Difference between revisions of "The cell in development and inheritance (1900) 5"

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=Chapter V Oogenesis and Spermatogenesis. Reduction of the Chromosomes=
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" ''Es kuromt also in der Generationenreihe der Keimzelle irgendwo zu einer Reduktion der ursprunglich vorhandenen Chromosomenzahl auf die Halfte, and diese ZaMtn-reduktion ist demnach nicht etwa nur ein theoretisches Postulat, sondern eine Thatsache.''"
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Boveri.
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* Zcllensiudien, III., p. 62. * Cf. Figs. 97, 116.
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Van Beneden's epoch-making discovery that the nuclei of the conjugating germ-cells contain each one-half the number of chromosomes characteristic of the body-cells has now been extended to so many plants and animals that it may probably be regarded as a universal law of development. The process by which the reduction in number is effected, forms the most essential part of the phenomena of maturation by which the germ-cells are prepared for their union. No phenomena of cell-life possess a higher theoretical interest than these. For, on the one hand, nowhere in the history of the cell do we find so unmistakable and striking an adaptation of means to ends or one of so marked a prophetic character, since maturation looks not to the present but to the future of the germ-cells. On the other hand, the chromatin-reduction suggests questions relating to the morphological constitution of nucleus and chromatin, which have an important bearing on all theories of the ultimate structure of living matter and now stand in the foreground of scientific discussion among the most debatable and interesting of biological problems.
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Two fundamentally different views have been held of the manner in which the reduction is effected. The earlier and simpler view, which was suggested by Van Beneden and adopted in the earlier works of Weismann, Boveri, and others, assumed an actual degeneration or casting out of half of the chromosomes during the growth of the germ-cells — a simple and easily intelligible process. Later researches conclusively showed, however, that this view cannot be sustained, and that reduction is effected by a rearrangement and redistribution of the nuclear substance without loss of any of its essential constituents. It is true that a large amount of chromatin is lost during the growth of the egg. 2 It is nevertheless certain that this loss is not directly connected with the process of reduction ; for, as Hertwig and others have shown, no such loss occurs during spermatogenesis, and even in the oogenesis the evidence is clear that an explanation must be sought in another direction. The attempts to find such an explanation have led to some of the most interesting researches of modern cytology ; and though only partially successful, they have raised many new questions which promise to give in the end a deeper insight into some of the fundamental questions of cell-morphology. For this reason they deserve careful consideration, despite the fact that taken as a whole the subject still remains an unsolved riddle in the face of which we can only return again and again to Boveri's remark that whatever be its theoretical interpretation the numerical reduction of the chromosomes is itself not a theory but a fact.
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Pig. 114. — Formation of the polar bodies before entrance of the spermatozoon, as seen in the living ovarian egg of the sea-urchin, Toxopneustes (X 365).
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A. Preliminary change of form in the germinal vesicle. B. The first polar body formed, the second forming. C. The ripe egg, ready for fertilization, after formation of the two polar bodies (p. b. 1, 2) ; e. the egg-nucleus. In this animal the first polar body fails to divide. For its division see Fig. 89.
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==A. General Outline==
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The general phenomena of maturation fall under two heads : viz. oogenesis, which includes the formation and maturation of the ovum, and spermatogenesis, comprising the corresponding phenomena in case of the spermatozoon. Recent research has shown that maturation conforms to the same type in both sexes, which show as close a parallel in this regard as in the later history of the germ-nuclei. Stated in the most general terms, this parallel is as follows : l In both sexes the final reduction in the number of chromosomes is effected in the course of the last two cell-divisions, or maturation-divisions, by which the definitive germ-cells arise, each of the four cells thus formed having but half the usual number of chromosomes. In the female but one of the four cells forms the " ovum " proper, while the other three, known as the polar bodies, are minute, rudimentary, and incapable of development (Figs. 89, 97, 114). In the male, on the other hand, all four of the cells become functional spermatozoa. This difference between the two sexes is probably due to the physiological division of labour between the germ-cells, the spermatozoa being motile and very small, while the egg contains a large amount of protoplasm and yolk, out of which the main mass of the embryonic body is formed. In the male, therefore, all of the four cells may become functional ; in the female the functions of development have become restricted to but one
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1 The parallel was first clearly pointed out by Platner in 1889, and was brilliantly demonstrated by Oscar Hertwig in the following year.
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Fig- 115. — Diagram showing the genesis of the egg. [After BOVERI.]
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of the four, while the others have become rudimentary (cf. p. 124). The polar bodies are therefore not only rudimentary cells (Giard, '76), but may further be regarded as abortive eggs — a view first put forward by Mark in 1881, and ultimately adopted by nearly all investigators. 1 The evidence is steadily accumulating that reduction is accomplished by two maturation-divisions throughout the animal kingdom, even in the unicellular forms ; though in certain Infusoria an additional division occurs, while in some other Protozoa only one maturation-division has thus far been made out. Among plants, also, two maturation
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1 A beautiful confirmation of this view is given by Francottes's ('97) observations on a turbellarian, Prostheceraus, The first polar body is here often abnormally large, all gradations having been observed from the normal size up to cells nearly as large as the egg itself. Such polar bodies are occasionally fertilized and develop into small gastrulas, first forming a single polar body like the second polar body of the egg. Here, therefore, two of the four cells are exceptionally capable of development. It may be added that Fol long ago observed the penetration of the small polar bodies by spermatozoa in the echinoderms; and this has been more recently observed by Kostanecki in mollusks.
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236 REDUCTION OF THE CHROMOSOMES
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divisions occur in all the higher forms (Muscineae, pteridophytes, and phanerogams), and in some, at least, of the lower ones. Here, however, the phenomena are complicated by the fact that the two divisions do not as a rule give rise directly to the four sexual germ-cells, but to four asexual spores which undergo additional divisions before the definitive germ-cells are produced. In the flowering plants there are only a few such divisions, which give rise to structures within the pollen-tube or embryo-sac. In the archegoniate cryptogams, on the other hand, each spore gives rise, by repeated divisions, to a "sexual generation " (prothallium, etc.) that intervenes between the process of reduction and that of fertilization. The following account deals primarily with reduction in animals, the plants being afterward considered.
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I. Reduction in the Female. Formation of tfie Polar Bodies
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As described in Chapter III., the egg arises by the division of cells descended from the primordial egg-cells of the maternal organism, and these may be differentiated from the somatic cells at a very early period, sometimes even in the cleavage-stages. As development proceeds, each primordial cell gives rise, by division of the usual mitotic type, to a number of descendants known as oogonia (Fig. 115), which are the immediate predecessors of the ovarian egg. At a certain period these cease to divide. Each of them then grows to form an ovarian egg, its nucleus enlarging to form the germinal vesicle, its cytoplasm becoming more or less laden with food-matters (yolk or deutoplasm), while egg-membranes may be formed around it. The ovum may now be termed the oocyte (Boveri) or ovarian egg.
