The cell in development and inheritance (1900) 6

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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 VI Some Problems of Cell-Organization

"Wir miissen dcshalb den lebenden Zellen, abgesehen von der Molecularstructur der organischen Verbindungen, welche sie enthalt, noch eine andere und in anderer Wcise complicirte Structur zuschreiben, und diese es ist, welche wir mit dem Namen Organization bezeichnen."


  • Eiementarorganismen % 1 861, p. 386.

" Was diese Zelle eigentlich ist, dariiber existieren sehr vcrschiedene Ansichten."


  • Anthropogcnie, 1 89 1, p. 104.

The remarkable history of the chromatic substance in the maturation of the germ-cells forces upon our attention the problem of the ultimate morphological organization of the nucleus, and this in its turn involves our whole conception of protoplasm and the cell. The grosser and more obvious organization is revealed to us by the microscope as a differentiation of its substance into nucleus, cytoplasm, and the like. But, as Strasburger has well said, it would indeed be a strange accident if the highest powers of our present microscopes had laid bare the ultimate organization of the cell. Brucke insisted more than thirty years ago that protoplasm must possess a far more complicated morphological organization than is revealed to us in the visible structure of the cell, repeating, though without accepting, an earlier suggestion of Henle's('4i) that the cell might be composed of more elementary vital units ranking between the molecule and the cell. Many biological thinkers since Briicke's time have in one form or other accepted this conception, which indeed lies at the root of nearly all recent attempts to analyze exhaustively the phenomena of cell-life. Without attempting to follow out the history of opinion in detail or to give any extended review of the various theories, 8 it may be pointed out that this conception was based both on theoretical a priori grounds and on the observed facts of cell-structure. On the former basis it was developed by Herbert Spencer 4 in his theory of " physiological units " by which he endeavoured to explain the phenomena of regeneration, development, and heredity ; while Nageli ('84) developed on the same general lines his theory of miccllce which has been so widely accepted by botanists. In the meantime Darwin l introduced a new element into the speculative edifice in his celebrated hypothesis of pangenesis, where for the first time appear the two assumptions of specific differences in the ultra-microscopic corpuscles ("gemmules") and the power of self-propagation by division. Darwin did not, however, definitely maintain that protoplasm was actually built of such bodies. The latter hypothesis was added by De Vries ('89), who remodelled the theory of pangenesis on this assumption, thus laying the basis for the theories of development which reached their climax in the writings of Hertwig and Weismann.

8 For an exhaustive review see Yves Delage, La structure du protoplasma et les theories sur rherediti. Paris, 1895. 4 Principles of Biology, 1864.

The views of Spencer and Darwin were based on purely theoretical grounds derived from the general phenomena of growth and inheritance. 2 Those of Nageli, De Vries, Wiesner, Altmann, and others were more directly based on the results of microscopical investigation. The view was first suggested by Henle ('41), and at a later period developed by Bechamp and Estor, by Maggi and especially by Altmann, that the protoplasmic granules might be actually organic units or bioblasts, capable of assimilation, growth, and division, and hence to be regarded as elementary units of structure standing between the cell and the ultimate molecules of living matter. By Altmann, especially, this view was pushed to an extreme limit, which lay far beyond anything justified by the known facts; and the theory of genetic continuity expressed by Redi in the aphorism "omne vivttm ex vivo,' 9 reduced by Virchow to " omnis cellula e cellula" finally appears in the writings of Altmann as " omne granu lit m e granulo" / 8

Altmann's premature generalization rested upon a very insecure foundation and was received with just scepticism. Except in the case of plastids, the division of the cytoplasmic granules was and still remains a pure assumption, and furthermore many of Altmann's "granules" (zymogen-granules of gland-cells, etc.) are undoubtedly metaplasmic bodies. 4 Yet the beautiful discoveries of Schimper ('85 > and others on the origin of plastids in plant-cells give evidence that these cells do in fact contain large numbers of bodies, other than the nuclei, that possess the power of growth and division. The division of the chlorophyll-bodies, observed long ago by Mohl, was shown by Schmitz and Schimper to be their usual if not their only mode of origin ; and Schimper was able to trace them back to minute colourless plastids, scarcely larger than " microsomes," that are present in large numbers in the protoplasm of the embryonic cells and of the &gg f and give rise not only to chlorophyll-bodies but also to the amyloplasts or starch-formers and the chromoplasts or pigment-bodies. While it still remains doubtful whether the plastids arise solely by division or also

1 Variation of Animals and Plants, 1868. a Cf Introduction, p. 12.

Die EUmentarorganismen, Leipsic, 1 894, p. 155. * Cf Lazarus, '98.

by new formation (as now seems to be the case with the centrosome), the foregoing observations on the plastids give a substantial basis for the hypothesis that protoplasm may be built of minute dividing bodies which form, its ultimate structural basis. It was these facts, taken in connection with the phenomena of particulate inheritance and variation (Galton), that led De Vries and his followers to the fundamental assumption of "pangens," "plasomes," " biophores," and the like as final protoplasmic units ; l but these were conceived not as the visible granules, plastids, etc., but as much smaller bodies, lying far beyond the limits of present microscopical vision, through the growth or aggregation of which the visible structures arise. This assumption has been harshly criticised ; yet when we recall that in one form or another it has been accepted by such men as Spencer, Darwin, Beale, Hackel, Michael Foster, Nageli, De Vries, Wiesner, Roux, Weismann, Oscar Hertwig, Verworn, and Whitman, and on evidence drawn from sources so diverse, we must admit that despite its highly speculative character it is not to be lightly rejected. In the present chapter we may inquire how far the known facts of cell-structure speak for or against this hypothesis, incidentally considering a number of detailed questions of cell-morphology which have not hitherto found a place.

A. The Nature of Cell-organs

The cell is, in Briicke's words, an elementary organism, which may by itself perform all the characteristic operations of life, as is the case with the unicellular organisms, and in a sense also with the germ-cells. Even when the cell is but a constituent unit of a higher grade of organization, as in multicellular forms, it is no less truly an organism, and in a measure leads an independent life, even though its functions be restricted and subordinated to the common life. It is true that the earlier conception of the multicellular body as a colony of one-celled forms cannot be accepted without certain reservations. 2 Nevertheless, all the facts at our command indicate that the tissue-cell possesses the same morphological organization as the egg-cell, or the protozoan, and the same fundamental physiological properties as well. Like these the tissue-cell has its differentiated structural parts or organs, and we have now to inquire how these cell-organs are to be conceived.

1 The following list includes only some of the various names that have been given to these hypothetical units by modern writers: Physiological units (Spencer); gem mules (Darwin); pangens (De Vries); plasomes (Wiesner); micella (Nageli); plastittules (Hackel and Elssberg); inotagmata (Kngelmann); biophores (Weismann); bioblasts (Beale); somacuUs (Foster) ; idioblasts (Hertwig); idiosomes (Whitman); biogens (Verworn); microzymas (Bechamp and Estor); gemma (Haacke). These names are not strictly synonymous, nor do all of the writers cited assume the power of division in the units. a Cf. p. 58.

The visible organs of the cell fall under two categories, according as they are merely temporary structures, formed anew in each successive cell-generation out of the common structural basis, or permanent structures whose identity is never lost, since they are directly handed on by division from cell to cell. 1 To the former category belong, in general, such structures as cilia, pseudopodia, and the like ; to the latter, the nucleus, perhaps also the centrosomes, and the plastids of plant-cells. A peculiar interest attaches to the permanent cell-organs. Closely interrelated as these organs are, they nevertheless have a remarkable degree of morphological independence. Tkey assimilate food, grow, divide, and perform their own characteristic actions like coexistent but independent organisms, of a lower grade than the cell, living together in colonial or symbiotic association. So striking is this morphological and physiological autonomy in the case of the green plastids or chromatophores that neither botanists nor zoologists are as yet able to distinguish with absolute certainty between those that form an integral part of the cell, as in the higher green plants, and those that are actually independent organisms living symbiotically within it, as is probably the case with the yellow cells of Radiolaria. Even so acute an investigator as Watase* ('93, 1) has seriously propounded the view that the nucleus itself — or rather the chromosome — should be regarded as a distinct organism living in symbiotic association with the cytoplasm, but having had, in an historical sense, a different origin. This rather fantastic view has not found much favour, and even were it true would teach us nothing of the origin of the power of division, which must for the present be taken as an elementary process forming one of the primary data of biology. Yet we may still inquire whether the power of division shown by such protoplasmic masses as plastids, chromosomes, centrosomes, nucleoli, and nuclei may not have its root in a like power residing in ultimate protoplasmic units of which they are made up. Could we accept such a view, we might much more easily meet some puzzling cytological difficulties. For under this assumption the difference between transient and permanent cellorgans would become only one of degree, depending on the degree of cohesion between their structural components ; and we could thus conceive, for example, how such a body as a centrosome might form, persist by division for a number of generations, and finally disintegrate. In connection with this it may be pointed out that even such a typical permanent organ as the nucleus does not persist as such during the ordinary form of division ; for it loses its boundary and many of its other structural characters, becoming resolved into a group of separate chromosomes. What persists is here not the structural unit, but the characteristic substance which forms its essential constituent, and a large part even of this substance may be lost in the process. The term "persistent organ" is therefore used in rather a figurative sense, and if too literally understood may easily mislead us.

1 Cf. footnote, p. 30.

With the foregoing considerations in mind let us turn to the actual structural relation of the cell-organs.

