a whole as the result of a physiological interaction of equivalent blastomeres, the transformation of each being due not to an inherent specific power of self-differentiation, as Roux's mosaic-theory assumed, but to the action upon it of the whole system of which it is a part. "According to my conception," said Hertwig, "each of the first two blastomeres contains the formative and differentiating forces not simply for the production of a half-body, but for the entire organism ; the left blastomere develops into the left half of the body only because it is placed in relation to a right blastomere." l Again, in a later paper : " The egg is a specifically organized elementary organism that develops epigenetically by breaking up into cells and their subsequent differentiation. Since every elementary part (i.e. cell) arises through the division of the germ, or fertilized Qgg f it contains also the germ of the whole, but during the process of development it becomes ever more precisely differentiated and determined by the formation of cytoplasmic products according to its position with reference to the entire organism (blastula, gastrula, etc.)." 2

An essentially similar view was advocated by the writer ('93, '94) nearly at the same time, and the same general conception was expressed with great clearness and precision by Driesch shortly after Hertwig: "The fragments (i.e. cells) produced by cleavage are completely equivalent or indifferent." "The blastomeres of the seaurchin are to be regarded as forming a uniform material, and they may be thrown about, like balls in a pile, without in the least degree impairing thereby the normal power of development." 3 " The relative position of a blastomere in the whole determines in general what develops from it ; if its position be changed, it gives rise to something different ; in other words, its prospective value is a f miction of its position. " 4

In this last aphorism the whole problem of development is brought to a focus. It is clearly not a solution of the problem, but only a highly suggestive restatement of it ; for everything turns upon how the relation of the part to the whole is conceived. Very little consideration is required to show that this relation cannot be a merely geometrical or rudely mechanical one, for in the eggs of different

1 '92, 1, p. 481.

2 '93» P- 793- It should be pointed out that Roux himself in several papers expressly recognizes the fact that development cannot be regarded as a pure mosaic-work, and that besides the power of self-differentiation postulated by his hypothesis we must assume a " correlative differentiation " or differentiating interaction of parts in the embryo. Cf. Roux, '92, '93, 1.

8 Studien IV., p. 25.

Studien IV., p. 39. Cf. His, " Es muss die Wachsthumserregbarkeit des Eies eine Function des Raumes sein." ('74, p. 153.)


416 INHERITANCE AND DEVELOPMENT

animals blastomeres may almost exactly correspond in origin and relative position, yet differ widely in their relation to the resulting embryo. Thus we find that the cleavage of polyclades, annelids, and gasteropods (Fig. 188) shows a really wonderful agreement in form, yet the individual cells differ markedly in prospective value. In all of these forms three quartets of micromeres are successively formed according to exactly the same remarkable law of the alternation of the spirals ; l and, in all, the posterior cell of a fourth quartet lies at the hinder end of the embryo in precisely the same geometrical relation to the remainder of the embryo ; yet in the gasteropods and annelids this cell gives rise to the mesoblast-bands and their products, in the polyclade to a part of the archenteron, while important differences also exist in the value of the other quartets. The relation of the part to the whole is therefore of a highly subtle character, the prospective value of a blastomere depending not merely upon its geometrical position, but upon its relation to the whole complex inherited organization of which it forms a part. The apparently simple conclusion stated in Driesch's clever aphorism thus leads to further problems of the highest complexity. It should be here pointed out that Driesch does not accept Hertwig's theory of the interaction of blastomeres as such, but, like Whitman, Morgan, and others, has brought forward effective arguments against that too simple and mechanical conception. That theory is, in fact, merely Schwann's cell-composite theory of the organism applied to the developing embryo, and the general arguments against that theory find some of their strongest support in the facts of growth and development. 2 This has been forcibly urged by Whitman ('93), who almost simultaneously with the statements of Driesch and Hertwig, cited above, expressed the conviction that the morphogenic process cannot be conceived as merely the sum total or resultant of the individual cell-activities, but operates as a unit without respect to cell-boundaries, precisely as De Bary concludes in the case of growing plant-tissues (p. 393), and the nature of that process is due to the organization of the egg as a whole.

While recognizing fully the great value of the results attained during the past few years in the field of experimental and speculative embryology, we are constrained to admit that as far as the essence of the problem is concerned we have not gone very far beyond the conclusions stated above ; for beyond the fact that the inherited organization is involved in that of the germ-cells we remain quite ignorant of its essential nature. This has been recognized by no one more clearly than by Driesch himself, to whose critical researches we owe so much in this field. At the climax of a recent elaborate analysis, the high interest of which is somewhat obscured by

1 Cf. p. 368. * Cf. pp. 38S-394.


NATURE AND CAUSES OF DIFFERENTIATION 417

its too abstruse form, Driesch can only reiterate his former aphorism, 1 finally taking refuge in an avowed theory of vitalism which assumes the localization of morphogenic phenomena to be determined by "a wholly unknown principle of correlation/' 2 and forms a problem sui generis? This conclusion recognizes the fact that the fundamental problem of development remains wholly unsolved, thus confirming from a new point of view a conclusion which it is only fair to point out has been reached by many others.