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In this condition the egg-cell remains until near the time of fertilization, when the process of maturation proper — i.e. the formation of the polar bodies — takes place. In some cases, e.g. in the sea-urchin, the polar bodies are formed before fertilization, while the Ggg is still in the ovary. More commonly, as in annelids, gasteropods, nematodes, they are not formed until after the spermatozoon has made its entrance ; while in a few cases one polar body may be formed before fertilization and one afterward, as in the lamprey-eel, the frog, and Amphioxus} In all these cases the essential phenomena are the same. Two minute cells are formed, one after the other, near the upper or animal pole of the ovum (Figs. 97, 1 16); and in many cases the first of these divides into two as the second is formed (Fig. 89).
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A group of four cells thus arises, namely, the mature egg, which gives rise to the embryo, and three small cells or polar bodies which take no part in the further development, are discarded, and soon die
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1 Cf. p. 189.
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GENERAL OUTLINE
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237
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without further change. The egg-nucleus is now ready for union with the sperm-nucleus.
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, ' r-: \
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D
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E H
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Fig. xx6. — Diagrams showing the essential facts in the maturation of the egg. The somatic number of chromosomes is supposed to be four.
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A. Initial phase; two tetrads have been formed in the germinal vesicle. B. The two tetrads have been drawn up about the spindle to form the equatorial plate of the first polar mitotic figure. C. The mitotic figure has rotated into position, leaving the remains of the germinal vesicle at g.v. D. Formation of the first polar body; each tetrad divides into two dyads. E. First polar body formed; two dyads in it and in the egg. F. Preparation for the second division. G. Second polar body forming and the first dividing; each dyad divides into two single chromosomes. H. Final result; three polar bodies and the egg-nucleus (9), each containing two single chromosomes (half the somatic number) ; c. the egg-centrosome which now degenerates and is lost
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238 REDUCTION OF THE CHROMOSOMES
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A study of the nucleus during these changes brings out the following facts. During the multiplication of the oogonia the number of chromosomes is the same as that occurring in the division of the somatic cells, and the same number enters into the formation of the chromatic reticulum of the germinal vesicle. During the formation of the polar bodies this number becomes reduced to one-half, the nucleus of each polar body and the egg-nucleus receiving the reduced number. In some manner, therefore, the formation of the polar bodies is connected with the process by which the reduction is effected. The precise nature of this process is, however, a matter which has been certainly determined in only a few cases.
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We need not here consider the history of opinion on this subject further than to point out that the early observers, such as Purkinje, Von Baer, Bischoff, had no real understanding of the process and believed the germinal vesicle to disappear at the time of fertilization. To Butschli ('76), Hertwig, and Giard ('76, '77) we owe the discovery that the formation of the polar bodies is through mitotic division, the chromosomes of the equatorial plate being derived from the chromatin of the germinal vesicle. 1 In the formation of the first polar body the group of chromosomes splits into two daughter-groups, and this process is immediately repeated in the formation of the second without an intervening reticular resting stage. The egg-nucleus therefore receives, like each of the polar bodies, one-fourth of the mass of chromatin derived from the germinal vesicle.
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But although the formation of the polar bodies was thus shown to be a process of true cell-division, the history of the chromosomes was found to differ in some very important particulars from that of the tissue-cells. The essential facts, which were first carefully studied in Ascaris by Van Beneden ('83, '87), and especially by Boveri ('87, 1 ), are in a typical case as follows (Figs. 116, 117): As the egg prepares for the formation of the first polar body, the chromatin of the germinal vesicle groups itself in a number of masses, each of which splits up into a group of four bodies united by linin-threads to form a " quadruple group" or tetrad (Vierergruppe). The number of tetrads is always one-half the usual number of chromosomes. Thus in Ascaris {mcgaloccphala, bivalcns) the germinal vesicle gives rise to two tetrads, the normal number of chromosomes in the earlier divisions being four ; in the mole-cricket there are six tetrads, the somatic number of chromosomes being twelve ; in Cyclops the respective numbers are twelve and twenty-four (one of the most frequent cases); while in Artemia there are eighty-four tetrads and one hundred and sixty 1 The early accounts asserting the disappearance of the germinal vesicle were based on the fact that in many cases only a small fraction of the chromatic network gives rise to chromosomes, the remainder disintegrating and being scattered through the yolk.
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GENERAL OUTLINE
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eight somatic chromosomes — the highest number thus far accurately counted. As the first polar body forms, each of the tetrads is halved to form two double groups, or dyads, one of which remains in the egg
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Fig. 117 A. The egg with the spermaloioiin just e shaped tetrads (only one clearly shown), the been (bur. B. The tetrads seen in profile.
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C. First polar body formed, contain) single chromosomes, completing the 1
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dyads. H. I. The dyads rotatii lg. A'. Each dyad has divided ir er stages see Fig. 90.)
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240
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REDUCTION OF THE CHROMOSOMES
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while the other passes into the polar body. Both the egg and the first polar body therefore receive each a number of dyads equal to one-half the usual number of chromosomes. The egg now proceeds at once to the formation of the second polar body without previous reconstruction of the nucleus. Each dyad is halved to form two single chromosomes, one of which, again, remains in the egg while its sister passes into the polar body. Both the egg and the second polar body accordingly receive two single chromosomes (one-half the usual number), each of which is one-fourth of an original tetrad group. From the two remaining in the egg a reticular nucleus, much smaller than the original germinal vesicle, is now formed. 1
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Primordial germ-cell.
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Spermatogonia.
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Primary spermatocyte.
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Secondary spermatocytes.
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Spermatids. Spermatozoa.
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Division-period (the number of divisions is much greater).
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• Growth-period.
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M aturation-period.
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Fig. zz8. — Diagram showing the genesis of the spermatozoon. [After Boveri.]
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Essentially similar facts have now been determined in a considerable number of animals, though, as we shall presently see, tetradformation is not of universal occurrence, nor is it always of the same type. For the moment we need only point out that the numerical reduction of chromatm-masses takes place before the polar bodies are actually formed, through processes which determine the number of tetrads within the germinal vesicle. The numerical reduction is therefore determined in the grandmother-cell of the egg. The actual divisions by which the polar bodies are formed merely distribute the elements of the tetrads.
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1 It is nearly certain that the division of the first polar body (which, however, may be omitted) is analogous to that by which the second is formed, i.e. each of the dyads is similarly halved. Cf. Griffin, '99.