B. Structural Basis of the Cell

In Chapter I. some of the reasons have been given for the conclusion that none of the obvious structural features of protoplasm (fibrillae, alveoli, granules, and the like) can be regarded as necessary or universal ; and we may now inquire whether there is any evidence that such structures may have such a common structural basis as De Vries's theory assumes. I shall here take as a point of departure my observations on the structure of protoplasm in echinoderm-eggs, already briefly reviewed at page 28. The beautiful alveolar structure of these eggs is entirely of secondary origin, and all the visible structural elements arise during the growth of the eggs by the deposit and subsequent enlargement of minute spherical bodies, all apparently liquid drops, in a homogeneous or finely granular basis which is itself a liquid. Some of these spheres enlarge to form the alveolar spheres, while the homogeneous basis or continuous substance remains as the interalveolar material. Others remain much smaller to constitute the " microsomes " scattered through the interalveolar walls ; and these bodies, like the alveolar spheres, are perfectly visible in life, as well as in section ; they are therefore not coagulation- products or artifacts. From these three elements arise all the other structures observed in these eggs, deutoplasm-spheres (Ophiura) and pigment-bodies (Arbacia) being formed by further enlargement and chemical alteration of the alveolar spheres, while astral rays and spindle-fibres are differentiated out of the inter-alveolar material and microsomes. 1 These various elements show a continuous gradation in size from the largest deutoplasm-spheres down to the smallest visible granules, the latter being the source of all the larger elements, and in their turn emerging into view from the " homogeneous " basis. Clearly, then, none of these bodies can be regarded as the ultimate structural units ; for the latter, if they exist, must lie in a region at present inaccessible to the microscope. This fact, however, no more disproves their existence than it does that of molecules and atoms. It only shows the futility of such attempts as those of Altmann and his predecessors to identify " granules " or "microsomes " as final morphological units, and compels us to turn to indirect instead of direct evidence. It may, however, again be pointed out that it would be quite irrational to conclude that the smaltest visible granules first come into existence when they first come within view of the microscope. The " homogeneous " substance must itself contain or consist of granules still smaller. The real question is not whether such ultra-microscopical bodies exist, but whether they are permanent organized bodies possessing besides the power of growth also the power of division. This question can be only indirectly approached ; and we shall find it convenient to do so by beginning at the opposite end of the series, through a reconsideration of the phenomena of nuclear division.

1 Cf. Wilson, '99.

C. Morphological Composition of the Nucleus

1. The Chromatin

(a) Hypothesis of the Individuality of the Chromosomes. — It may now be taken as a well-established fact that the nucleus is never fortned de novo, but always arises by the division of a preexisting nucleus. In the typical mode of division by mitosis the chromatic substance is resolved into a group of chromosomes, always the same in form and number in a given species of cell, and having the power of assimilation, growth, and division, as if they were morphological individuals of a lower order than the nucleus. That they are such individuals or units has been maintained as a definite hypothesis, especially by Rabl and Boveri. As a result of careful study of mitosis in epithelial cells of the salamander, Rabl ('85) concluded that the chromosomes do not lose their individuality at the close of division, but persist in the chromatic reticulum of the resting nucleus. The reticulum arises through a transformation of the chromosomes, which give off anastomosing branches, and thus give rise to the appearance of a network. Their loss of identity is, however, only apparent. They come into view again at the ensuing division, at the beginning of which "the chromatic substance flows back, through predetermined paths, into the primary chromosome-bodies " (Kernfaden), which reappear in the ensuing spireme-stage in nearly or quite the same position they occupied before. Even in the resting nucleus, Rabl believed that he could discover traces of the chromosomes in the configuration of the network, and he described the nucleus as showing a distinct polarity having a "pole" corresponding with the point toward which the apices of the chromosomes converge {i.e. toward the centrosome), and an " antipole " (Gegenpol) at the opposite point {i.e. toward the equator of the spindle) (Fig. 22). Rabl's hypothesis was precisely formulated and ardently advocated by Boveri in 1887 and 1888, and again in 1891, on the ground of his own studies and those of Van Beneden on the early stages of Ascaris. The hypothesis was supported by extremely strong evidence, derived especially from a study of abnormal variations in the early development of Ascaris % the force of which has, I think, been underestimated by the critics of the hypothesis. Some of this evidence may here be briefly reviewed. In some cases, through a miscarriage of the mitotic mechanism, one or both of the chromosomes destined for the second polar body are accidentally left

Fig. 143. — Evidence of the individuality of the chromosomes. Abnormalities in the fertilization of Ascaris. [BOVERI.J

A. The two chromosomes of the egg-nucleus, accidentally separated, have given rise each to a reticular nucleus ($, $) ; the sperm-nucleus below (<?). B. Later stage of the same, a single chromosome in each egg-nucleus, two in the sperm-nucleus. C An egg in which the second polar body has been retained ; p.b* the two chromosomes arising from it ; 9 * ne egg-chromosomes ; d the sperm-chromosomes. D. Resulting equatorial plate with six chromosomes.

in the egg. These chromosomes give rise in the egg to a reticular nucleus, indistinguishable from the egg-nucleus. At a later period this nucleus gives rise to the same number of chromosomes as those that entered into its formation, i.e. either one or two. These are drawn into the equatorial plate along with those derived from the germnuclei, and mitosis proceeds as usual, the number of chromosomes being, however, abnormally increased from four to five or six (Fig. 143,

C, Z>). Again, the two chromosomes left in the egg after removal of the second polar body may accidentally become separated- In this case each chromosome gives rise to a reticular nucleus of half the usual size, and from each of these a single chromosome is afterward formed (Fig. 143, A, B). Finally, it sometimes happens that the two germ-nuclei completely fuse, while in the reticular state, as is normally the case in sea-urchins and some other animals (p. 188). From the cleavage- nucleus thus formed arise four chromosomes.

The same general result is given by the observations of Zur Strassen ('98) on the history of giant embryos in Ascaris. These embryos arise by the fusion, either before or after the fertilization, of previously separate eggs, and have been shown to be capable of development up to a late stage. Not only in the first but also in some, at least, of the later mitoses, such eggs show an increased number of chromosomes proportional to the number of nuclei that have united. Thus in monospermic double eggs (variety bivalent) the number is six instead of four; in dispermic double eggs the number is increased to eight (Fig. 144).

These remarkable observations show that whatever be the number of chromosomes entering into the formation of a reticular nucleus, the same number afterward issues from it — a result which demonstrates that the number of chromosomes is not due merely to the chemical composition of the chromatin-substance, but to a morphological organization of - Giam-embryo of Auaris, the nucleus. A beautiful confirmation sing from a double- Q f tn j s conclusion was afterward made tgR. showing eight ,

t Strom*). by Boven ( 93, 95, i ) and Morgan ( 95,

4), in the case of echinoderms, by rearing larvae from enucleated egg-fragments, fertilized by a single spermatozoon (p. 194). All the nuclei of such larva? contain but half the typical number of chromosomes, — i.e. in Echinus nine instead of eighteen, — since all are descended from one germ-nucleus instead of two !

Equally striking is the remarkable fact, described at page 275, that all of the cells in the sexual generation (oophore) of the higher cryptogams show half the number of chromosomes characteristic of the sporophyte, the explanation being that while reduction occurs at the time of spore-formation, the spores develop without fertilization, the reduced chromosome- number persisting until fertilization occurs long afterward. Attention may be again called to the surprising case of Anemia, described at page 281, which gives a strong argument in favour of the hypothesis.

In addition to the foregoing evidence, Van Beneden and Boveri were able to demonstrate in Ascaris that in the formation of the spireme the chromosomes reappear in the same position as those which entered into the formation of the reticulum, precisely as Rabl

Fig. MS- — Evidence of the individuality of the chromosomes in the egg of Ascarii. [BOVRM.) E. Anaphase of the first cleavage. F. Two-cell stage with lolled nuclei, the lobes formed by the ends of the chromosome*. C, Early prophase of the ensuing division ; chromosomes re-forming, ccntrosomes dividing. //. Later prophase, the chromosomes lying with their ends in the same position as before; centrosomes divided.

maintained. As the long chromosomes diverge, their free ends are always turned toward the middle plane (Fig. 31), and upon the reconstruction of the daughter-nuclei these ends give rise to corresponding lobes of the nucleus, as in Fig. [45, which persist throughout the resting state. At the succeeding division the chromosomes reappear exactly in the same position, their ends lying in the nuclear lobes as before (Fig. 145, G, H\ On the strength of these facts Boveri concluded that the chromosomes must be regarded as " individuals" or " elementary organisms," that have an independent existence in the cell. During the reconstruction of the nucleus they send forth pseudo podia which anastomose to form a network in which their identity is lost to view. As the cell prepares for division, however, the chromosomes contract, withdraw their processes, and return to their "resting state," in which fission takes place. Applying this conclusion to the fertilization of the egg, Boveri expressed his belief that

Fig. 146.— Independence of paternal _ Cyclops. [A-C. from ROCKKkT; D, from H

A. Firs! cleavage- figure in chromosomes. B. Resulting I«  Mill in double groups. D. Bias

, , independence of paternal and 1

with double nuclei. C. Second cleavage; chror h double nuclei from the eight-cell stage of C. in

" wc may identify every chromatic element arising from a resting nucleus with a definite element that entered into the formation of that nucleus, from which the remarkable conclusion follows that in all cells derived in the regular course of division front the fertilised egg, one-half of the chromosomes are of strictly paternal origin, the other half of maternal." 1

I'ol.p. 410.

Boveri's hypothesis has been criticised by many writers, especially by Hertwig, Guignard, and Brauer, and I myself have urged some objections to it. Recently, however, it has received a support so strong as to amount almost to a demonstration, through the remarkable observations of Ruckert, Hacker, Herla, and Zoja on the independence of the paternal and maternal chromosomes. These observations, already referred to at page 208, may be more fully reviewed at this point. Hacker ('92, 2) first showed that in Cyclops strenuns, as in Ascaris and other forms, the germ-nuclei do not fuse, but give rise to two separate groups of chromosomes that lie side by side near the equator of the cleavage-spindle. In the two-cell stage (of Cyclops tenuicornis) each nucleus consists of two distinct though closely united halves, which Hacker believed to be the derivatives of the two respective germ-nuclei. The truth of this surmise was demonstrated three years later by Ruckert ('95, 3) in a species of Cyclops, likewise identified as C. strenutis (Fig. 146). The number of chromosomes in each germ-nucleus is here twelve. Ruckert was able to trace the paternal and maternal groups of daughter-chromosomes not only into the respective halves of the daughter-nuclei of the two-cell stage, but into later cleavage -stages. From the bilobed nuclei of the two-cell stage arise, in each cell, a double spireme and a double group of chromosomes, from which are formed bilobed or double nuclei in the four-cell stage. This process is repeated at the next cleavage, and the double character of the nuclei was in many cases distinctly recognizable at a late stage when the germ-layers were being formed.

Finally Victor Herla's ('93) and Zoja's ('95, 2) remarkable observations on Ascaris showed that in Ascaris not only the chromatin of the germ-nuclei, but also the paternal and maternal chromosomes, remain perfectly distinct as far as the twelve-cell stage — certainly a brilliant confirmation of Boveri's conclusion. Just how far the distinction is maintained is still uncertain, but Hacker's and Riickert's observations give some ground to believe that it may persist throughout the entire life of the embryo. Both these observers have shown that the chromosomes of the germinal vesicle appear in two distinct groups, and Ruckert suggests that these may represent the paternal and maternal elements that have remained distinct throughout the entire cycle of development, even down to the formation of the egg !