But while the fundamental nature of the morphogenic process thus remains unknown, we have learned some very interesting facts regarding the conditions under which it takes place, and which show that Driesch's aphorism loses its meaning unless carefully qualified. The experiments referred to at pages 353, 410, show that up to a certain stage of development the blastomeres of the early echinoderm, Amp/iioxus or medusa-embryo, are " totipotent " (Roux), or "equipotential" (Driesch), i.e. capable of producing any or all parts of the body. Even in these cases, however, we cannot accept the early conclusion of Pfliiger ('83), applied by him to the frog's egg, and afterward accepted by Hertwig, that the material of the egg f or of the blastomeres into which it splits up, is absolutely "isotropic," i.e. consists of quite uniform indifferent material, devoid of preestablished axes. Whitman and Morgan, and Driesch himself, showed that this cannot be the case in the echinoderm Qgg\ for the ovum possesses a polarity predetermined before cleavage begins, as proved by the fact that at the fourth cleavage a group of small cells or micromeres always arises at a certain point, which may be precisely located before cleavage by reference to the eccentricity of the first cleavage-nucleus, 4 and which, as Morgan showed, 6 is indicated before the third, and sometimes before the second cleavage, by a migration of pigment away from the micromere-pole. These observers are thus led to the assumption of a primary polarity of the egg-protoplasm, to which Driesch, in the course of further analysis of the phenomena, is compelled to add the assumption of a secondary polarity at right angles to the first. 6 These polarities, inherent not only in the entire egg, but also in each of the blastomeres into which it divides, form the primary conditions under which the bilaterally symmetrical organism develops by epigenesis. To this extent, therefore, the material of the blastomeres, though " totipotent,' ' shows a certain predetermination with respect to the adult body.

1 '99, pp. 86-87.

2 This phrase is cited by Driesch from an earlier work ('92, p. 596) as giving a correct though " unanalytical " statement of his view. It may be questioned whether many readers will regard as an improvement the " analytical " form it assumes in his last work.

I.e., p. 90. 4 Cf. Fig. 103. 6 '94, p. 142. 6 See Driesch, '93, pp. 229, 241 ; '96, and '99, p. 44. 2 £


4l8 INHERITANCE AND DEVELOPMENT

We now proceed to the consideration of experiments which show that in some animal eggs such predetermination may go much farther, so that the development does, in fact, show many of the features of a mosaic-work, as maintained by Roux. The best-determined of these cases is that of the ctenophore-egg, as shown by the work of Chun,



ihore Brroi. [DmEscH and MORGAN.] n isolated blaslomere. B. Resuming larva, with (our i

ows of pl.iles and two gastric pouches. E. '. trie pouches, from a nucleated fragment of a


Driesch, and Morgan ('95), and Fischel ('98). These observers have demonstrated that isolated blastomeres of the two-, four-, or eight-cell stage undergo a cleavage which, through the earliest stages, is exactly like that which it would have undergone if forming part of a com


NATURE AND CAUSES OF DIFFERENTIATION


419


plete embryo, and gives rise to a defective larva, having only four, two, or one row of swimming-plates (Fig. 189); and Fischer s observations give strong reason to believe that each of the eight micromeres of the sixteen-cell stage is definitely specified for the formation of one of the rows of plates. In like manner Crampton ('96) found that in case of the marine gasteropod Ilyanassa isolated blastomeres of twocell or four-cell stages segmented exactly as if forming part of an entire embryo, and gave rise to fragments of a larva, not to complete dwarfs, as in the echinoderm (Fig. 190). Further, in embryos from which the " yolk-lobe w (a region of that macromere from which the primary mesoblast normally arises) had been removed, no mesoblastbands were formed. Most interesting of all, Driesch and Morgan discovered that if a part of the cytoplasm of an unscgmented ctenophore-egg were removed, the remainder gave rise to an incomplete larva, showing definite defects (Fig. 189, E, F).

In none of these cases is the embryo able to complete itself, though it should be remarked that neither in the ctenophore nor in the snail is the partial embryo identical with a fragment of a whole embryo, since the micromeres finally enclose the macromeres, leaving no surface of fracture. This extreme is, however, connected by a series of forms with such cases as those of Amphioxus or the medusa, where the fragment develops nearly or quite as if it were a whole. In the tunicates the researches of Chabry ('87), Driesch ('94), and Crampton C97) show that an isolated blastomere of the two-cell stage undergoes a typical half-cleavage (Crampton), but finally gives rise to a nearly perfect tadpole larva lacking only one of the asymmetrically placed sense-organs (Driesch). Next in the series may be placed the frog, where, as Roux, Endres, and Walter have shown, a blastomere of the two-cell stage may give rise to a typical half-morula, half-gastrula, and half-embryo 1 (Fig. 182), yet finally produces a perfect larva. A further stage is given by the echinoderm-egg, which, as Driesch showed, undergoes a half-cleavage and produces a haif-blastula, which, however, closes to form a whole before the gastrula-stage (Fig. 183). Perfectly formed though dwarf larvae result. Finally, we reach Amphioxus and the hydromasae in which a perfect " whole development " usually takes place from the beginning, though it is a very interesting fact that the isolated blastomeres of Amphioxus sometimes show, in the early stages of cleavage, peculiarities of development that recall their behaviour when forming part of an entire embryo. 2

We see throughout this series an effort, as it were, on the part of the isolated blastomere to assume the mode of development characteristic of a complete egg, but one that is striving against conditions that