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GENERAL OUTLINE 24 1
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2. Reduction in the Male. Spermatogettesis
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The researches of Platner ('89), Boveri, and especially of Oscar Hertwig ('90, 1) have demonstrated that reduction takes place in the male in a manner almost precisely parallel to that occurring in the female. Platner first suggested ('89) that the formation of the polar bodies is directly comparable to the last two divisions of the sperm mother-cells (spermatocytes). In the following year Boveri reached the same result in Ascaris, stating his conclusion that reduction in the male must take place in the " grandmother-cell of the spermatozoon, just as in the female it takes place in the grandmother-cell of the egg," and that the egg-formation and sperm-formation really agree down to the smallest detail ('90, p. 64). Later in the same year appeared Oscar Hcrtwig's splendid work on the spermatogenesis of Ascaris, which established this conclusion in the most striking manner. Like the ova, the spermatozoa are descended from primordial germ-cells which by mitotic division give rise to the spermatogonia from which the spermatozoa are ultimately formed (Fig. 118). Like the oogonia, the spermatogonia continue for a time to divide with the usual (somatic) number of chromosomes, i.e. four in Ascaris mcgalocephala bivalens. Ceasing for a time to divide, they now enlarge considerably to form spermatocytes, each of which is morphologically equivalent to an unripe ovarian ovum, or oocyte. Each spermatocyte finally divides twice in rapid succession, giving rise first to two daughter-spermatocytes and then to four spermatids, each of which is directly converted into a single spermatozoon. The history of the chromatin in these tzvo divisions is exactly parallel to that in the formation of the polar bodies (Figs. 119, 120). From the chromatin of the spermatocyte are formed a number of tetrads equal to one-half the usual number of chromosomes. Each tetrad is halved at the first division to form two dyads which pass into the respective daughter-spermatocytes. At the ensuing division, which occurs without the previous formation of a resting reticular nucleus, each dyad is halved to form two single chromosomes which enter the respective spermatids (ultimately spermatozoa). From each spermatocyte, therefore, arise four spermatozoa, and each sperm-nucleus receives half the usual number of single chromosomes. The parallel with the egg-reduction is complete.
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These facts leave no doubt that the spermatocyte is the morphological equivalent of the oocyte or immature ovarian egg, and that the group of four spermatozoa to which it gives rise is equivalent to the ripe egg plus the three polar bodies. Hertwig was thus led to the following beautifully clear and simple conclusion : " The polar bodies are abortive eggs which are formed by a final process of
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242
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REDUCTION OF THE CHROMOSOMES
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division from the egg-mother-cell (oocyte) in the same manner as the spermatozoa are formed from the sperm-mother-cell (spermatocyte). But while in the latter case the products of the division are all used as functional spermatozoa, in the former case one of the products
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Pig. 119. — Diagrams showing the essential facts of reduction in the male. The somatic number of chromosomes is supposed to be four.
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A. Ii. Division of one of the spermatogonia, showing the full number (four) of chromosomes. C. Primary spermatocyte preparing for division ; the chromatin forms two tetrads. D. E. F. First division to form two secondary spermatocytes each of which receives two dyads. G. H. Division of the two secondary spermatocytes to form four spermatids. Each of the latter receives two single chromosomes and a centrosome which passes into the middle-piece of the spermatozoon.
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1
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of the egg-mother-cell tecomes the egg, appropriating to itself the entire mass of the yolk at the cost of the others which persist in rudimentary form as the polar bodies." l
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1 *9o> i> P* I2 & '
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GENERAL OUTLINE 243
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3. Weismanris Interpretation of Reduction
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Up to this point the facts are clear and intelligible. Before coming to closer quarters with them it will be useful to make a digression in order to consider some of the theoretical aspects of reduction ; though the reader must be warned that this will lead us into very uncertain ground traversed by a labyrinth of conflicting hypotheses from which no exit has yet been discovered.
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The process of reduction is very obviously a provision to hold constant the number of chromosomes characteristic of the species ; for if it did not occur, the number would be doubled in each succeeding generation through union of the germ-cells. 1 A number of writers have contented themselves with this simple interpretation, Oscar Hertwig, for example, regarding reduction as " merely a process to prevent a summation through fertilization of the nuclear mass and of the chromatic elements." 2 A moment's reflection reveals the entire inadequacy of such an explanation. As far as the chromatin-mass is concerned, it does not agree with the facts ; for in reduction with tetrad-formation the chromatin-mass is reduced not to one-half, but to one-fourth. That reduction must mean more than mere mass-reduction is moreover proved by the fact that the bulk of the nucleus may enormously increase or decrease at different periods in the same cell, irrespective of the number of chromosomes. The real problem is why the number of chromosomes should be held constant. The
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1 Of the many earlier attempts to interpret the meaning of the polar bodies, we need only consider at this point the very interesting suggestion of Minot ('77), afterward adopted by Van Beneden ('83), that the ordinary cell is hermaphrodite, and that maturation is for the purpose of producing a unisexual germ-cell by dividing the mother-cell into its sexual constituents, or "genoblasts." Thus, the male element is removed from the egg in the polar bodies, leaving the mature egg a female. In like manner he believed the female element to be cast out during spermatogenesis (in the " Sertoli cells "), thus rendering the spermatozoa male. By the union of the germ-cells in fertilization, the male and female elements are brought together so that the fertilized egg or oosperm is again hermaphrodite or neuter. This ingenious view was independently advocated by Van Beneden in his great work on Ascaris ('83). A fatal objection to it, on which both Strasburger and Weismann have insisted, lies in the fact that male as well as female qualities are transmitted by the egg-cell, while the sperm-cell also transmits female qualities. The germ-cells are therefore non-sexual. The researches of many observers show, moreover, that all of the four spermatids derived from a spermatocyte become functional spermatozoa. Minot's hypothesis must, therefore, in my opinion, be abandoned.
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Balfour doubtless approximated more nearly to the truth when he said, " In the formation of the polar cells part of the constituents of the germinal vesicle, which are requisite for its functions as a complete and independent nucleus, is removed to make room for the supply of the necessary parts to it again by the spermatic nucleus" ('80, p. 62). He fell, however, into the same error as Minot and Van Beneden in characterizing the germ-nuclei as " male " and "female"; and, as shown at pages 194, 353, it has been found that a single germnucleus is able to carry out development of an embryo without union with another.
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2 '90, I, p. 1 12. Cf. Hartog, '91, p. 57.