Leaving aside all doubtful cases (such as the above suggestion of Riickert's), the well-determined facts form an irresistible proof of the general hypothesis ; and it is one with which every general analysis of the cell has to reckon. I believe, however, that the hypothesis has received an unfortunate name ; for, except in a few special cases, 1

1 Cf. p. 273.

almost no direct evidence exists to show that the chromosomes persist as " individuals " in the chroma tin- reticulum of the resting cell. The facts indicate, on the contrary, that in the vast majority of cases the identity of the chromosomes is wholly lost in the resting nucleus, and the attempts to identify them through the polarity or other morphological features of the nuclear network have on the whole been futile. It is therefore an abuse of language to speak of a persistent " individ

— Hybrid fertiliiatie

W.WKM&W. [HfiKLA.]

A. The germ-nuclei shortly before ur as given rise lo one chromosome (J).

i still shown in the primordial germ-cell o:

uality " of chromosomes. But this verbal difficulty should not blind us to the extraordinary interest and significance of the facts. It is difficult to suppose that the tendency of the chromatin to resolve itself into a particular number of chromosomes is directly due to its chemical or molecular structure, or is analogous to crystallization ; for in the chromatin of the same species, or even in that of the same egg, this tendency varies, not with chemical, but with purely morphological


conditions, i.e. with the number of chromosomes that enter the nucleus. Neither can we assume that it is due merely to the total mass of the chromatin in each case ; for this varies in different nuclei of the same species, or even in the nucleus of the same cell at different periods (as in the egg-cell), yet the same number of chromosomes is characteristic of all. Indeed, we seek in vain for an analogy to these phenomena and can only admit our entire inability to explain them. No phenomena in the history of the cell more clearly indicate the existence of a morphological organization which, though resting upon, is not to be confounded with, the chemical and molecular structure that underlies it ; and this remains true even though we are wholly ignorant what that organization is.

(6) Composition of the Chromosomes. — We owe to Roux l the first clear formulation of the view that the chromosomes, or the chromatinthread, consist of successive regions or elements that are qualitatively different (p. 244). This hypothesis, which has been accepted by Weismann, Strasburger, and a number of others, lends a peculiar interest to the morphological composition of the chromatic substance. The facts are now well established ( 1 ) that in a large number of cases the chromatin-thread consists of a series of granules (chromomeres) embedded in and held together by the linin-substance, (2) that the splitting of the chromosomes is caused by the division of these more elementary bodies, (3) that the chromatin-grains may divide at a time when the spireme is only just beginning to emerge from the reticulum of the resting nucleus. These facts point unmistakably to the conclusion that these granules are perhaps to be regarded as independent morphological elements of a lower grade than the chromosomes. That they are not artifacts or coagulation-products is proved by their uniform size and regular arrangement in the thread, especially when the thread is split. A decisive test of their morphological nature is, however, even more difficult than in the case of the chromosomes ; for the chromatin-grains often become apparently fused together so that the chromatin-thread appears perfectly homogeneous, and whether they lose their individuality in this close union is undetermined. Observations on their number are still very scanty, but they point to some very interesting conclusions. In Boveri's figures of the eggmaturation of Ascaris each element of the tetrad consists of six chromatin-discs arranged in a linear series (Van Beneden's figures of the same object show at most five) which finally fuse to form an apparently homogeneous body. In the chromosomes of the germ-nuclei the number is at least double this (Van Beneden). Their number has been more carefully followed out in the spermatogenesis of the same animal (variety bivalens) by Brauer. At the time the chromatin-grains

1 Bedeutung der Kcrnthcilung$figuren> 1883, p. 15.

divide, in the reticulum of the spermatocyte-nucleus, they are very numerous. His figures of the spireme-thread show at first nearly forty granules in linear series (Fig. 120, B). Just before the breaking of the thread into two the number is reduced to ten or twelve ( Fig. 120, C). Just after the division to form the two tetrads the number is four or five (Fig. 120, D) y which finally fuse into a homogeneous body. 1

It is certain, therefore, that the number of chromomeres is not constant in a given species, but it is a significant fact that in Ascaris the final number, before fusion, appears to be nearly the same (four to six) both in the oogenesis and the spermatogenesis. The facts regarding bivalent and plurivalent chromosomes (p. 87) at once suggest themselves, and one cannot avoid the thought that the smallest chromatin-grains may successively group themselves in larger and larger combinations of which the final term is the chromosome. Whether these combinations are to be regarded as " individuals " is a question which can only lead to a barren play of words. The fact that cannot be escaped is that the history of the chromatin-substance reveals to us, not a homogeneous substance, but a definite morphological organization in which, as through an inverted telescope, we behold a series of more and more elementary groups, the last visible term of which is the smallest chromatin-granule, or nuclear microsome, beyond which our present optical appliances do not allow us to see. Are these the ultimate dividing units, as Brauer suggests (p. 113)? Here again we may well recall Strasburger's warning, and hesitate to identify the end of the series with the limits reached t>y our best lenses. Somewhere, however, the series must end in final chromatic units which cannot be further subdivided without the decomposition of chromatin into simpler chemical substances ; and these units must be capable of assimilation, growth, and division without loss of their specific character. It is in these ultimate units that we must seek the " qualities," if they exist, postulated in Roux's hypothesis ; but the existence of such qualitative differences is a physiological assumption that in no manner prejudices our conclusion regarding the ultimate morphological composition of the chromatin.

D. Chromatin, Linin, and Cytoplasm

What, now, is the relation of the chromatin-grains to the linin-network and the cytoplasm ? Van Beneden long ago maintained 2 that

1 Eisen ('99) finds that the chromosomes of the spermatogonia of Batrachoseps always consist of six " chromomeres," each of which consists of three smaller granules or " chromioles." The latter persist as the chromatin-granules of the resting nucleus; and it is through their successive aggregation that the chromomeres and chromosomes are formed.

2 '83, pp. 580, 583.


the achromatic network, the nuclear membrane, and the cell-meshwork have essentially the same structure, all consisting of microsomes united by connective substance, and being only " parts of one and the same structure." But, more than this, he asserted that the chromatic and achromatic microsomes might be transformed into one another, and were therefore of essentially the same morphological nature. " They pass successively, in the course of the nuclear evolution, through a chromatic or an achromatic stage, according as they imbibe or give off the chromophilous substance." l Both these conclusions are borne out by recent researches. Heidenhain ('93, '94), confirmed by Reinke and Schloter, finds that the nuclear network contains granules of two kinds differing in their staining-capacity. The first are the basi-chromatin granules, which stain with the true nuclear dyes (basic tar-colours, etc.), and are identical with the " chromatin-granules " of other authors. The second are the oxychromatin-granules of the linin-network, which stain with the plasma-stains (acid colours, etc.), and are closely similar to those of the cytoreticulum. These two forms graduate into one another, and are conjectured to be different phases of the same elements. This conception is furthermore supported by many observations on the behaviour of the nuclear network as a whole. The chromatic substance is known to undergo very great changes in staining-capacity at different periods in the life of the nucleus (p. 338), and is known to vary greatly in bulk. In certain cases a very large amount of the original chromatic network is cast out of the nucleus at the time of the division, and is converted into cytoplasm. And, finally, in studying mitosis in sea-urchin eggs I found reason to conclude ('95, 2) that a considerable part of the linin-network, from which the spindle-fibres are formed, is actually derived from the chromatin.

From the time of the earlier writings of Frommann ('65, '67), Arnold ('67), Heitzmann ('73), and Klein ('78). down to the present, an increasing number of observers have held that the nuclear reticulum is to be conceived as a modification of the same structural basis as that which forms the cytoplasm. The latest researches indicate, indeed, that true chromatin (nuclein) is confined to the nucleus. 2 But the whole weight of the evidence now goes to show that the lininnetwork is of the same nature as the cell-meshwork, and that the achromatic nuclear membrane is formed as a condensation of the same substance. Many investigators, among whom may be named Frommann, Leydig, Klein, Van Beneden, Carnoy, and Reinke, have described the fibres of both the intra- and extra-nuclear network as terminating in the nuclear membrane ; and the membrane itself is described by these and other observers as being itself reticular in structure, and by some (Van Beneden) as consisting of closely crowded

1 U. p. 583. 2 Cf. Hammarsten C95).

microsomes arranged in a network. The clearest evidence is, however, afforded by the origin of the spindle-fibres in mitotic division ; for it is now well established that these may be formed either inside or outside the nucleus, and at the close of mitosis the central portion of the spindle appears always to give rise to a portion of the cytoplasm lying between the daughter-nuclei. In such a case as that of the sea-urchin (see above) we have, therefore,. evidence of a direct transformation of chromatin into linin-substance, of the latter into spindlefibres, and, finally, of these into cytoplasm.

When all these facts are placed in connection, we find it difficult to escape the conclusion that no definite line can be drawn between the cytoplasmic granules at one extreme and the chromatin-granules at the other. And inasmuch as the latter are certainly capable of growth and division, we cannot deny the possibility that the former may themselves have, or arise from elements having like powers. But while we may take this as a fair working hypothesis, we should clearly recognize that the base of well-determined fact on which it rests is approached by a circuitous route ; that in case of most of the cytoplasmic granules there is not the slightest evidence that they multiply by division ; and that even though some of them may have such powers, we cannot regard them as the ultimate structural units, for the latter must be bodies far more minute.

E. The Centrosome

From our present point of view the centrosome possesses a peculiar interest as a cell-organ which may be scarcely larger than a cytomicrosome, yet possesses specific physiological properties, assimilates, grows, divides, and may persist from cell to cell without loss of identity. Nearly all observers of the centrosome have found it lying in the cytoplasm, outside the nucleus; but apart from the Protozoa (p. 94) there is at least one well-established case in which it lies within the nucleus, namely, that of Ascaris, where Brauer made the interesting discovery that /;/ one variety (univalens) the centrosome lies inside the nucleus y in the other variety (bivalens) outside — a fact which proves that its position is non-essential (cf. Figs. 120 and 148).