1 This is not invariably the case, as described beyond.

2 Cf. Wilson, '93, pp. 590, 608.


420


INHERITANCE AND DEVELOPMENT


tend to confine its operations to the rdle it would have played if still forming part of an entire developing egg. In Amphioxus or Ctytia this tendency is successful almost from the beginning. In other forms the limiting conditions are only overcome at a later period, while in the ctenophore or snail they seem to afford an insurmount


A. Normal eight-cell stage. B. Normal sixteen sola ted hi asto mere of Ihe mo-cell stage. D. Halfr n the cleavage of an isolated hlastomere of the four-cell stage jelow a one-fourth sixteen-cell stage.


c. C. Half eight-cell stage, from stace succeeding. E. Two stains ahove a one-fourth eight-cell stage.


able barrier to complete development. What determines the limitations of development in these various cases ? They cannot be due to nuclear specification ; for in the ctenophore the fragment of an Hi/segmented egg, containing the normal egg-nucleus, gives rise to a defective larva; and my experiments on Nereis show that even in a highly


NATURE AND CAUSES OF DIFFERENTIATION


421


determinate cleavage, essentially like that of the snail, the nuclei may be shifted about by pressure without altering the end-result. Neither can they lie in the form of the dividing mass as some authors have assumed ; for in Crampton's experiments the half or quarter blastomere does not retain the form of a half or quarter sphere, but rounds



Pig. 191. — Double embryos of frog developed from eggs in' JO. Schui.tzeJ

A. Twins with heads turned in opposite directions. B. Twi united by their ventral sides. D. Double-headed tadpole.


rted when in the Iwo-cell stage, < united back to back. C. Twin*


off to a spheroid like the egg. But if the limiting conditions lie neither in the nucleus nor in the form of the mass, we must seek them in the cytoplasm ; and if we find here factors by which the tendency of the part to develop into a whole may be, as it were, hemmed in, we shall reach a proximate explanation of the mosaic-like character of cleavage shown in the forms under consideration, and the mosaic


422 INHERITANCE AND DEVELOPMENT

theory of cytoplasmic localization will find a substantial if somewhat restricted basis.

That we are here approaching the true explanation is indicated by certain very remarkable and interesting experiments on the frog's egg t which prove that each of the first two blastomeres may give rise either to a half-embryo or to a whole embryo of half size, according to circumstances, and which indicate, furthermore, that these circumstances lie in a measure in the arrangement of the cytoplasmic materials. This most important result, which we owe especially to Morgan, 1 was reached in the following manner. Born had shown, in 1885, that if frogs' eggs be fastened in an abnormal position, — e.g. upside down, or on the side, — a rearrangement of the egg-material takes place, the heavier deutoplasm sinking toward the lower side, while the nucleus and protoplasm rise. A new axis is this established in the egg-, which has the same relation to the body-axes as in the ordinary development (though the pigment retains its original arrangement). This proves that in eggs of this character (telolecithal) the distribution of deutoplasm, or conversely of protoplasm, is one of the primary formative conditions of the cytoplasm ; and the significant fact is that by artificially changing this distribution the axis of the embryo is shifted. Oscar Schultze ('94) discovered that if the egg be turned upside down when in the two-cell stage, a whole embryo (or half of a double embryo) may arise from each blastomere instead of a half-embryo as in the normal development, and that the axes of these embryos show no constant relation to one another (Fig. 191). Morgan ('95, 3) added the important discovery that either a half-embryo or a whole half-sized dwarf might be formed, according to the position of the blastomere. If, after destruction of one blastomere, the other be allowed to remain in its normal position, a half -embryo always results, 2 precisely as described by Roux. If, on the other hand, the blastomere be inverted, it may give rise either to a half-embryo 3 or to a whole dwarf. 4 Morgan therefore concluded that the production of whole embryos by the inverted blastomeres was, in part at least, due to a rearrangement or rotation of the egg-materials under the influence of gravity, the blastomere thus returning, as it were, to a state of equilibrium like that of an entire ovum.

This beautiful experiment gives most conclusive evidence that each of the two blastomeres contains all the materials, nuclear and cytoplasmic, necessary for the formation of a whole body ; and that these materials may be used to build a whole body or half-body, according to the grouping that they assume. After the first cleavage takes

1 Anat. Anz., X. 19, 1895. 8 Three cases.

a Eleven cases observed. * Nine cases observed.


NATURE AND CAUSES OF DIFFERENTIATION 423

place, each blastomere is set, as it were, for a half-development, but not so firmly that a rearrangement is excluded.

I have reached a nearly related result in the case of both Amp hioxus and the echinoderms. In Amphioxus the isolated blastomere usually segments like an entire ovum of diminished size. This is, however, not invariable, for a certain number of such blastomeres show a more or less marked tendency to divide as if still forming part of an entire embryo. The sea-urchin Toxopnetistes reverses this rule, for the isolated blastomere of the two-cell stage usually shows a perfectly typical half-cleavage, as described by Driesch, but in rare cases it may segment like an entire ovum of half-size(Fig. 183, Z?)and give rise to an entire blastula. We may interpret this to mean that in Amphioxus the differentiation of the cytoplasmic substance is at first very slight, or readily alterable, so that the isolated blastomere, as a rule, reverts at once to the condition of the entire ovum. In the seaurchin, the initial differentiations are more extensive or more firmly established, so that only exceptionally can they be altered. In the snail and ctenophore we have the opposite extreme to Amphioxus, the cytoplasmic conditions having been so firmly established that they cannot be readjusted, and the development must, from the outset, proceed within the limits thus set up.