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„, REDUCTION OF THE CHROMOSOMES
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o. k ,. :icaiiin^ of the phenomena was first seriously considered by »c-«a.i.i;i in his essays of 1885 and 1887 ; and, although his conclun-'o* *«•« 01 j. highly speculative character, they nevertheless gave so
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ffg. i»o. — Reduction in the spermatogenesis of Aicaril mtgafocephiila.vai.6ii aitn
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A-tl. Successive stages in the division of the primary sperm atocyte. The onxin.n: telicul umkrxocs a very early division of the chromatin -pa nulrs which then form a doubly split spim HuviUl, /'. This shortens ((.'),. ind breaks in two to form (he two tetrads (D in profile. £ viev endwise). F. G. H. First division to form two secondary spermatocytes, each rrcciring t«o dya /. Secondary spermatocyte. J, K. The same dividing. L. Two resulting spermatids, each «
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great a stimulus to the study of the entire problem that his views deserve special attention. Weismann's interpretation was based on a remarkable paper published by Wilhelm Roux in 1883,' in which are
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1 For division of the spermatogonia see Fig. 55; for the corresponding phf univalent see Fig. 148.
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Ubtr Jit BiJeutung Jer Kirnthtilungsfigurin.
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GENERAL OUTLINE 245
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developed certain ideas which afterward formed the foundation of Weismann's whole theory of inheritance and development. Roux argued that the facts of mitosis are only explicable under the assumption that chromatin is not a uniform and homogeneous substance, but differs qualitatively in differerit regions of the nucleus ; that the collection of the chromatin into a thread and its accurate division into two halves is meaningless unless the chromatin in different regions of the thread represents different qualities which are to be divided and distributed to the daughter-cells according to some definite law. He urged that if the chromatin were qualitatively the same throughout the nucleus, direct division would be as efficacious as indirect, and the complicated apparatus of mitosis would be superfluous. Roux and Weismann, each in his own way, subsequently elaborated this conception to a complete theory of inheritance and development, but at this point we may confine our attention to the views of Weismann. The starting-point of his theory is the hypothesis of De Vries that the chromatin is a congeries or colony of invisible self-propagating vital units or biophorcs somewhat like Darwin's "gemmules" (p. 12), each of which has the power of determining the development of a particular quality. Weismann conceives these units as aggregated to form units of a higher order known as " determinants," which in turn are grouped to form " ids," each of which, for reasons that need not here be specified, 1 is assumed to possess the complete architecture of the germ-plasm characteristic of the species. The "ids" finally, which are identified with the visible chromatin-granules, are arranged in linear series to form " idants " or chromosomes. It is assumed further that the " ids " differ slightly in a manner corresponding with the individual variations of the species, each chromosome therefore being a particular group of slightly different germ-plasms and differing qualitatively from all the others.
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We come now to the essence of Weismann's interpretation. The end of fertilization is to produce new combinations of variations by the mixture of different ids. Since, however, their number, like that of the chromosomes which they form, is doubled by the union of two germ-nuclei, an infinite complexity of the chromatin would soon arise did not a periodic reduction occur. Assuming, then, that the " ancestral germ-plasms" (ids) are arranged in a linear series in the spiremethread or the chromosomes derived from it, Weismann ventured the prediction ('87) that two kinds of mitosis would be found to occur. The first of these is characterized by a longitudinal splitting of the thread, as in ordinary cell-division, " by means of which all the ancestral germ-plasms are equally distributed in each of the daughter-nuclei after having been divided into halves." This form of division, which
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1 Cf. the Germ-plasm, p. 60.
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246 REDUCTION OF THE CHROMOSOMES
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he called equal division (Aequationstheilung), was then a known fact. The second form, at that time a purely theoretical postulate, he assumed to be of such a character that each daughter-nucleus should receive only half the number of ancestral germ-plasms possessed by the mother-nucleus. This he termed a reducing division (Reduktionstheilung), and suggested that this might be effected either by a transverse division of the chromosomes, or by the elimination of entire chromosomes without division. 1 By either method the number of " ids M would be reduced ; and Weismann argued that such reducing divisions must be involved in the formation of the polar bodies, and in the parallel phenomena of spermatogenesis.
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The fulfilment of Weismann's prediction is one of the most interesting results of recent cytological research. It has been demonstrated, in a manner which seems to be incontrovertible, that the reducing divisions postulated by Weismann actually occur, though not precisely in the manner conceived by him. Unfortunately for the general theory, however, transverse divisions have been certainly determined in only a few types, while in others, of which Ascaris is the best-known example, the facts thus far known seem clearly opposed to the assumption. On the whole, the evidence of reducing divisions, i.e. such as involve a transverse and not a longitudinal division of the chromatin-thread, has steadily increased ; but it remains quite an open question whether they have the significance attributed to them by Weismann.
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B. Origin of the Tetrads
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I. General Sketch
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In considering the origin of the tetrads or their equivalents, it should be borne in mind that true tetrad-formation, as described above, has only been certainly observed in a few groups (most clearly in the nematodes and arthropods). But even in cases where the chromatin does not condense into actual tetrads these bodies are represented by chromosomes in the form of rings, crosses, and the like, which are closely similar, and doubtless equivalent, to those from which actual tetrads arise, and present us with the same problems. With a few apparent exceptions, described hereafter, the tetrads of their equivalents always arise by a double division of a single primary chromatin-rod or mass. Nearly all observers agree further that the number of primary rods at their first appearance in the germinal vesicle or in the spermatocyte-nucleus is one-half the usual number of chromosomes, and that this numerical reduction is due to the fact that the spireme-thread segments into one-half the
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1 Essay VI., p. 375.
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ORIGIN OF THE TETRADS
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247
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/
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5 6
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a
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usual number of pieces. Apparently, however, there are two radically different types of tetrad-formation as follows.
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In the first type the tetrad arises by one longitudinal and one transverse division of each primary ckromatin-rod, the latter effecting the reduction demanded by _
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Weismann , shypothesis(Fig. /\ £j Ls U
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I2i,l). To give the usual graphic representation, let us, for the sake of discussion, assume the somatic number of chromosomes to be four, designating the spireme-thread as a b c d, each letter representing a chromosome, each of which we may in turn assume to consist of a series of four granulesor "ids "(Fig. 121). In ordinary mitosis the spireme would segment into a — b — c — d t which then would divide lengthwise to form pairs of identical sister
 +
 +
, abed chromosomes — •
 +
 +
abed To form the tetrad, on the other hand, the spireme first segments into two rods ab and cd, each of which, in view of its subsequent history, may be regarded as
 +
 +
bivalent, representing tWO Pig. 121. — Diagrams of tetrad-formation; I, with
 +
 +
chromosomes United end to one transverse an d one longitudinal division (copepod
 +
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, __ .. type); II. with two longitudinal divisions (Ascaris type).