An intra-nuclear origin of the centrosome has also been asserted by Julin ('93) in the primary spermatocytes of Styleopsis y by Riickert ('94) in the eggs of Cyclops, Mathews ('95) in those of Asterias, Carnoy and Le Brun ('97, 2) in Ascaris, Van der Stricht C98) in the eggs of Thysanozobn, by R. Hertwig ('98) in Actinosph<zrium y Calkins ('98, 1) in Noctiluca, and Schaudinn ('96, 3) in spore-producing buds of Acanthocystis, though in the last-named form the centrosome of the vegetative forms is extra-nuclear (p. 92).

As already stated, 1 it is still undetermined whether a true centrosome may ever arise de novo, but the evidence in favour of such a possibility has of late rapidly increased. Carnoy ('86) long since showed that the egg of Asearis, during the formation of the polar bodies, sometimes showed numerous accessory asters scattered through the cytoplasm. Reinke ('94) described somewhat similar asters in peritoneal cells of the salamander, distinguishing among them three orders of magnitude, the largest containing distinct centrosomes or " primary centres," while the smaller contained " secondary " and " tertiary " centres, the last named being single

Fig. 148. — Mitosis with inlra-nu clear centres

ome, in the spermatocytes of

ctphala, var. mmivaUni. [Brauek.]

A. Nucleus containing a quadruple group or

tetrad of chromosomes (/), nu

cenlrosome (t). B.C. Division of the cenlrosom

e. D.E.F.G. Formation of ll

centrosomes escaping from the nucleus in G.

microsomes at the nodes of the cytoreticulum. By successive aggregations of the tertiary and secondary centres arise true centrosomes as new formations. Watase ('94-95) also finds in the egg of Macrobdella, besides the normal aster containing an undoubted centrosome, numerous smaller asters graduating downwards to such "tertiary asters " as Reinke describes with a microsome at the centre of each, and on this basis concludes that the true centrosome differs from a microsome only in degree and may arise de novo. Mottier ('97, 2) finds in pollen- mother-cells numerous minute " cyto-asters " having no direct relation to the spindle-formation (Fig. 133). Again Juel

1 Cf. pp. 53, 214.

('97) finds that an isolated chromosome, accidentally separated from the equatorial plate (pollen-m other-cells of Hcnterocallis), may give rise to a small vesicular nucleus which may subsequently divide by mitosis, though it is quite out of relation to the spindle-poles of the preceding mitosis (Fig. 149). Strong evidence of the same character as the last is given by the facts in the hehozobn Acanthocystis, as shown by Schaudinn ('96, 3), the ordinary vegetative cells containing a persistent extra-nuclear centrosome, while in the bud-formation of the swarm-spores a centrosome is formed de novo, without relation to that of the mother-cell, inside the nucleus of the bud (Fig. 41).

The strongest case in favour of the independent origin of centrosomes is, however, given by the observations of Mead on Ckcetopterus ('98) and the remarkable experiments of R. Hertwig ('95, '96) and

Morgan ('96, 1; '99, i)on theeggs of echinoderms and other animals. When eggs of C/iaJtofterHs are taken from the body-cavity and placed in sea-water, a multitude of small asters appear in the cytoplasm, two of which are believed to persist as those of the polar spindle, while the others degenerate (Fig. 150). Mead is therefore convinced that the polar centrosomes arise in this case separately and de novo. 1 R. Hertwig showed that when unfertilized eggs of sea-urchins (Sfrongylocetitrotits, Echinus) are kept for some time in sea-water or treated with dilute solutions of strychnine the nuclei undergo some of

1 A numher of other authors (eg. Griffin, Tkalassima, Coe, Ccrt/ira/ului) have likewise fouiul the first polar asters widely separated at their first appearance. On the other hand, Mathews ('05), whose preparations 1 have seen, limls the polar ccntrusomes in Asteriai close together, and Krancotte ('97, '98) has demonstrated that in Cyrlnperas and Prosthae raajlhey arise by the division of a single primary centrusome. The same is stated by Gardiner ('98) to he the case in Polyth<rrHs. It should be noted, further, thai Mead could find no undoubted centrosomes save in the " primary " or definitive polar asters.

the changes of mitosis, the chromatin-network giving rise to a group of chromosomes and a spindle, or more frequently a fan-shaped half-spindle, arising from the achromatic substance. In some cases not only a complete spindle appeared but also asters at the poles, though no centrosomes were observed (Fig. 151). Morgan's experiments along the same lines were mainly performed upon the seaurchin Arbacia, but included also the eggs of Asterias, Sipunculus, and Cerebratulus (Figs. 150, 151). In these eggs numerous asters may arise in the cytoplasm, if they are allowed to He some time in sea

Fig. 150. — Formation dc novo (1) of centrosomes. [A. W,MeaI>; C, Moroan.]

A. Unfertilized egg of Chatopttnis wilh " secondary asters " developed a few minutes after the

f gR i* placed in sea-water. /(. Slightly later stage with two definitive polar asters and centrosomes.

C. Large " sun " (transformed polar aster) containing numerous small " secondary asters " and

centrosomes, from unferiiliied egg of Ctribrotulm after aa hours in 1.5 % sodium chloride


water or treated by weak solutions of sodium or magnesium chloride. These asters often contain deeply staining, central granules indistinguishable from the centrosomes of the normal asters ; and, what is of high interest, such of them as lie near the nucleus take part in the irregular nuclear division that ensues, forming centres toward which the chromosomes pass. These divisions continue for some time, the chromosomes being irregularly distributed through the egg, and giving rise to nuclei of various sizes apparently dependent upon the number of chromosomes each receives. After a variable number of such


divisions the asters disappear, yet the irregular nuclear divisions continue, nuclear spindles with distinct centrosomes being formed at each division, but apparently without relation to the older asters, and they

. after tff, hours in 1.5% solut

1 asters in unfertilized echinoderm-eges. [A, B.

on of sodium chloride, then 5 hours in sea-water: formed after 6% hours in NaCI. C-E. Echinus afier treatment with 0.5 '!/, stry^hninc-so'iit-iin. showing various forms of astral formations (fanshaped asler. half spindle, and complete mitotic figure).

are believed by Morgan to arise dc novo from the egg substance. 1 In the meantime irregular cleavage of the egg occurs, though no embryo is produced. 2 Loeb, however, in the remarkable experiments

1 '99. p. 479

Morgan makes Ihe important observation, which harmonizes with that of Boreri, reported at page toK, that the divisions ocrur with rcspttt to the number and position of Iht nutlet, not oj the asters, concluding that the former must therefore play an essential rile as centres of division, and that the activity of (he asters is in itself not sufficient to account for division of the cytoplasm.

referred to at page 215, finds that after treatment with magnesium chloride unfertilized sea-urchin eggs (Ariacta) may give rise to perfect Pluteus larvae — a result which if well founded seems to place the new formation of true centrosomes beyond question.

Taken together, these researches give strong ground for the conclusion that true {i.e. physiological) centrosomes may arise de novo from either the cytoplasmic or the nuclear substance and may play the usual rdle (whatever that may be) in mitosis. If this conclusion be sustained by future research, we shall no longer be able to accept Van Beneden's and Boveri's conception of the centrosome as a persistent organ in the same sense as the nucleus ; but on the other hand we shall have gained important ground for further inquiry into the nature and source of that power of division which is so characteristic of living things and upon which the law of genetic continuity rests.

Morphology of the Centrosome. — In its simplest form (Fig. 152, A) the centrosome appears under the highest powers as nothing more than a single granule of extraordinary minuteness which stains intensely with iron-haematoxylin, and can scarcely be distinguished from the cyto-microsomes except for the fact that it lies at the focus of the astral rays. In this form it always appears at the centre of the very young sperm-asters during fertilization (Figs. 97, 99), in the early phases of ordinary mitosis ( Figs. 27, 32), and in some cases also in the resting cell, for example, in leucocytes and connective tissue corpuscles (Figs. 8, 49), where, however, it is often triple or quadruple. In the course of division the centrosome often increases in size and assumes a more complex form, becoming also surrounded by various structures involved in the aster-formation. The relation of these structures to the centrosome itself has not yet been fully cleared up and there is still much divergence of opinion regarding the cycle of changes through which the centrosome passes. It is, therefore, not yet possible to give a very consistent account of the centrosome, still less to frame a satisfactory morphological definition of it.

It is convenient to take up as a starting-point Boveri's (*88) account of the centrosomes in the egg of Ascaris, supplemented by Brauer's ('93) description of those in the spermatocytes of the same animal. During the early prophases of the first cleavage Boveri found the centrosome as a minute granule which steadily enlarges as the spindle forms, until shortly before the metaphase it becomes a rather large, well-defined sphere in the centre of which a minute central granule or centriole appears (Fig. 152, B, C). From this time onward the centrosome decreases in size until in the daughter-cells it is again reduced to a small granule which divides into two and goes through a similar cycle during the second cleavage and so on. The centrosome is at all stages surrounded by a clear zone (" Heller Hof ") in which the astral rays are thinner and stain less deeply than farther out. Brauer's account is substantially the same, though no definite " Heller Hof " was found, and the astral rays were traced directly in to the boundary of the centrosome. He added, however, two important observations, viz. ( i ) that the central granule is visible at every period ; and (2) division of the centrosome is prccedal by division of the central granule (Fig. 148) — an observation recently extended by Boveri to the division of the egg-centrosome. 1 Van Beneden and Neyt ('87), on the other hand, gave a quite different account of the

e enclosing a central granuli

surrounded by a " Heller Hof; tx. Iloveri's account of the centrosome of the Ascaris egg. D. Central granule surrounded by a

tx. polar spindles of ThysanotoSn, Van der Stricht. E. Ceniral granule ("centrosome") surrounded by medullary and cortical radial lones, each bounded by a microsome-circle ; tx. polar spinille of Unit, LIUie. P. Van Bent- don's representation of aster of the Aiearil egg '. I'l« the last.

granules surrounded by a ■'Heller Hof; tx. the echinoilenn-egg. //. "Centrosome" (central granule) surrounded by a vague larger body lying in a reticulated centrosphere ; tx. Thataatma. [GKIKP1N.]

structures at the centre of the aster. The " corpuscule central " (usually assumed by later writers to be the centrosome), described as a "mass of granules," is surrounded by two well-defined astral zones, formed as modifications of the inner part of the aster, and constituting the "attraction-sphere." These are an inner "medullary zone," and an outer " cortical zone," each bounded by a very distinct layer of microsomes (Fig. 152, F).