Through this conclusion we reconcile, as I believe, the theories of cytoplasmic localization and mosaic development with the hypothesis of cytoplasmic totipotence. Primarily the egg-cytoplasm is totipotent in the sense that its various regions stand in no fixed relation with the parts to which they respectively give rise, and the substance of each of the blastomeres into which it splits up contains all of the materials necessary to the formation of a complete body. Secondarily, however, development may assume more or less of a mosaic-like character through differentiations of the cytoplasmic substance involving local chemical and physical changes, deposits of metaplasmic material, and doubtless many other unknown subtler processes. Both the extent and the rate of such differentiations seem to vary in different cases; and here probably lies the explanation of the fact that the isolated blastomeres of different eggs vary so widely in their mode of development. When the initial differentiation is of small extent or is of such a kind as to be readily modified, cleavage is indeterminate in character and may easily be remodelled (as in Amphioxus). When they are more extensive or more rigid, cleavage assumes a mosaic-like or determinate character, 1 and qualitative division, in a certain sense, becomes a fact. Conklin's ('99) interesting observations on the highly determinate cleavage of gasteropods {Crepiduld)

1 The convenient terms indeterminate and determinate cleavage were suggested by Conklin ('98).


424


INHERITANCE AND DEVELOPMENT


show that here the substance of the attraction-spheres is unequally distributed, in a quite definite way, among the cleavage-ceils, each sphere of a daughter-cell being carried over bodily into one of the granddaughter-cells (Fig. 192). We have here a substantial basis for the conclusion that in cleavage of this type qualitative division of the cytoplasm may occur.

It is important not to lose sight of the fact that development and differentiation do not in any proper sense first begin with the cleavage of the ovum, but long before this, during its ovarian history. 1 The primary differentiations thus established in the cytoplasm form the immediate conditions to which the later development must conform; and the difference between Amphioxus on the one hand, and the



A B

Fig. 191. — Two successive stages in the third cleavage of the egg of CrtpiduL upper pole. [CoNKLIN.]

In both figures the old spheres (dotted) lie at the upper pole of the embryo, i cleavage they pass into the four respective cells of the first quartet of micromeri somes are seen in the neu spheres.


ind at the third


snail or ctenophore on the other, simply means, I think, that the initial differentiation is less extensive or less firmly established in the one than in the other.

The origin of the cytoplasmic differentiations existing at the beginning of cleavage has already been considered (p. 386). If the conclusions there reached be placed beside the above, we reach the following conception. The primary determining cause of development lies in the nucleus, which operates by setting up a continuous series of specific metabolic changes in the cytoplasm. This process begins during ovarian growth, establishing the external form of the egg, its primary polarity, and the distribution of substances within it. The cytoplasmic differentiations thus set up form as it were a frame1 See Wilson ('96), Driesch ('98, 1).


THE NUCLEUS IN LATER DEVELOPMENT 425

work within which the subsequent operations take place in a course which is more or less firmly fixed in different cases. If the cytoplasmic conditions be artificially altered by isolation or other disturbance of the blastomeres, a readjustment may take place and development may be correspondingly altered. Whether such a readjustment is possible depends on secondary factors — the extent of the primary differentiations, the physical consistency of the eggsubstance, the susceptibility of the protoplasm to injury, and doubtless a multitude of others. The same doubtless applies to the later stages of development ; and we must here seek for some of the factors by which the power of regeneration in the adult is determined and limited. It is, however, not improbable, as pointed out below, that in the later stages differentiation may occur in the nuclear as well as in the cytoplasmic substance.

G. The Nucleus in Later Development

The foregoing conception, as far as it goes, gives at least an intelligible view of the more general features of early development and in a measure harmonizes the apparently conflicting results of experiment on various forms. But there are a very large number of facts relating especially to the later stages of differentiation, which it seems to leave unexplained* and which indicate that the nucleus as well as the cytoplasm may undergo progressive changes of its substance. It has been assumed by most critics of the Roux- Weismann theory that all of the nuclei of the body contain the same idioplasm, and that each therefore, in Hertwig's words, contains the germ of the whole. It is, however, doubtful whether this assumption is well founded. The power of a single cell to produce the entire body is in general limited to the earliest stages of cleavage, rapidly diminishes, and as a rule soon disappears entirely. When once the germ-layers have been definitely separated, they lose entirely the power to regenerate one another save in a few exceptional cases. In asexual reproduction, in the regeneration of lost parts, in the formation of morbid growths, each tissue is in general able to reproduce only a tissue of its own or a nearly related kind. Transplanted or transposed groups of cells (grafts and the like) retain more or less. completely their autonomy and vary only within certain well-defined limits, despite their change of environment. All of these statements are, it is true, subject to exception ; yet the facts afford an overwhelming demonstration that differentiated cells possess a specific character, that their power of development and adaptability to changed conditions becomes in a greater or less degree limited with the progress of development. As indicated above, this progressive specification of the tissue-cells