 +
 +
end (VOm Rath, RUckert, A _ D succes sive stages; chromatin-granules num Hacker). Eagh of these beredfrom 1 to 8. The two types diverge at C. In D
 +
 +
divides once longitudinally, giving the identical pairs or
 +
 +
dyads — , and once transversely, giving the tetrads
 +
 +
ab cd a
 +
 +
 +
 +
U
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 +
 +
 +
8
 +
 +
 +
 +
 +
ab ab
 +
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ab ab
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 +
 +
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the granules of each constituent of the tetrad fuse to form a homogeneous sphere.
 +
 +
 +
 +
a
 +
 +
 +
 +
b b
 +
 +
 +
 +
d m d
 +
 +
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Inspection of Fig. 121, 1, shows that through the second or transverse division, each member of the tetrad receives only half the number of ids contained in the original segment. This number, four, is the same as that assumed for a single chromosome ; and, since each of the two tetrads contributes one chromosome to the germ-cell, the latter receives
 +
 +
 +
 +
248 REDUCTION OF THE CHROMOSOMES
 +
 +
but half the usual number both of chromosomes and of ids. This mode of tetrad-formation has been most clearly demonstrated in insects and copepods, and an equivalent process occurs also in mollusks, annelids, turbellarians, and some other animals, as described beyond. In the second type, illustrated especially by Ascaris, the tetfad is apparently formed by two longitudinal divisions of each primary chromatin-rod, and no reducing division occurs. If, therefore, we adopt the same terminology as before, we have first ab and cd, then
 +
 +
ab cd
 +
 +
ab cd
 +
 +
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ab cd j r n ab — , and finally
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cd
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— , by two longitudinal divisions. In
 +
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cd
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 +
 +
 +
ab cd ab
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 +
this case, according to Brauer's careful studies, each chromatin-granule (" id") divides at each longitudinal division of the primary rod. The four chromosomes of the tetrad are therefore exactly equivalent, being derived from the same region of the spireme-thread, and containing the undiminished number of " ids " (Fig. 121, II).
 +
 +
The contradiction may be stated in a different way. In the first type of tetrad formation, the number both of granules and of chromosomes is first doubled (i.e. in the assumed case, through the formation of two tetrads, each consisting of four chromosomes, or eight in all), and then reduced to half that number by the two successive maturation-divisions. In the second type, on the other hand, the number of chromosomes is likewise doubled, but that of the granules is quadrupled, so that, although in both types the two maturation-divisions reduce the number of chromosomes to one-half, only in the first type do they reduce the number of granules or " ids," as Weismann's hypothesis demands. We must therefore distinguish sharply between the reduction of the chromosomes and that of the "ids." The former is primarily effected by the segmentation of the primary spiremethread, or the resolution of the nuclear reticulum, into one-half the usual number of segments (i.e. the "pseudo-reduction" of Riickert); and here the real secret of the reduction of the chromosomes lies. The reduction of the "ids," if they have any real existence, is a distinct, and as yet unsolved, question.
 +
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2. Detailed Evidence
 +
 +
We may now consider some of the phenomena in detail, though the limits of this work will only allow the consideration of a few typical cases.
 +
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(a) Tetradfonnation with one Longitudinal and one Transverse Division. — In many of the cases of this type the tetrads arise from ring-shaped bodies which are analogous to the ring-shaped chromosomes occurring in heterotypical mitosis (p. 86). First observed by HenkingCgi) in Pyrrhocoris, tetrad-origin of this type has since been found in other insects by Vom Rath, Toyama, Paulmier, and others,
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ORIGIN OF THE TETRADS
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249
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in copepods by Riickert, Hacker, and Vom Rath, in pteridophytes by Calkins and Osterhout, in the onion, Allium, by Ishikawa, and in various other forms where their history has been less clearly made out. The genesis of the ring was first determined by Vom Rath in the mole cricket {Gryllotalpa, '92), and has been thoroughly elucidated by the later work of Riickert ('94), Hacker ('95, 1), and Paulmier ('99). All these observers have reached the same conclusion ; namely, that the ring arises by the longitudinal splitting of a primary chro matin-rod, the two halves remaining united by their ends, and opening out to form a ring. The ring-formation is, in fact, a form of
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ontogenesis of the mole-cricket
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Pig. 132. — Origin of the tetrads by ting- formation Gryllotalpa. [VOM RATH.]
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somes united end to end anrl longitudinally split cxct-pl at Ihc free ends. B. C. Opening out of (he double rods to form rings. I), Concentration of the rings. E. The rings broken up into letrads. /'. First division -figure established.
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heterotypical mitosis (p. 86). The breaking of the ring into four parts involves, first, the separation of these two halves (corresponding with the original longitudinal split), and second, the transverse division of each half, the latter being the reducing division of Weismann. The number of primary rods, from which the rings arise, is one-half the somatic number. Hence each of them is conceived by Vom Rath, Hacker, and Riickert as bivalent or double ; i.e. as representing two chromosomes united end to end. This appears with the greatest clearness in the spermatogenesis of Gryllotalpa (Fig. 122). Here the spireme-thread splits lengthwise before its segmentation into rods. It then divides transversely to form six double rods (half the usual number of chromosomes), which open out to form six closed rings. These become small and thick, break each into four parts, and thus
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Fif. 113.— Formation of the tetrads and polar bodies in Cyclops, slightly si full number of tetrads is no! shown.) [Rt'c:KKKT.]
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A. Germinal vesicle containing eight longitudinally split chromatin-rods (half the number). II. Shortening of the rods; transverse division (to furm the tetrads) in C. Position of the tetrads in the first polar spindle, the longitudinal split horizontal, phase: longitudinal divisions of the tetrads. E. The first polar liody formed; seco spindle with the eight dyads in position fur the ensuing division, which will be a trait
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9. Ana
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give rise to six typical tetrads. An essentially similar account of the ring-formation is given by Vom Rath in Euchceta and Calanus, and by Riickert in Hetcrocopc and Diaptomus.
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That the foregoing interpretation of the rings is correct, is beautifully demonstrated by the observations of Hacker, and especially of
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Ruckert, on a number of other copepods {Cyclops, Canthocamptus\ in which rings are not formed, since the splitting of the primary chromatin-rods is complete. The origin of the tetrads has here been traced with especial care in Cyclops strcnuus y by Ruckert C94), whose observations, confirmed by Hacker, are quite as convincing as those
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Pig. 134. — Diagrams of various modes of tetrad-formation. [HACKER.]