• Reported by Flint, '9S, p. III.

The discrepancy between these results on the part of the two pioneer investigators of the centrosome has led to great confusion in the terminology of the subject, which has not yet been fully cleared away. Many of the observers who followed Boveri (Flemming, Hermann, Van der Stricht, Heidenhain, etc.) found the centrosome, in various cells, as a much smaller body than he had described, often as a single or double minute granule, staining intensely with iron-haematoxylin. Heidenhain ('93, '94) and Driiner ('94, '95) found further that the asters in leucocytes and other forms often show several concentric circles of microsomes, and that the sphere bounded by the innermost circle often stains more deeply than the outer portions and may appear nearly or quite homogeneous (Fig. 156). To this sphere, with its contained central granule or granules Heidenhain applies the term microcentrum C94, p. 463), while Kostanecki and Siedlecki suggest the term microsphere ('96, p. 217). Still later Kostanecki and Siedlecki ('97) found that even in Ascaris, as in other forms, sufficient extraction of the colour (iron-haematoxylin) reduces the centrosome to a minute granule to which the astral rays converge, and which is presumably identical with Boveri's "central granule." Heidenhain ('93, '94) found that in leucocytes the central granule is often double, triple, or even quadruple, while in giant-cells of certain kinds there are numerous deeply staining granules (Fig. 14). He therefore proposed to restrict the term centrosome to the individual granules, whatever be their number, applying the term microcentrum to the entire group ('94, p. 463).

With these facts in mind we can gain a clear view of the manner in which both the confusion of terminology and the contradiction of results has arisen. Brauer ('93 ) found in Ascaris (see above) that division of the central granule precedes division of the "centrosome" and therefore suggested that only the former is equivalent to Van Beneden's "corpuscule central," while the body called "centrosome" by Boveri is really the medullary astral zone, the " Heller Hof " being the cortical zone. This is substantially the same conclusion reached by Heidenhain, Rawitz, Lenhoss£k, Kostanecki and Siedlecki, Erlanger, Van der Stricht, Lillie, and several others. The confusion of the subject is owing, on the one hand, to the fact that those who have accepted this conclusion continue to use the word centrosome in two quite different senses, on the other hand to the fact that the conclusion is itself repudiated by Boveri ('95), MacFarland C97), and Furst C98).

As regards the terminology we find that most recent writers agree with Heidenhain, Kostanecki and Siedlecki, in restricting the word centrosome to the minute, deeply staining granules, whether one or more, at the centre of the aster. On the other hand, Brauer, Fran

cotte, Van der Stricht, Meves, and others apply the term to the central granule or granules plus the surrounding sphere (" centrosome " of Boveri), which they regard as equivalent to the medullary zone of Van Beneden, the " corpuscule central " of the last-named author being identified with the central granule or "centriole" of Boveri, though the latter structure is considerably smaller than the former as described by Van Beneden.

The matter of fact turns largely on the question whether the astral rays traverse the larger sphere to the central granule. That such is the case in Ascaris is positively asserted by Kostanecki and Siedlecki, ('97) and as positively denied by Furst ('98) with whose observations


A. Mitotic figure, formation of first polar body. B. Inner aster granule double within the " centrosome." C. Elongation of c" polar spindle.

those of MacFarland ('97) on gasteropod-eggs agree. On the other hand, in the turbellarians the observations of Francotte ('97, '98) and Van der Stricht ('98, 1) seem to leave no doubt that the larger sphere ("centrosome"), here very sharply defined and staining deeply in iron-hrematoxylin, is traversed by well-defined astral rays converging to the central corpuscle, and both these observers agree further that both the corpuscle and the sphere divide to persist as the "cetttrosomes" of the daughter-cells — a result in conformity with Van Beneden's conclusion in the case of Ascaris.

LilHe's valuable observations on the polar asters of Unto ('98) afford, I believe, conclusive evidence as to the nature of the sphere. In the earlier stages the aster has exactly the structure described by Van Beneden in Ascaris, except that the innermost body {i.e. the " corpuscule central ") is a single minute granule. This Is surrounded by typical medullary and cortical zones, through both of which the

Kg- 154- A. Alter o( the first polar (enrosphere) and cortical (ectc . entosphere bounded \>y continuous of central spindle within and Irum tV

B. Late anaphase oi . C, D. Prophases <

of the old cntospheri


esof Unie. [LlLLtB.]

>some) surrounded by medullary

rays pass (Fig. 152, E, Fig. 154). The inner sphere, consisting of a dense and deeply staining substance, has at first a typical radiate structure and is bounded by a microsome-circle. In later stages (late anaphase) the central granule divides into two and afterward into four or more granules, of which, however, only one or two actually persist. The inner sphere is now bounded by a definite membrane, and its radiate structure becomes obscure, the astral rays extending only to the boundary of the sphere, though a few rays persist within it (Fig. 154, B). It is clear from this that the inner sphere and central granule pass through phases that bridge the gap between Van Beneden's and Boveri's descriptions. Lillie's observations fully sustain the conclusion that the central granule ("centriole" of Boveri) corresponds to the " corpuscule central" of Van Bcnedcn, and the inner sphere {medullary zone) to Boveri's "centrosome." A comparison of the polar aster of Unio with that of Thysanozoon, as described by Van der Stricht ('98), leaves hardly room for doubt that the cortical zone represents Boveri's " Heller Hof " ; for in both forms the rays of the cortical zone are much thinner and lighter than the more peripheral portions, thus giving a clear zone, which in Unio is bounded by only a fairly definite microsome-circle and in Thysanozoon by none. Lastly, we must recognize the justice of the view urged by Kostanecki, Griffin, Mead, Lillie, Coe, and others, that the term centrosotne should be applied to the central granule and not to the sphere surrounding it (medullary zone), despite the fact that historically the word was first applied by Boveri to the latter structure. For in both Diaulula (MacFarland) and Unio (Lillie) the second polar spindle arises from the substance of the inner sphere, while the central granule, becoming double, gives rise to the centrosomes at its poles. By following Boveri's terminology, therefore, MacFarland is driven to the strange conclusion that the second polar spindle is nothing other than an enormously enlarged " centrosome " — a result little short of a reductio ad absurdum when we consider that in Ascaris the polar spindle arises by a direct transformation of the germinal vesicle (p. 277). The obvious interpretation is that the central granule is the only structure that should be called a centrosome, the surrounding sphere being a part of the aster, or rather of the attraction-sphere. Thus regarded, the origin of the spindle in Diaulula presents nothing anomalous and a similar interpretation may be placed on the polar spindles of Ascaris as described by Fiirst C98). 1

1 In echinoderms the concurrent results of Reinke ('95), Boveri ('95), myself (*96-'97), show that the " centrosome " is a well-defined sphere containing a large group (ten to twentv) of irregularly scattered, deeply staining granules. I have shown in this case that in the early prophases there is but one such granule, which then becomes double and finally multiple, forming a pluricorpuscular centrum (Fig. 52) not unlike that described by Heidenhain in giant-cells. Kostanecki, who asserts that the centrosome of echinoderms is a single granule ('96, 1, '96, 2, p. 24S), has not sufficiently studied the later phases of mitosis. Cf. also Erlanger ('98). The centrosomes described in nerve cells by Lenhossek ('95) are apparently of somewhat similar type. Until the facts are more fully known the exact nature of these " centrosomes " remains an open question. Lillie's observations on Unio show that here, too (first polar spindle), the centrosome divides to form a considerable number of

The genesis of the concentric spheres surrounding the centrosome will be considered in the following section. We may here only emphasize the remarkable fact that the centres of the dividing system are bodies which are in many cases so small as to lie almost at the limits of microscopical vision, and which in the absence of the surrounding structures could not be distinguished from other protoplasmic granules. Full weight should be given to this fact in every estimate of the centrosome theory, and it is no less interesting in its bearing upon the corpuscular theory of protoplasm.

Watas£ ('93, '94) made the very interesting suggestion that the centrosome is itself nothing other than a microsome of the same morphological nature as those of the astral rays and the general meshwork, differing from them only in size and in its peculiar powers. 1 Despite the vagueness of the word " microsome," which has no well-defined meaning, Watas^'s suggestion is full of interest, indicating as it does that the centrosome is morphologically comparable to other elementary bodies existing in the cytoplasmic structure, and which, minute though they are, may have specific chemical and physiological properties.

An interesting hypothesis regarding the historical origin of centrosome is that of Biitschli ('91) and R. Hertwig (92), who suggest that it may be a derivative of a body comparable with the micro-nucleus of Infusoria, which has lost its chromatin but retained the power of division ; and the last-named author has suggested further that the so-called " archoplasmic loops" discovered by Platner in pulmonates may be remnants of the chromatic elements. A similar view has been advocated by Heidenhain ('93, '94) and Lauterborn C96). Heidenhain regards central spindle and centrosomes as forming essentially a unit ("microcentrum ,, ) homologous with the micro-nucleus of the Infusoria, the centrodesmus (p. 79) representing a part of the original achromatic elements. The metazoan nucleus is compared to the protozoan macro-nucleus. The improbability of a direct derivation of the Metazoa from Infusoria, urged by Boveri ('95) and Hertwig C96), has led Lauterborn ('96) to the view that the metazoan centrosome and nucleus are respectively derivatives of two equivalent nuclei, such as Schaudinn ('95) describes in Amoeba binucUata, the "Nebenkorper" of Paramceba (cf. p. 94), being regarded as an intermediate step, and the micro-nucleus of Infusoria a side-branch. R. Hertwig ('96), on the other hand, regards the metazoon centrosome as a derivative of an intra-nuclear body such as the '* nucleolo-centrosome " of Euglena (p. 91), which has itself arisen through a condensation of the general achromatic substance. With this view Calkins ('98), on the whole, agrees ; but he regards it as probable that the " nucleolo-centrosome "

granules of which one or two remain as the persistent centrosome, while others are converted into microsomes or other cytoplasmic structures. It is probable that something similar occurs in the echinoderms.

1 The microsome is conceived, if I understand Watase rightly, not as a permanent morphological body, but as a temporary varicosity of the thread, which may lose its identity in the thread and reappear when the thread contracts. The centrosome is in like manner not a permanent organ like the nucleus, but a temporary body formed at the focus of the astral rays. Once formed, however, it may long persist even after disappearance of the aster, and serve as a centre of formation for a new aster.

of Euglena and Amoeba and the sphere of Noctiluca and Paramaba are to be compared with the attraction-sphere, while the centrosome may have had a different origin.