426 INHERITANCE AND DEVELOPMENT

is no doubt due in part to differentiation of the cytoplasm. There is, however, reason to suspect that, beyond this, differentiation may sooner or later involve a specification of the nuclear substance. When we reflect on the general role of the nucleus in metabolism and its significance as the especial seat of the formative power, we may well hesitate to deny that this part of Roux's conception may be better founded than his critics have admitted. Nageli insisted that the idioplasm must undergo a progressive transformation during development, and many subsequent writers, including such acute thinkers as Boveri and Nussbaum, and many pathologists, have recognized the necessity for such an assumption. Boveri's remarkable observations on the nuclei of the primordial germ-cells in Ascaris demonstrate the truth of this view in a particular case ; for here all oft/ie somatic nuclei lose a portion of their chromatin, and only the progenitors of the germ-neclei retain the entire ancestral heritage. Boveri himself has in a measure pointed out the significance of his discovery, insisting that the specific development of the tissue-cells is conditioned by specific changes in the chromatin that they receive, 1 though he is careful not to commit himself to any definite theory. It hardly seems possible to doubt that in Ascaris the limitation of the somatic cells in respect to the power of development arises through a loss of particular portions of the chromatin. One cannot avoid the thought that further and more specific limitations in the various forms of somatic cells may arise through an analogous process, and that we have here a key to the origin of nuclear specification without recourse to the theory of qualitative division. We do not need to assume that the unused chromatin is cast out bodily ; for it may degenerate and dissolve, or may be transformed into linin-substance or into nucleoli.

This suggestion is made only as a tentative hypothesis, but the phenomena of mitosis seem well worthy of consideration from this point of view. Its application to the facts of development becomes clearer when we consider the nature of the nuclear "control" of the cell, i.e. the action of the nucleus upon the cytoplasm. Strasburger, following in a measure the lines laid down by Nageli, regards the action as essentially dynamic, i.e. as a propagation of molecular movements from nucleus to cytoplasm in a manner which might be compared to the transmission of a nervous impulse. When, however, we consider the role of the nucleus in synthetic metabolism, and the relation between this process and that of morphological synthesis, we must regard the question in another light ; and opinion has of late strongly tended to the conclusion that nuclear "control" can only be explained as the result of active exchanges of material between nucleus and cytoplasm. De Vries, followed by Hertwig,

1 '9>» p. 433


THE NUCLEUS IN LATER DEVELOPMENT 427

assumes a migration of pangens from nucleus to cytoplasm, the character of the cell being determined by the nature of the migrating pangens, and these being, as it were, selected by circumstances (position of the cell, etc.). But, as already pointed out, the pangenhypothesis should be held quite distinct from the purely physiological aspect of the question, and may be temporarily set aside; for specific nuclear substances may pass from the nucleus into the cytoplasm in an unorganized form. Sachs, followed by Loeb, has advanced the hypothesis that the development of particular organs is determined by specific " formative substances " which incite corresponding forms of metabolic activity, growth, and differentiation. It is but a step from this to the very interesting suggestion of Driesch that the nucleus is a storehouse of ferments which pass out into the cytoplasm and there set up specific activities. Under the influence of these ferments the cytoplasmic organization is determined at every step of the development, and new conditions are established for the ensuing change. This view is put forward only tentatively as a " fiction " or working hypothesis ; but it is certainly full of suggestion. Could we establish the fact that the number of ferments or formative substances in the nucleus diminishes with the progress of differentiation, we should have a comparatively simple and intelligible explanation of the specification of nuclei and the limitation of development. The power of regeneration might then be conceived, somewhat as in the Roux-Weismann theory, as due to a retention of idioplasm or germ-plasm — i.e. chromatin — in a less highly modified condition, and the differences between the various tissues in this regard, or between related organisms, would find a natural explanation.

Development may thus be conceived as a progressive transformation of the egg-substance primarily incited by the nucleus, first manifesting itself by specific changes in the cytoplasm, but sooner or later involving in some measure the nuclear substance itself. This process, which one is tempted to compare to a complicated and progressive form of crystallization, begins with the youngest ovarian egg and proceeds continuously until the cycle of individual life has run its course. Cell-division is an accompaniment but not a direct cause of differentiation. The cell is no more than a particular area of the germinal substance comprising a certain quantity of cytoplasm and a mass of idioplasm in its nucleus. Its character is primarily a manifestation of the general formative energy acting at a particular point under given conditions. When once such a circumscribed area has been established, it may, however, emancipate itself in a greater or less degree from the remainder of the mass, and acquire a specific character so fixed as to be incapable of further change save within the limits imposed by its acquired character.