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a. Common starting-point, a double spireme-thread in the germinal vesicle ; d. common result, the typical tetrads ; b. c. intermediate stages : at the left the ring-formation (as in Diaptomus, Gryllotalpa, Heterocopt) ; middle series, complete splitting of the rods (as in Cyclops according to Ruckert, and in Canthocamptus) ; at the right by breaking of the V-shaped rods (as in Cyclops strenuus, according to Hacker.
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of Brauer on Ascaris, though they led to a diametrically opposite result.
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The normal number of chromosomes is here twenty-two. In the germinal vesicle arise eleven threads, which split lengthwise (Fig. 123), and finally shorten to form double rods, manifestly equivalent to the closed rings of Diaptomus. Each of these now segments transversely to form a tetrad group, and the eleven tetrads then place themselves in the equator of the spindle for the first polar body (Fig. 123, C), in such a manner that the longitudinal split is transverse to the axis of the spindle. As the polar body is formed, the longitudinal halves of the tetrad separate, and the formation of the first polar body is thus demonstrated to be an "equal division" in Weismann's sense. The eleven dyads remaining in the eggs now rotate (as in Ascaris),
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B. Later stage of the same, condensation and segmentation of the rings. [ROcKERT.]
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C. " fyefops tlnauus," illustrating Hacker's account of the tetrad-formation from elongate double rods; a group of " accessory nucleoli." [H ACKER.]
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/). Germinal vesicle of an annelid (Ophryotrocka} showing nucleolus and four chromosomes. [K.ORSCHKLT.]
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 +
so that the transverse division lies in the equatorial plane, and are halved during the formation of the second polar body. The division is accordingly a " reducing division," which leaves eleven single chromosomes in the egg. Paulmier's work on Anasa and other Hemiptera ('99) gives the same result as the above in regard to the origin of the tetrads (Figs. 126, 127). The process is, however, slightly complicated by the fact that no continuous spi rem e-th read is formed, while the rings are often bent or twisted and never open out to a circular form. They finally condense into true tetrads which are successively divided into dyads and monads by the two divisions; but it is an interesting fact that the order of division occurring in the copepods appears here to be reversed, the first division being the transverse and the second the longitudinal one — a result agreeing with Henking's earlier conclusion in the case of Pyrrochoris. Osterhout C97) and Calkins C97) independently discovered tetrads in the vascular cryptogams (Equisctutn, Pteris), and the last-named observer finds that in Pteris they may arise either from rings, as in Gryllotalpa or Heterocope, or from double rods as in Cyclops, the halves in the latter case being either parallel or forming a cross. This longitudinal split, occurring in the spireme, is followed by a transverse division by which the tetrad is formed. Tetrads having an essentially similar mode of origin are also described by Atkinson ('99) in Ariscema, and tetrad-formation is nearly approached in Allium according to Ishikawa C99). 1 These cases are considered at page 263.
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 +
Re'sume'. In all the foregoing cases the tetrads arise from a spireme which splits lengthwise, segments into one-half the somatic number of rods (each longitudinally divided) and each of the latter divides transversely to form the tetrad. When the ends of the daughter-chromosomes resulting from the longitudinal split remain united (as in insects) ring-forms result, and the earlier phases of tetradformation are thus identical with those of heterotypical mitosis. When the split is complete, so that the ends remain free, double rods result; while, if the daughter-chromosomes remain temporarily united at the middle or at the end, X-, Y-, and V-shaped figures may arise. In all these forms tetrad-formation is completed by the complete separation of the daughter-rods, the transverse division of each in the middle, and the condensation of the four resulting bodies into a quadruple mass. As will be shown in Section C (p. 258) the transverse division is in many forms delayed until after separation of the longitudinal halves. In such cases no actual tetrads are formed, though the result is the same.
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(b) Second Type. Tetrad-formation zuit/t two Longitudinal Divisions. — The only accurately known case of this type is Ascaris y the object in which tetrads were first discovered by Van Beneden in 1883. Carnoy ('86, 2) reached the conclusion that the tetrads in some other nematodes (Op//iostomum t Ascaris clavata, A. lumbricoides) arose by a double longitudinal splitting of the primary chromatin-rods.
 +
 +
1 Vom Rath ('93, '59) has endeavoured to show that a process involving the formation of true tetrads occurs in the salamander and the frog, but the later and more accurate studies of Meves ('96) seem to leave little doubt that this was an error, and that the tetrads observed in these forms are not of normal occurrence, as Hemming ('87) had earlier concluded. €/. p. 259.
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In the first of his classical celt-studies Boveri ('87, 1) reached the same result through a careful study of Ascaris megalocephala, showing that each tetrad appears in the germinal vesicle in the form of four parallel rods, each consisting of a row of chrom at in -granules (Fig. 117, A-C). He believed these rods to arise by the double longitudinal splitting of a single primary chromatin- rod, each cleavage being a
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A. Resting spermatogonium with single plasmosome ; irial plate of dividing s]>ermatogonium : twenty large a [jcrmatogonium- division. /'-/. Prophases of first malt; ingle chromalin-ilucleolus. /'. Segmented split spireme. /. /. Concentration of the rings to form tetrads.
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nam. [PAULMIER.]
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1 two chromatin-nucleoli. B. Equatwo small chromosomes. C. Final lion-division. D. £. Synapsis, with C. H. Formation of the tetrad-rings.
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preparation for one of the polar bodies. In his opinion, therefore, the formation of the polar bodies differs from ordinary mitosis only in the fact that the chromosomes split very early, and not once, but twice, in preparation for two rapidly succeeding divisions without an intervening resting period. He supported this view by further observations in 1890 on the polar bodies of Sagitta and several gasteropods, in which he again determined, as he believed, that the tetrads arose by double longitudinal splitting. An essentially similar view of the tetrads was taken by Hertwig in 1890, in the spermatogenesis of Ascaris, though he couid not support this conclusion by very convincing evidence. In 1893, finally, Brauer made a most thorough and apparently exhaustive study of their origin in the spermatogenesis of Ascaris, which seemed to leave no doubt of the correctness of Boveri's result. Every step in the origin of the tetrads from the reticulum of the resting spermatocytes was traced with the most painstaking care. In the early prophases of the first division the nuclear reticulum breaks up more or Jess completely into granules, which
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Fig. 137. — Maturat ion-divisions in an insect, Anaia. [Paui.mier.] A. Primary spermatocyte in metaphase. B. Equatorial plate, showing ten large tetrads and one small one ; " odd chromosome " al o, C. Separation of the dyads. D. Telophase, which is also a prophase of the second division. E. Secondary spermatocyte; division of the dyads; small dyad shown undivided. F. Final anaphase ; small dyad near the lower chromosome-group. - (The figures are numbered from left to right. For later stales, see Fig. 8a.)