It appears to me that none of these views rests upon a very substantial basis, and they must be taken rather as suggestions for further work than as well-grounded conclusions.

F. The Archoplasmic Structures

1. Hypothesis of Fibrillar Persistence

The asters and attraction-spheres have a special interest for the study of cell-organs ; for they are structures that may divide and persist from cell to cell or may lose their identity and re-form in successive cell-generations, and we may here trace with the greatest clearness the origin of a cell-organ by differentiation out of the structural basis. Two sharply opposing views of these structures have been held, represented among the earlier observers on the one hand by Boveri, on the other by Butschli, Klein, Van Beneden, and Carnoy. The latter observers held that the astral rays and spindle-fibres, and hence the attraction-sphere, arise through a morphological rearrangement of the preexisting protoplasmic meshwork, under the influence of the centrosome. This view, which may be traced back to the early work of Fol ('73) and Auerbach ('74)> was first clearly formulated by Butschli ('76), who regarded the aster as the optical expression of a peculiar physico-chemical alteration of the protoplasm primarily caused by diffusion-currents converging to the central area of the aster. 1 An essentially similar view is maintained in Butschli's recent great work on protoplasm, 2 the astral " rays " being regarded as nothing more than the meshes of an alveolar structure arranged radially about the centrosomes (Fig. 10, B). The fibrous appearance of the astral rays is an optical illusion, for they are not fibres, but flat lamellae forming the walls of elongated closed chambers. This view has recently been urged, especially by Erlanger ('97, 4, etc.), who sees in all forms of asters and spindles nothing more than a modified alveolar structure.

The same general conception of the aster is adopted by most of those who accept the fibrillar or reticular theory of protoplasm, the astral rays and spindle-fibres being regarded as actual fibres forming part of the general network. One of the first to frame such a conception was Klein {'78), who regarded the aster as due to " a radial arrangement of what corresponds to the cell-substance," the latter

1 For a very careful review of the early views on this subject, see Mark, Li max, 1881. 2 '92, 2, pp. 158-169.

being described as having a fibrillar character. 1 The same view is advocated by Van Beneden in 1883. With Klein, Heitzman, and Frommann he accepted the view that the intra-nuclear and extranuclear networks were organically connected, and maintained that the spindle-fibres arose from both. 2 "The star-like rays of the asters are nothing but local differentiations of the protoplasmic network. 8 . . . In my opinion the appearance of the attraction-spheres, the polar corpuscle (centrosome), and the rays extending from it, including the achromatic fibrils of the spindle, are the result of the appearance in the egg-protoplasm of two centres of attraction comparable to two magnetic poles. This appearance leads to a regular arrangement of the reticulated protoplasmic fibrils and of the achromatic nuclear substance with relation to the centres, in the same way that a magnet produces the stellate arrangement of iron filings." 4

This view is further developed in Van Beneden's second paper, published jointly with Neyt ('87). " The spindle is nothing but a differentiated portion of the asters." 6 The aster is a " radial structure of the cell-protoplasm, whence results the image designated by the name of aster." 6 The operations of cell-division are carried out through the " contractility of the fibrillae of the cell-protoplasm and their arrangement in a kind of radial muscular system composed of antagonizing groups." 7

An essentially similar view of the achromatic figure has been advocated by many later workers. Numerous observers, such as Rabl, Flemming, Carnoy, Watas£, Wilson, Reinke, etc., have observed that the astral fibres branch out peripherally into the general meshwork and become perfectly continuous with its meshes, and tracing the development of the aster, step by step, have concluded that the rays arise by a direct progressive modification of the preexisting structure. The most extreme development of this view is contained in the works of Heidenhain ('93, '94), Buhler ('95), Kostanecki and Siedlecki ('97), which are, however, only a development of the ideas suggested by Rabl in a brief paper published several years before. Rabl ('89, 2) suggested that neither spindle-fibres nor astral rays really lose their identity in the resting cell, being only modified in form to constitute the mitome or filar substance (meshwork), but still being centred in the centrosome. Fission of the centrosome is followed by that of the latent spindle-fibres (forming the lininnetwork); hence each chromosome is connected by pairs of daughter 1 It is interesting to note that in the same place Klein anticipated the theory of fibrillar contractility, both the nuclear and the cytoplasmic reticulum being regarded as contractile (/.<:., p. 417).

2 '83. P- 592. 4 '83. P. 55°- 6 lc * P- 2 75 •'83, p. 576. 6, 87, p. 263. 7 Lc. t p. 280.

fibres with the respective centrosomes. Heidenhain, adopting the first of these assumptions, builds upon it an elaborate theory of cellpolarity and cell-division already considered in part at pages 103-105. Sometimes the astral rays (" organic radii n ) retain their radial arrangement throughout the life of the cell (leucocytes, Fig. 49) ; more commonly they are disguised and lost to view in the cytoplasmic mesh work. All, however, are equal in length and in tension — assumptions based on the one hand on the occurrence of concentric circles of microsomes in the aster, on the other hand on the analogy of the artificial model described at page 104. Btihler C95) and Kostanecki and Siedlecki C97) likewise unreservedly accept the view that besides the centrosome the entire system of " organic radii," including astral rays, mantle-fibres, and central spindle-fibres, persists in the resting cell in modified form, and is centred in the centrosome. Kostanecki finally ('97 ) takes the last step, logically necessitated by the foregoing conclusion, and apparently supported also by the crossing of the astral rays opposite the equator of the spindle and the relations of their peripheral ends, concluding that the monocentric astral system is converted into the dicentric system (amphiaster) by longitudinal fission of the rays} Thus the entire mitome of the mother-cell divides into equal halves for daughter-cells ; and since the radii consist of microsomes, each of these must likewise divide into two. 2

Could this tempting hypothesis be established, Roux's interpretation of nuclear division (p. 224) could be extended also to the cytoplasm ;. and the aster- and amphiaster-formation, with the spireme-formation, might be conceived as a device for the meristic division of the entire cell-substance — a result which would place upon a substantial basis the general corpuscular theory of protoplasm. Unfortunately, however, the hypothesis rests upon a very insecure foundation : first, because it is based solely upon the fibrillar theory of protoplasm ; second, because of the very incomplete direct evidence of such a splitting of the rays ; third, because there is very strong evidence that in many cases the old astral rays wholly disappear, to be replaced by new ones. 3 We may best consider this adverse evidence in connection with a general account of the opposing archoplasm-hypothesis.

2. The A re hop las m Hypothesis

Entirely opposed to the foregoing conception are the views of Boveri and his followers, the starting point of which is given by

1 '97, p. 680.

2 This view had been definitely stated also by O. Schultze in 1890.

3 There is, however, no doubt that the aster as a whole does, in some cases, divide into two — for instance, in the echinoderm-egg, Fig. 95.

Boveri's celebrated archoplasm-hypothesis. Boveri has from the first maintained that the amphiastral fibres are quite distinct from the general cell-meshwork. In his earlier papers he maintained ('88, 2) that the attraction-sphere of the resting cell is composed of a distinct substance, "archoplasm" consisting of granules or microsomes aggregated about the centrosome as the result of an attractive force exerted by the latter. From the material of the attraction-sphere arises the entire achromatic figure, including both the spindle-fibres and the astral rays, and these have nothing to do with the general reticulum of the cell. They grow out from the attraction-sphere into the reticulum as the roots of a plant grow into the soil, and at the close of mitosis are again withdrawn into the central mass, breaking up into granules meanwhile, so that each daughter-cell receives one-half of the entire archoplasmic material of the parent-cell. Boveri was further inclined to believe that the individual granules or archoplasmic microsomes were " independent structures, not the nodal points of a general network," and that the archoplasmic rays arose by the arrangement of these granules in rows without loss of their identity. 1 In a later paper on the sea-urchin this view underwent a considerable modification through the admission that the archoplasm may not preexist as formed material, but that the rays and fibres may be a new formation, crystallizing, as it were, out of the protoplasm about the centrosome as a centre, but having no organic relation with the general reticulum ; though Boveri still held open the possibility that the archoplasm might preexist in the form of a specific homogeneous substance distributed through the cell, though not ordinarily demonstrable by reagents. 2 In this form the archoplasm-theory approaches very nearly that of Strasburger, described below.

There are three orders of facts that tell in favour of Boveri's modified theory: first, the existence of persistent archoplasm-masses or attraction-spheres from which the amphiasters arise ; second, the origin of amphiasters in alveolar protoplasm ; and, third, the increasing number of accounts asserting the replacement of the old asters by others of quite new formation. In at least one case, namely, that of Noctiluca % the entire achromatic figure is formed from a permanent attraction-sphere lying outside the nucleus and perfectly distinct from the general cell-meshwork. 8 Other cases of this kind are very rare, and in most cases the attraction-sphere sooner or later disintegrates, 4 but in the formation of the spermatozoa we have many examples of archoplasmic masses (Nebenkern, attraction-sphere, idiozome), which apparently consist of a specific substance having a special relation to the achromatic figure.

1 '88, 2, p. 80. 8 Ishikawa, '94, '98; Calkins, '9$, 2.

'95, 2, p. 40. * Cf. p. 323.

The amphiastral formation in alveolar protoplasm gives very clear evidence against the theory of fibrillar persistence. Here the fibrillar rays can be seen growing out through the walls of the alveoli ' quite distinct from, though embedded in, them. At the close of mitosis every trace of the fibrillar formation may disappear, e.g. in echinoderm-eggs after formation of the polar bodies, the protoplasm retaining only a typical alveolar structure.

Fig. 155. — Siages in the first cleavage of Ihe egg in Cttebral«l«i (A -C, Cof.) and Thalaistma

{D-F. Griffin).

A. Firs! appearance of rhe clcavaj>e-ccntrosome at the poles of Ihe fused perm-nuclei : cleavageasirrs iorminj; within i!ie ricgeneralinj; sperm -asters. B. Final anaphase of first cleavage, showing persistent eentrosomes anil new aslirs forming. C. Immediately after division. D-F. Three stages of Ihe laic anaphase in Thalnsstatii, showing formation of new asters wiihin the old. {Cf. Fig. 99.)