INHERITANCE AND DEVELOPMENT


H. The External Conditions of Development

We have thus far considered only the internal conditions of development which are progressively created by the germ-cell itself. We must now briefly glance at the external conditions afforded by the environment of the embryo. That development is conditioned by the external environment is obvious. But we have only recently

to realize how intimate the relation is; and it has been especially the service of Loeb, Herbst, and Driesch to show how essential a part is played by the environment in the development of specific organic forms. The limits of this work will not admit of any adequate review of the vast array of known facts in this field, for which the reader is referred to the works especially of Herbst. I shall only consider one or two cases which may serve to bring out the general principle that they involve. Every living organism at every stage of its existence reacts to its environment by physiological and morphological changes. The developing embryo, like the adult, is a moving equilibrium — a product of the response of the inherited organization to the external stimuli working upon it. If these stimuli be altered, development is altered. This is beautifully shown by the experiments of Herbst and others on the development of sea-urchins. Pouchet and Chabry showed that if the embryos of these animals be made to develop in sea-water containing no lime-salts, the larva fails to develop not only its calcareous skeleton, but also its ciliated arms, and a larva thus results that resembles in some particulars an entirely different specific form ; namely, the Tornaria larva of Balanoglossus. This result is not due simply to the lack of necessary material : for Herbst showed that the same result is attained if a slight excess of potassium chloride be added to sea-water containing the normal amount of lime (Fig. 193). In the latter case the specific metabolism of the protoplasm is altered by a particular chemical stimulus, and a new form results.



Fig. 193-



[HERHST.J


slight ;


THE EXTERNAL CONDITIONS OF DEVELOPMENT 429

The changes thus caused by slight chemical alterations in the water may be still more profound. Herbst ('92) observed, for example, that when the water contains a very small percentage of lithium chloride, the blastula of sea-urchins fails to invaginate to form a typical gastrula, but evaginates to form an hour-glass-shaped



A. Polyp (CtriaMfiHi), prodi B. Hydroid ( TWn/ar/j), general water. C. D. Similar generalion of heai

larva, one half of which represents the archenteron, the other half the ectoblast. Moreover, a much larger number of the blastula-cells undergo the differentiation into entoblast than in the normal development, the ectoblast sometimes becoming greatly reduced and occasionally disappearing altogether, so that the entire blastula is


430 INHERITANCE AND DEVELOPMENT

differentiated into cells having the histological character of the normal entoblast ! One of the most fundamental of embryonic differentiations is thus shown to be intimately conditioned by the chemical environment.

The observations of botanists on the production of roots and other structures as the result of local stimuli are familiar to all. Loeb's interesting experiments on hydroids give a similar result ('91). It has long been known that Tubularia, like many other hydroids, has the power to regenerate its " head " — i.e. hypostome, mouth, and tentacles — after decapitation. Loeb proved that in this case the power to form a new head is conditioned by the environment. For if a Tnbnlaria stem be cut off at both ends and inserted in the sand upside down, i.e. with the oral end buried, a new head is regenerated at the free (formerly aboral) end. Moreover, if such a piece be suspended in the water by its middle point, a new head is produced at each end (Fig. 194); while if both ends be buried in the sand, neither end regenerates. This proves in the clearest manner that in this case the power to form a definite complicated structure is called forth by the stimulus of the external environment.

These cases must suffice for our purpose. They prove incontestably that normal development is in a greater or less degree the response of the developing organism to normal conditions ; and they show that we cannot hope to solve the problems of development without reckoning with these conditions. But neither can we regard specific forms of development as directly caused by the external conditions ; for the egg of a fish and that of a polyp develop, side by side, in the same drop of water, under identical conditions, each into its predestined form. Every step of development is a physiological reaction, involving a long and complex chain of cause and effect between the stimulus and the response. The character of the response is determined, not by the stimulus, but by the inherited organization. While, therefore, the study of the external conditions is essential to the analysis of embryological phenomena, it serves only to reveal the mode of action of the germ and gives but a dim insight into its ultimate nature.

I. Development, Inheritance, and Metabolism

In bringing the foregoing discussion into more direct relation with the general theory of cell-action, we may recall that the cell-nucleus appears to us in two apparently different roles. On the one hand, it is a primary factor in morphological synthesis and hence in inheritance, on the other hand an organ of metabolism especially concerned with the constructive process. These two functions we may with


DEVELOPMENT, INHERITANCE, AND METABOLISM 431

Claude Bernard regard as but different phases of one process. The building of a definite cell-product, such as a muscle-fibre, a nerveprocess, a cilium, a pigment-granule, a zymogen-granule, is in the last analysis the result of a specific form of metabolic activity, as we may conclude from the fact that such products have not only a definite physical and morphological character, but also a definite chemical character. In its physiological aspect, therefore, inheritance is the recurrence, in successive generations, of like forms of metabolism ; and this is effected through the transmission from generation to generation of a specific substance or idioplasm which we have seen reason to identify with chromatin. The validity of this conception is not affected by the form in which we conceive the morphological nature of the idioplasm — whether as simply a mixture of chemical substances, as a microcosm of invisible germs or pangens, as assumed by De Vries, Weismann, and Hertwig, as a storehouse of specific ferments as Driesch suggests, or as a complex molecular substance grouped in micellae as in Nageli's hypothesis. It is true, as Verworn insists, that the cytoplasm is essential to inheritance ; for without a specifically organized cytoplasm the nucleus is unable to set up specific forms of synthesis. This objection, which has already been considered from different points of view, by both De Vries and Driesch, disappears as soon as we regard the egg-cytoplasm as itself a product of the nuclear activity ; and it is just here that the general rdle of the nucleus in metabolism is of such vital importance to the theory of inheritance. If the nucleus be the formative centre of the cell, if nutritive substances be elaborated by or under the influence of the nucleus while they are built into the living fabric, then the specific character of the cytoplasm is determined by that of the nucleus, and the contradiction vanishes. In accepting this view we admit that the cytoplasm of the egg is, in a measure, the substratum of inheritance, but it is so only by virtue of its relation to the nucleus, which is, so to speak, the ultimate court of appeal. The nucleus cannot operate without a cytoplasmic field in which its peculiar powers may come into play ; but this field is created and moulded by itself.