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become in part aggregated in a mass at one side of the nucleus {"synapsis," p. 276), from which delicate threads extend through the remaining nuclear space (Fig. 120, A). Even at this period the granules of the threads are divided into four parts. As the process proceeds the chromatin resolves itself into a single spireme-thread, consisting of four parallel rows of granules, which break in two to form the two tetrads (var. diva/ens), or is directly converted into a single tetrad (var. utiivalens) (Fig. 120). From these observations Brauer concludes that each tetrad arises from a rod, doubly split lengthwise by a process initiated at a very early period through the double fission of the chromatin-granules. If this be correct, there can be no reduction in Wcismann's sense ; for the four products of each primary chromatin-granule are equally distributed among the four daughter-cells. A similar conclusion, based on much more incomplete evidence, was reached by Brauer ('92) in the phyllopod Branchipus.
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Brauer's evidently conscientious figures very strongly sustain his conclusion, which, reinforced by the earlier work of Hertwig and Boveri, has until now seemed to rest upon an unassailable basis. The recent work of Sabaschnikoff ('97) nevertheless raises the possibility of a different interpretation. Brauer himself justly urges that the essence of the process lies in the double fission of the chromatingranules to which the formation of chromosomes is secondary. 1 Everything, therefore, turns on the manner in which the quadruple granules arise ; and Sabaschnikoff's work gives some ground for the view that they may arise, not by a double fission, but in some other way.
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 +
According to this author there is a period (in the oogenesis) at which the nuclear threads wholly disappear, the entire chromatin being broken up into granules. From this state the granules emerge in quadruple form to arrange themselves in the doubly split spireme exactly as Brauer describes : and a few observations are given (regarding the size and arrangement of the granules) which suggest the possibility that the quadruple granules may arise by the conjugation either of four separate granules or of two pairs of double granules. Since there is ground for the view that tetrads may arise by the conjugation of chromosomes (see following section), there is no a priori objection to such a conclusion. Could it be sustained, the maturationdivisions of Ascaris would in fact involve a true reduction in Weismann\s sense ; for despite the fact that the chromosomes are only longitudinally divided, the four longitudinal constituents of each tetrad would not be equivalent with respect to the granules, and it is the reduction of the latter (" ids ") that forms the essence of Weismann's hypothesis (p. 245). Another consideration, suggested to me by Professor T. H. Morgan, opens still another possibility, which seems well worthy of test by further research. As already stated (p. 88). the long chromosomes of Ascaris are plurivalcnt, since in all but the germ-cells each breaks up into a much larger number of smaller chromosomes (Fig. 73, p. 148). If, therefore, the latter correspond to the chromosomes of other forms in which tetrads occur (c.%. Cyclops or Ar/emia), the so-called "tetrad" of Ascaris is a compound body: and the true process of reduction must be sought in the origin of the smaller elements of which it is composed, which are, perhaps, directly comparable with Sabaschnikoff's i; granules." Until the questions thus opened have been further studied, the case for Ascaris must remain open : and it is perhaps worth suggesting that a new point of view may here be found for further study also of reduction in the vertebrates. 2
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1 Cf. p. 113.
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2 Bodies closely resembling tetrads arc sometimes formed in mitosis, where no reduction should occur. Thus, R. Hertwig ('95) has observed tetrads in the first cleavage-spindle of echinoderm-eggs after treatment with dilute poisons (p. 306). Klinckowstr8m figures them in the second polar spindle of Prostheccraus eggs, while Moore ('95) describes in the elasmobranchs small ring-shaped chromosomes, not only in the first but also in the second spermatocyte- divisions, concluding that no reduction occurs in either division.
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(c) The Formation of Tetrads by Conjugation. — A considerable number of observers have maintained that reduction may be effected by the union or conjugation of chromosomes that were previously separate. This view agrees in principle with that of Riickert, Hacker, and Vom Rath ; for the bivalent chromosomes assumed by these authors may be conceived as two conjugated chromosomes. It seems to be confirmed by the observations of Born and Fick on Amphibia and those of Riickert on selachians {Pristiurus) ; for in all these cases the number of chromatin-masses at the time the first polar body is formed is but half the number observed in younger stages of the germinal vesicle. In Pristiurus there are at first thirtysix double segments in the germinal vesicle. At a later period these give rise to a close spireme, which then becomes more open, and is found to form a double thread segmented into eighteen double segments ; i.e. the reduced number. In this case, therefore, the preliminary pseudo-reduction is almost certainly effected by the union of the original thirty-six double chromosomes, two by two. The most specific accounts of such a mode of origin have, however, been given by Calkins (earthworm) and Wilcox (grasshopper). The latter author asserts ('95) that in Caloptcnus the spireme of the first spermatocyte gives rise without longitudinal division to twenty-four chromosomes (double the somatic number). These then become associated in pairs, and still later the twelve pairs conjugate two and two to form six tetrads. There is, therefore, no longitudinal splitting of the chromosomes. The a priori improbability of such a conclusion is increased by the studies of Paulmier on the Hemiptera, which demonstrate the occurrence of a longitudinal division in a number of these forms and confirm the original studies of Vom Rath on Gryllotalpa}
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The second case, which is perhaps better founded, is that of the earthworm (Lumbricus terrcstris), as described by Calkins ('95, 2), whose work was done under my own direction. Calkins finds that the spireme splits longitudinally and then divides transversely into 32 double segments. These then unite, two by two, to form 16 tetrads. The 32 primary double segments therefore represent chromosomes of the normal number that have split longitudinally,
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i.e. — -p etc., and the formula for a tetrad is - , or — -. Such a b a o a\x
 +
 +
a tetrad, therefore, agrees as to its composition with the formulas
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of Hacker, Vom Rath, and Riickert, and agrees in mode of origin
 +
 +
with the process described by Riickert in the eggs of Pristiurus.
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While these observations are not absolutely conclusive, they never 1 Montgomery, who has denied the occurrence of a longitudinal division in Pentatoma ('98, i), has subsequently found such a division in the nearly related if not identical genus Euchistis ('99).
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theless rest on strong evidence, and they do not stand in actual contradiction of what is known in the copepods and vertebrates. The possibility of such a mode of origin in other forms must, I think, be held open.
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Under the same category must be placed Korschelt's unique results in the egg-reduction of the annelid Ophryotrocha ('95), which are very difficult to reconcile with anything known in other forms. The typical somatic number of chromosomes is here four. The same number of chromosomes appear in the germinal vesicle (Fig. 125, D). They are at first single, then double by a longitudinal split, but afterward single again by a reunion of the halves. The four chromosomes group themselves in a single tetrad, two passing into the first polar body, while two remain in the egg, but meanwhile each of them again splits into two. Of the four chromosomes thus left in the egg two are passed out into the second polar body, while the two remaining in the egg give rise to the germ-nucleus. From this it follows that the formation of the first polar body is a reducing division — a result which agrees with the earlier conclusions of Henking on Pyrrochoris, and with those of Paulmier on the Hemiptera.