The strongest evidence against fibrillar persistence is, however, given by recent studies on mitosis, showing on the one hand that the new astral centres do not coincide with the old ones, on the other that the old rays degenerate /;/ situ, to be replaced by new ones. Aside from many earlier observers, who believed the entire aster to disappear at the close of mitosis, the first to assert the wholly new » Cf. Reinke ('95). Wilson ('99).

formation of the rays was Drtiner, who maintained in the case of the mitosis of salamander testis-cells, that " not a single fibre of the astral system of the mother-cell is carried over unchanged into the organism of the daughter-cell" ('95, p. 309). The same conclusion was soon afterward supported by Braus ('95) in the case of the cleavagemitoses of Triton. The most convincing evidence of this fact has been given by studies on the maturation and fertilization of the egg by Griffin ('96, '99), Mac Far land C97), Lillie ('99), and Coe C99), all of whom find that the new astral centres, arising by division of the centrosome, move away from the old position, to which, however, the old rays still converge while the new asters are independently forming (Fig. 155). This is shown with especial clearness in the egg of Ccrebratulus (Coe), where the peripheral portions of the old asters persist until the new amphiaster is completely formed. This observation seems conclusively to overturn Kostanecki's hypothesis of the persistence and division of the rays, and together with the work of MacFarland gives a very strong support to Boveri's later view.

It still remains an open question whether the rays actually arise from the substance of the centrosome, from a specific surrounding archoplasm, or by differentiation out of the general substance of the meshwork. The first of these possibilities has been urged in a very interesting way by Watase ('94), who believes that the centrosome " spins out the cytoplasmic filaments " l of the spindle and aster, and that ordinary microsomes may in like manner spin out the fibrillae of ordinary cytoplasmic networks. 2 This view is sustained by the mode of origin of the axial filament in the spermatozoa and that of the cilia in plant spermatozoids. It is, on the other hand, opposed by the almost infinitesimal bulk of the centrosome as compared with that of the aster that may form about it, and by the formation of the spindles in higher plants in the apparent absence of centrosomes. On the whole, the facts do not seem at present to warrant the acceptance of Watas^'s ingenious hypothesis, and the most probable view is that of Druner and Boveri, that the rays are differentiated out of the walls of the meshwork. In cases where the protoplasm is reticular or fibrillar the differentiation of the rays may be indistinguishable from a mere rearrangement of the thread-work; in alveolar protoplasm they may be seen as new formations, while in either case the material of the old aster may be more or less directly utilized in the building of the new. The feature common to all is the periodic activity either of the centre itself or of the surrounding protoplasm, and the coincidence or non-coincidence of the new aster with the old is apparently a secondary matter.

1 /.r., p. 283.

8 See the same paper for a suggestive comparison of the astral fibrillae to muscle-fibres. Y

In its original form the archoplasm hypothesis, as stated by Boveri, was developed with reference only to the material of the spindlefibres and astral rays. Later writers have greatly extended the conception on the basis of Boveri's earlier view that archoplasm is a specific form of protoplasm, possessing specially active properties. Strasburger ('92-98), whose views have already been considered in part, believes the protoplasm to consist of, or to show a tendency to differentiate itself into, two distinct substances, namely, a specially active fibrillar kinoplastn and a less active alveolar trophoplasm. The former gives rise to the mitotic fibrillae, constitutes the peripheral cell layer, or Hautschicht, from which the membrane arises, forms the substance of the centrosomes, and gives origin to the contractile substance of cilia and flagella. The kinoplasm is thus mainly concerned with the motor phenomena of the cell, the trophoplasm with those of nutrition ; and this physiological difference is morphologically expressed in the fact that the former has in general a fibrillar structure, the latter an alveolar. Beyond this the two forms of protoplasm show a difference of staining-reaction, the kinoplasmic fibrillae staining deeply with gentian-violet and iron-haematoxylin, while the trophoplasm is but slightly stained.

Prenant ('98, '99) still further extends the hypothesis, adopting the view that the " ergastoplasmic " (Gamier) fibrillae of gland-cells l are equivalent to the kinoplasmic or archoplasmic fibrillae of the mitotic figure, and to the fibrillae of nerve- and muscle-fibres as well. He is thus led to the conception of a dominating or " superior " cytoplasm (including "archoplasm," "kinoplasm," "ergastoplasm"), which arises by differentiation out of the general cytoplasm, plays the leading role in the elaboration of active cell-elements ("cytosomes"), such as mitotic, neural, and glandular fibrillae, and finally, its rdle accomplished, may disappear. Under the same category with the foregoing structures are placed the centrosome, attraction-sphere, mid-body, idiozome, Nebenkern, and yolk-nucleus.

Such a generous expansion of the archoplasm-hypothesis brings it perilously near to a rcductio ad absurdum ; for the step is not a great one to the identification of the " superior protoplasm " with the active cell-substance in general, which would render the whole hypothesis superfluous. Physiologically, we can draw no definite line of demarcation between the more and the less active protoplasmic elements, and it may further be doubted whether such a boundary exists even between the latter and the metaplasmic substances. 2 It is further quite unjustifiable to infer physiological likeness from similarity in staining-reaction 3 or in fibrillar structure. For these reasons the hypothesis of " superior protoplasm " seems one of doubtful utility.

1 Cf. the pancreas, p. 44. 2 Cf. p. 29. 8 Cf. p. 335.

In its more restricted form, however, the archoplasm or kinoplasm hypothesis is of high interest as indicating a common element in the origin and function of the mitotic fibrillae, the centrosome and midbody, and the contractile substances of cilia, flagella, and musclefibres. The main interest of the hypothesis seems to me to lie in the definite genetic relations that have been traced between the archoplasmic structures of successive cell-generations (as is most clearly shown in the phenomena of maturation and fertilization). It has been pointed out at various places in the preceding chapters l how many apparently contradictory phenomena in cell-division, fertilization, and related processes can be brought into relation with one another under the assumption of a specific substance, carried by the centrosome or less definitely localized, which gives the stimulus to division, which is concerned in the formation of the mitotic figure and of contractile elements, and which may be transmitted from cell to cell without loss of its specific character. There seems, however, to be clear evidence that such substance (or substances), if it exists, is not to be regarded as being necessarily a permanent constituent of the cell, but only as a phase, more or less persistent, in the general metabolic transformation of the cell-substance. 2

3. The Attraction-sphere

As originally used by Van Beneden 8 the term attraction-sphere was applied (in Ascaris) to the central mass of the aster surrounding the " corpuscule central " and consisting of medullary and cortical zones, as already described (p. 310). The cortical zone is bounded by a distinct circle of microsomes from which the astral rays proceed ; and at the close of cell-division the rays were stated to fade away, leaving only the attraction-sphere, which, like the centrosome, was regarded as a permanent cell-organ. Later researches have conclusively shown that the attraction-sphere cannot be regarded as a permanent organ, since in many cases it disintegrates and disappears. This occurs, for example, in the early prophases of mitosis in the testis-cells of the salamander, 4 where the sphere breaks up and scatters through the cell as the new amphiaster forms (Fig. 27). A very interesting case of this kind occurs in the cleavage of the ovum in Crepidula, as described by Conklin C99). The spheres here persist for a considerable period after division (Fig. 192), but have no direct relation to those of the ensuing division, finally disappearing in situ. The new spheres are formed about the centrosomes, which Conklin believes to migrate out of the old spheres (somewhat as occurs in the spermatid, p. 167) to their new position. The interesting point here is that the old sphere

1 Cf. pp. in, 215. 2 6y.p. 171. 8 '83, p. 548.

4 Drtiner, '95, Rawitz, '96, Meves, '96.

takes up such a position as to pass entirely into one of the granddaughter-cells, while the new sphere-substance is equally distributed between them and in its turn passes into one of the cells of the ensuing division. 1

In Crepidula, as in Ascaris, the attraction-sphere represents only the central part (centrosphere)of the aster. In some cases, however, e.g. in leucocytes, the entire aster may persist, and the term attraction-sphere has by some authors been applied to the whole structure. Later workers have proposed different terminologies, which are at present in a state of complete confusion. Fol ('91 ) proposed to call the centrosome the astrocentre, and the spherical mass surrounding it (attraction-sphere of Van Beneden) the astrosphere. Strasburger accepted the latter term but proposed the new word ccntrosphcre for the astrosphere and the centrosome taken together. 2 A new complication was introduced by Boveri ('95 ), who applied the word "astrosphere " to the entire aster exclusive of the centrosome, in which sense the phrase " astral sphere " had been employed by Mark in 188 1 . The word " astrosphere " has therefore a double meaning and would better be abandoned in favour of Strasburger's convenient term centra sphere, which may be understood as equivalent to the ** astrosphere " of Fol.

Besides these terms we have HeidenhahVs microcentrum (p. 311), equivalent to the centrosome or group of centrosomes at the centre of the aster, with its surrounding sphere : 3 Kostanecki's and Siedlecki's microsphere* applied to the central region of the aster surrounding the centrosome whether bounded by a distinct microsome-circle or not ; 4 Erlanger's centrop/asm, equivalent to microsphere ; 5 Ziegler's cctosphere and entosphere % applied to the cortical and medullar}' zones respectively : and M eves' s idiozome % applied to the attraction-sphere " of the spermatids. 6 This profusion of technical terms has arisen through the desire to avoid ambiguity in the use of the term *• attraction-sphere," which, like the word *" Nebenkern " (p. 163 \, has been applied to bodies of quite different origin and fate. If we adhere to Van Beneden's original use of the term it must be confined to the body surrounding the centrosome, forming a part of. or directly derived from, an aster, and giving rise wholly or in part to the succeeding aster. Meves('96), Rawitz (96X Erlanger ( '07, 21 and others have, however, clearly shown that the "attraction-sphere" surrounding the centrosome (in testis-cells)may not only contain other material derived from the cytoplasm, e.g. the %i centrodeutop'asm " of Erlanger. but may take no direct part in the succeeding aster- formation, disintegrating and scattering through the cell as the new aster forms ( Fig. 2JV In

Cf. p. 424- * '94- P- 403. * 96. 3. p. S. 1 '9i p. 5- « 9k p. 217. * "97. 4- P- 5*5

other cases a sphere closely simulating an attraction-sphere may arise in the cytoplasm without apparent relation to the centrosomes or to the preceding aster, e.g. the yolk-nucleus or the sphere from which the acrosome arises in mammalian spermatogenesis. 1 To call such structures "attraction-spheres" or " archoplasm-masses " is to beg an important question ; and in all such doubtful cases the simple word sphere should be used. 2 In case of the aster itself we may, for descriptive purposes, employ Strasburger's convenient and non-committal term centrosphere, to designate in a somewhat vague and general way the central mass of the aster surrounding the centrosome, leaving its exact relation to Van Beneden's attraction-sphere to be determined in each individual case. Where the centrosphere shows two concentric zones (medullary and cortical), they may be well designated with Ziegler as entosphere (" centrosome " of Boveri) and ectosphere.