J. Preformation and Epigenesis. The Unknown Factor in Development

We have now arrived at the farthest outposts of cell-research, and here we find ourselves confronted with the same unsolved problems before which the investigators of evolution have made a halt. For we must now inquire what is the guiding principle of embryological development that correlates its complex phenomena and directs them to a definite end. However we conceive the special mechanism of development, we cannot escape the conclusion that the power behind it is involved in the structure of the germ-plasm inherited from foregoing generations. What is the nature of this structure and how has it been acquired ? To the first of these questions we have as yet no certain answer. The second question is merely the general problem of evolution stated from the standpoint of the cell-theory. The first question raises once more the old puzzle of preformation or epigenesis. The pangen-hypothesis of De Vries and Weismann recognizes the fact that development is epigenetic in its external features ; but like Darwin's hypothesis of pangenesis, it is at bottom a theory of preformation, and Weismann expresses the conviction that an epigenetic development is an impossibility. 1 He thus explicitly adopts the view, long since suggested by Huxley, that "the process which in its superficial aspect is epigenesis appears in essence to be evolution in the modified sense adopted in Bonnet's later writings ; and development is merely the expansion of a potential organism or 'original preformation ' according to fixed laws." 2 Hertwig ('92, 2), while accepting the pangen-hypothesis, endeavours to take a middle ground between preformation and epigenesis, by assuming that the pangens (idioblasts) represent only cell-characters, the traits of the multicellular body arising epigenetically by permutations and combinations of these characters. This conception certainly tends to simplify our ideas of development in its outward features, but it does not explain why cells of different characters should be combined in a definite manner, and hence does not reach the ultimate problem of inheritance.

What lies beyond our reach at present, as Driesch has very ably urged, is to explain the orderly rhythm of development — the coordinating power that guides development to its predestined end. We are logically compelled to refer this power to the inherent organization of the germ, but we neither know nor can we even conceive what that organization is. The theory of Roux and Weismann demands for the orderly distribution of the elements of the germ-plasm a prearranged system of forces of absolutely inconceivable complexity. Hertwig's and De Vries's theory, though apparently simpler, makes no less a demand ; for how are we to conceive the power which guides the countless hosts of migrating pangens throughout all the long and complex events of development? The same difficulty confronts us under any theory we can frame. If with Herbert Spencer we assume the germ-plasm to be an aggregation of like units, molecular or supra-molecular, endowed with predetermined polarities which lead to their grouping in specific forms, we but throw the problem one stage farther back, and, as Weismann himself has pointed out, 1 substitute for one difficulty another of exactly the same kind.

1 Germ-plasm, p. 14. 2 Evolution, Science, and Culture* p. 296.


The truth is that an explanation of development is at present beyond our reach. The controversy between preformation and epigenesis has now arrived at a stage where it has little meaning apart from the general problem of physical causality. What we know is that a specific kind of living substance, derived from the parent, tends to run through a specific cycle of changes during which it transforms itself into a body like that of which it formed a part ; and we are able to study with greater or less precision the mechanism by which that transformation is effected and the conditions under which it takes place. But despite all our theories we no more know how the organization of the germ-cell involves the properties of the adult body than we know how the properties of hydrogen and oxygen involve those of water. So long as the chemist and physicist are unable to solve so simple a problem of physical causality as this, the embryologist may well be content to reserve his judgment on a problem a hundred-fold more complex.

The second question, regarding the historical origin of the idioplasm, brings us to the side of the evolutionists. The idioplasm of every species has been derived, as we must believe, by the modification of a preexisting idioplasm through variation, and the survival of the fittest. Whether these variations first arise in the idioplasm of the germ-cells, as Weismann maintains, or whether they may arise in the body-cells and then be reflected back upon the idioplasm, is a question to which the study of the cell has thus far given no certain answer. Whatever position we take on this question, the same difficulty is encountered ; namely, the origin of that coordinated fitness, that power of active adjustment between internal and external relations, which, as so many eminent biological thinkers have insisted, overshadows every manifestation of life. The nature and origin of this power is the fundamental problem of biology. When, after removing the lens of the eye in the larval salamander, we see it restored in perfect and typical form by regeneration from the posterior layer of the iris, 2 we behold an adaptive response to changed conditions of which the organism can have had no antecedent experience either ontogenetic or phylogenetic, and one of so marvellous a character that we are made to realize, as by a flash of light, how far we still are from a solution of this problem. It may be true, as Schwann himself urged, that the adaptive power of living beings differs in degree only, not in kind, from that of unor 1 Germinal Selection, January, 1896, p. 284. 8 See Wolff, '95, and Miiller, '96.


ganized bodies. It is true that we may trace in organic nature long and finely graduated series leading upward from the lower to the higher forms, and we must believe that the wonderful adaptive manifestations of the more complex forms have been derived from simpler conditions through the progressive operation of natural causes. But when all these admissions are made, and when the conserving action of natural selection is in the fullest degree recognized, we cannot close our eyes to two facts : first, that we are utterly ignorant of the manner in which the idioplasm of the germ-cell can so respond to the influence of the environment as to call forth an adaptive variation ; and second, that the study of the cell has on the whole seemed to widen rather than to narrow the enormous gap that separates even the lowest forms of life from the inorganic world.

I am well aware that to many such a conclusion may appear reactionary or even to involve a renunciation of what has been regarded as the ultimate aim of biology. In reply to such a criticism I can only express my conviction that the magnitude of the problem of development, whether ontogenetic or phylogenetic, has been underestimated ; and that the progress of science is retarded rather than advanced by a premature attack upon its ultimate problems. Yet the splendid achievements of cell-research in the past twenty years stand as the promise of its possibilities for the future, and we need set no limit to its advance. To Schleiden and Schwann the present standpoint of the cell-theory might well have seemed unattainable We cannot foretell its future triumphs, nor can we doubt that the way has already been opened to better understanding of inheritance and development.


Literatuire

Barfurth, D. — Regeneration und Involution: Merkel u. Bonnet, Ergeb. y I .-VI 1 1.

1891-99. Boveri, Th. — Ein geschlechtlich erzeugter Organismus ohne mutterliche Eigen schaften: Sit z.- Iter. d. Ges.f. Morph. und Phys. in Munchen, V. 1889. Sec

also Arch. f. Entw. 1895. Brooks, W. K. — The Law of Heredity. Baltimore, 1883. Id. — The Foundations of Zoology. New York, 1899.

Davenport, C. B. — Experimental Morphology: I., II. New York, 1897, 1899. Driesch, H. — Analvtische Theorie der organischen Entwicklung. Leipzig* 1894. Id. — Die Localisation morphogenetischer Vorgange: Arch. Entw., VII. 1. 1899. Id. — Resultate und Probleme der Entwickelungs-physiologie der Tiere : Merkel u.

Bonnet, Frgeb., VIII., 1898. (Full literature.) Herbst, C. — Uber die Bedeutung der Reizphysiologie fur die kausale Auffassung

von Vorgangen in der tierischen Ontogenese: Biol. Centralb., XIV., XV.

1894-95. Eertwig, 0. — Altere und neuere Entwicklungs-theorien. Berlin, 1892.


Hertwig, 0. — Urmund und Spina Bifida: Arch. mik. Anat., XXXIX. 1892. Id. — Uber den Werth der Ersten Furchungszellen fiir die Organbildung des Embryo: Arch. mik. Anat., XLII. 1893. Id. — Zeit und Streitfragen der Biologic I. Berlin, 1894. II. Jena, 1897. Id. — Die Zelle und die Gewebe, II. Jena, 1898. His, W. — Unsere Korperform und das physiologische Problem ihrer Entstehung.

Leipzig* 1874. Loeb» J. — Untersuchungen zur physiologischen Morphologie : I. Heteromorphosis.

IViirzburg, 1891. II. Organbildung und Wachsthum. WUrzburg, 1892. Id. — Some Facts and Principles of Physiological Morphology: Wood's Holl Biol.

Lectures. 1893. Morgan, T. H. — Experimental Studies of the Regeneration of Phanaria Maculata :

Arch. Entw., VII. 2, 3. 1898. Id. — The Development of the Frog's Egg. New York, 1897. Nageli, C. — Mechahisch-physiologische Theorie der Abstammungslehre. Afiin chen u. Leipzig, 1884. Rous, W. — Uber die Bedeutung der Kernteilungsfiguren. Leipzig, 1883. Id. — Ober das kunstliche Hervorbringen halber Embryonen durch Zerstorung einer

der beiden ersten Furchungskugeln, etc. : Virchovfs Archiv, 114. 1888. Id. — Fiir unsere Programme und seine Verwirklichung : Arch. Entw., V. 2. 1897.

(See also Gesammelte Abhandlungen liber Entwicklungsmechanik der Organ ismen, 1895.) Sachs, J. — Stoflf und Form der Pflanzenorgane : Ges. Abhandlungen, II. 1893. Weismann, A. — Essays upon Heredity, First Series. Oxford, 1891. Id. — Essays upon Heredity, Second Series. Oxford, 1892. Id. — Aussere Einfllisse als Entwicklungsreize. Jena, 1894. Id. — The Germ-plasm. New York* 1893.

Whitman, C. 0. — Evolution and Epigenesis : Wood's Moll Biol. Lectures. 1894. Wilson, Edm. B. — On Cleavage and Mosaic-work: Arch, fur Entwicklungsm. y

III. 1. 1896. See also Literature, VIII., p. 394.)



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   Cell development and inheritance (1900): Introduction | List of Figures | Chapter I General Sketch of the Cell | Chapter II Cell-division | Chapter III The Germ-cells | Chapter IV Fertilization of the Ovum | Chapter V Reduction of the Chromosomes, Oogenesis and Spermatogenesis | Chapter VI Some Problems of Cell-organization | Chapter VII Some Aspects of Cell-chemistry and Cell-physiology | Chapter VIII Cell-division and Development | Chapter IX Theories of Inheritance and Development | Glossary

Wilson EB. The Cell in Development and Inheritance. Second edition (1900) New York, 1900.


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