As regards the structure of the centrosphere, two well-marked types have been described. In one of these, described by Van Beneden in Ascaris, by Heidenhain in leucocytes, by Druner and Braus in dividing cells of Amphibia, and by Francotte, Van der Stricht, Lillie, Kostanecki, and others, in various segmenting eggs, the centrosphere has a radiate structure, being traversed by rays which stretch between the centrosome and the peripheral microsome-circle (Fig. 152, D, E t F), when the latter exists. In the other form, described by Vejdovsky in the eggs of Rhynchelmis, by Solger and Zimmermann in pigment-cells, by myself in Nereis, by Riickert in Cyclops, by Mead in Chaetoptems, Griffin in Tfialassema, Coe in Cerebratulus, Gardiner in Po/ychcerus, and many others, the centrosphere has a non-radial reticular or vesicular structure, in which the centrosomes lie (Figs. 152, H> 155). Kostanecki and others have endeavoured to show that such structures are artifacts, insisting that in perfectly fixed material the astral rays always traverse the centrosphere to the centrosome. This interpretation is, however, contradicted by the fact that the new asters developing in the centrospheres during the anaphases and telophases of such forms as Thalassema or Cerebratulus (Figs. 99, 155) show perfect fixation of the rays. The reticular centrosphere almost certainly arises as a normal differentiation of the interior of the aster, which, as Griffin ('96) has suggested, probably marks the beginning of the degeneration of the whole astral apparatus, to make way for the newly developing system.

The radial centrosphere is in Ascaris divided into cortical and medullary zones, as already described (p. 310), the aster being bounded by a distinct circle of microsomes. The true interpretation of these zones was given through HeidenhahVs beautiful studies on the asters in leucocytes, and the still more thorough later work of Druner on the sper 1 Cf. p. 170. 2 Cf. Lenhossck, '98.

matocyte-divisions of the salamander. In leucocytes (Fig. 49) the large persistent aster has at its centre a well-marked radial sphere bounded by a circle of microsomes, as described by Van Beneden, but without division into cortical and medullary zones. The astral rays, however, show indications of other circles of microsomes lying outside the centrosphere. Driiner found that a whole series of such concentric circles might exist (in the cell shown in Fig. 1 56 no less than nine), but that the innermost two are often especially distinct, so as to mark off a centrosphere composed of a medullary and a cortical zone precisely as described by Van Beneden. These observations show conclusively that the centrosphere of the radial type is merely the innermost portion of the aster, which acquires a boundary through the especial development of a ring of microsomes, or otherwise, and which often further acquires an intense staining-capacity so as to appear like acentrosome ^tart^uS^'tf d^^^k <P- 3'3)- I" Tkystni0soif*(Vm der

circles of microsome!). The area within the Stricht) Only a Single ring of microsecond drclc probably represents the "altrac- somes exists, and this lies at the Uoii-spherc of Van Eteneden. '

boundary between the medullary and cortical zones (Fig. 152, D), the latter differing from the outer region only in the greater delicacy of the rays and their lack of stain ing-capacity, thus producing a " Heller Hof." In other cases, no " microsome-circles " exist ; but even here a clear zone often surrounds the centrosome (e.g. in P/tysa, t. Kostanecki and Wierzejski), like that seen in the cortical zone of ThysauosoSn.

There are some observations indicating that the entosphere (medullary zone) may be directly derived from the centrosome (central granule). This is the conclusion reached by Lillie in the case of Vnio referred to above, where, during the prophases of the second polar spindle, the central granule enlarges and breaks up into a group of granules from which the new entosphere is formed. Van der Stricht ('98) reaches a similar conclusion in case of the first polar spindle of Tliysanosoon. We may perhaps give the same interpretation to the large pluricorpiisciilar centrum of echinoderms (p. 314). This observation may be used in support of the probability that the astral rays may be actually derived from the centrosome (p. 321) ; but Lillie finds in some cases that in the same mitosis the entosphere is formed by a different process, arising by a differentiation of the cytoplasm around the central granule. The former case, therefore, may be interpreted to mean simply that the centrosome may give rise to other cytoplasmic elements (as has already been shown in the formation of the spermatozoon, p. 172), the material of which may then contribute either directly or indirectly to the building of the aster ; and the facts do not come into collision with the view that the astral rays are in general formed from the cytoplasmic substance.

Pig. Ijjfi. — Spermatogonium der. [DHUNEK.J

The nucleus lies below. Abov

G. Summary and Conclusion

A minute analysis of the various parts of the cell leads to the conclusion that all cell-organs, whether temporary or " permanent," are local differentiations of a common structural basis. Temporary organs, such as cilia or pseudopodia, are formed out of this basis, persist for a time, and finally merge their identity in the common basis again. Permanent organs, such as the nucleus or plastids, are constant areas in the same basis, which never are formed de novo, but arise by the division of preexisting areas of the same kind. These two extremes are, however, connected by various intermediate gradations, examples of which are the contractile vacuoles of Protozoa, which belong to the category of temporary organs, yet in many cases are handed on from one cell to another by fission, and the attraction-spheres and asters, which may either persist from cell to cell or disappear and re-form about the centrosome. There is now considerable evidence that the centrosome itself may in some cases have the character of a permanent organ, in others may disappear and re-form like the asters.

The facts point toward the conclusion, which has been especially urged by De Vries and Wiesner, that the power of division, not only of the cell-organs, but also of the cell as a whole, may have its root in a like power on the part of more elementary masses or units of which the structural basis is itself built, the degree of permanence in the cellorgans depending on the degree of cohesion manifested by these elementary bodies. If such bodies exist, they must, however, in their primary form, lie beyond the present limits of the microscope, the visible structures arising by their enlargement or aggregation. The cell, therefore, cannot be regarded as a colony of "granules or other gross morphological elements. The phenomena of cell-division show, however, that the dividing substance tends to differentiate itself into several orders of visible morphological aggregates, as is most clearly shown in the nuclear substance. Here the highest term is the plurivalent chromosome, the lowest the smallest visible dividing basichromatin-grains, while the intermediate terms are formed by the successive aggregation of these to form the chromatin-granules of which the dividing chromosomes consist. Whether any or all of these bodies are " individuals " is a question of words. The facts point, however, to the conclusion that at the bottom of the series there must be masses that cannot be further split up without loss of their characteristic properties, and which form the elementary morphological units of the nucleus. In case of the cytoplasm the evidence is far less satisfactory. Could Rabl's theory of fibrillar persistence, as developed by Heidenhain and Kostanecki, be established, we should indeed have almost a demonstration of panmeristic division in the cytoplasm. At present, however, the facts do not admit the acceptance of that theory, and the division of the visible cytoplasmic granules must remain a quite open question. Yet we should remember that the dividing plastids of plant-cells are often very minute, and that in the centrosome we have a body, no larger in many cases than a " microsome," which is positively known to be in some cases a persistent morphological element, having the power of growth, division, and persistence in the daughter-cells. Probably these powers of the centrosome would never have been discovered were it not that its staining-capacity renders it conspicuous and its position at the focus of the astral rays isolates it for observation. When we consider the analogy between the centrosome and the basichromatin-grains, when we recall the evidence that the latter graduate into the oxychromatin-granules, and these in turn into the cytomicrosomes, we must admit that Briicke's cautious suggestion that the whole cell might be a congeries of selfpropagating units of a lower order is sufficiently supported by fact to constitute a legitimate working hypothesis.


  • See also Literature, I., II., IV., V.

Van Beneden, E. — (See List IV.)

Van Beneden and Julin. — La segmentation chez les Ascidiens et ses rapports avec

l'organisation de la larve: Arch. Biol., V. 1884. Boveri, Th. — Zellenstudien. (See List IV.)

Briicke, C — Die Elementarorganismen : Wiener Siiz.-fier.. XLIV. 1861. Butschli, 0. — Protoplasma. (See List I.) Delage, Yves. — La structure du protoplasma. et les thdories sur rhe'rddite'. Paris*

^95Hacker, V. — Uber den heutigen Stand der Centrosomenfrage : Vcrh. d. deutsch.

Zool. Ges. 1894. Heidenhain, M. — (See List I.) Herla, V. — Etude des variations de la mitose chez Tascaride megalocephale : Arch.

BioLXllh 1893.

Morgan, T. H. — The Action of Salt-solutions on the Fertilized and Unfertilized

Eggs of Arbacia and Other Animals. Arch. Entw.* VIII. 3. 1898. Kostanecki, K. — Ueber die Bedeutung der Polstrahung wahrend der Mitose. Arch.

mik. Ana/., XLIX. 1897. Nussbaum, M. — Uber die Teilbarkeit der lebendigen Materie : Arch. mik. Anat.,

XXVI. 1886. Prenant, A. — Sur le protoplasma supeVieure (archiplasme. kinoplasme. ergastro plasme) : Journ. Ana/, et Phys., XXIV.-V. 1898-99. (Full Literature-lists.) Rabl, C. — Uber Zellteilung: Morph. Jahrb., X. 1885. Anat. Anzeiger,W. 1889. Ruckert, J. — (See List IV.)

De Vries, H. — Intracellular Pangenesis: Jena % 1889.

Watasl, S. — Homology of the Centrosome : Journ. Morph., VIII. 2. 1893. Id. — On the Nature of Cell-organization : Woods Holl Biol. lectures. 1893. Wiesner, J. — Die Elementarstruktur und das Wachstum der lebenden Substanz :

W'ien, 1892. Wilson, Edm. B. — Archoplasm, Centrosome, and Chromatin in the Sea-urchin Egg :

Journ. Morph., Vol. XI. 1895.

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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.

Cite this page: Hill, M.A. (2020, October 25) Embryology The cell in development and inheritance (1900) 6. Retrieved from

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