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==C. Reduction without Tetrad-formation==
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As already stated (p. 246), the formation of actual tetrads is of relatively rare occurrence, being thus far certainly known only in the arthropods, nematodes, and some annelids. In the greater number of cases the two divisions of the primary chromatin-masses {i.e. of the primary oocyte or spermatocyte) are separated by a considerable interval, during which the first maturation cell-division takes place or is initiated, and hence no actual tetrads are formed. This obviously differs only in degree from tetrad-formation, the latter occurring only when the two divisions are simultaneous or occur in rapid succession.
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In the cases now to be considered the length of the pause between the maturation-divisions varies considerably, and in some forms (vertebrates, flowering plants) it is so prolonged that the nucleus is partially reconstructed. In all, or nearly all, these cases the first maturationdivision is of the hetcrotypical form, the chromosomes having the form of rings and arising by a process that agrees in most of its features with that leading to tetrad-formation. There is here, however, exactly the same contradiction of results as in the case of tetrad-formation described at page 247, and a bewildering confusion of the subject still exists. In brief, it may be stated that most observers of reduction of this type in the lower animals (flat-worms, annelids, mollusks) have found one transverse and one longitudinal, division ; most of those who have studied the vertebrates find two longitudinal divisions; while opinion regarding the plants is still divided.
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{a) Animals. — In the gephyrean Thalassema and the mollusk Zirp/uea (Figs. 128-130) Griffin ('99) finds that the rings, arising as described above, place themselves in the equator of the spindle with the longitudinal division in the equatorial plane. They are then drawn out toward the spindle-poles from the middle point, first assuming the form of a double cross, then of elongated ellipses, and finally break into two daughter-U's or -Vs. The first division is therefore longitudinal. During the late anaphase the V's break at the apex, the two limbs come close together, so as to give the decep
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Fig. 128. — Diagrams of reduction in the types represented by Thalassema (A) and Solamandra (/?). In both the first division is heterotypical. The second division (6) is transverse in the first and longitudinal in the second.
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tive appearance of a longitudinal split, and are separated by the second division (following immediately upon the first without intervening resting stage). The latter is therefore a transverse division (Fig. 130). An essentially similar result, though less completely worked out, is independently reached by Bolles Lee (*97) in Helix ; by Klinckowstrom ('97) in the turbellarian Prosthccerceus ; and by Francotte('97)and Van der Stricht ('98, 1) in Thysanzoon. Klinckowstrom shows that there is much variation in the way in which the rings open out and break apart, though the result is the same in all. In case of the vertebrates, Flemming ('87) long since described and figured typical tetrads in the salamander, but regarded them as "anomalies." Vom Rath's later conclusion ('93, '95) that they are normal tetrads has not been sustained by the still more recent work of Mevcs ('96), whose careful studies, together with those of Moore, Lenhossek, and others, thus far give no evidence of tetrad-formation, and seem opposed to the occurrence of reducing divisions in the vertebrates. Meves's work in the main confirms the earlier results of Flemming, except that he shows that, as in so many other animals, only two generations of spermatocytes exist. At the first division the nuclear reticulum resolves itself into twelve (the reduced number) segments, which split lengthwise, the halves remaining united to form elongated rings (Figs. 27, 37). These do not, however, contain dense into tetrads, but break apart during the first division at the points corresponding with the ends of the united halves. The first div'^ionSs th ere fore an equation -division. As the V-shaped halves separate theya&ain split lengthwise (Fig. 131), each of the secondary spermatocytes receiving twelve double V's or dyads. In the telophases and ensuing renting stage, however, all traces of this splitting are lost, the nuclei partially returning to the resting stage, but retaining traces of a spireme-like arrangement (Fig. 131). In the second division twelve double V's reappear, showing a longitudinal division which Flemming and Meves believe to be directly related to that seen during the foregoing anaphases. There is therefore no evidence of a transverse division. McGregor ('99) describes a nearly similar process in Amphiuma, where the longitudinal division of the
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A. A few moments after entrance of the spcrmatoii forming. B. Early prophase of first polar mitosis with c wall. D, Cenrralspindleeslablished; elimination of nuclei laler stage viewed from above. F. First polar spindle est
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Pig- >30. — Maturalion in Ihe lamellibranch Zitfhaa 4-E, Zirfkaa; F-I, Thalaiitma.
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in Thalaiitma. [GUI
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A. Unfcrtiliied egg. ring-shaped and cross-shaped chromosomes. B, Prophase of first polar mitosis. C. First polar spindle ; double crosses. D. Slightly later stage. E. The double crosses have broken apart (equation-division). G. Ensuing stage; daughter-V's broken apart at the apex. H. Telophase of first, early prophase of second, division; limbs of the v's separate bul closely opposed. F. Later prophase of second division. /. Second polar spindle in metaphase.
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daughter-V's is seen with the greatest clearness throughout the anaphases.
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The weak point in both the foregoing cases is the fact that all traces of the second longitudinal division are lost during the ensuing resting period ; and I do not think that even the observations of Flemming ('97), who has published the fullest evidence in the case, completely establish the occurrence of a subsequent longitudinal divi
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ng. 131. — {Compare Fig. 17). Maturation-division! in Salamc the others from Meves.]
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A. First division in metap base, showing heterotype rings, B. Anaphase; longiim ting of the daughter-loops. C. Telophase. D. Ensuing pause. E. Early prophase division with longitudinally divided segmented spireme. F. Later prophase. G. Mel second division.
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[/; from Flrmminc,
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sion of the chromosomes in the second mitosis. In Desmognathns, however, where the resting stage is less complete, Kingsbury ('99) finds the longitudinal split in the persistent chromosomes of the pause following the first division; and he believes this to be the same division as that seen during the anaphase. Carnoy and Le Brun ('99) reach the same result in the formation of the polar bodies in Triton, though their general account of the heterotypical mitosis differs very considerably from that of other authors, the rings being stated to arise by a double instead of a single longitudinal split These observers describe the rings of the early anaphase as having almost exactly the same double cross-form as those in Thalassema or Zirphcea (Griffin, '99), but believe them to arise in a manner nearly in accordance with Strasburger's abandoned view of 1895, 1 and with Guignard's ('98, 2) and Gr^goire's ('99) latest results on the flowering plants, the ring being stated to arise by a double longitudinal splitting, as explained at page 265.
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In the elasmobranch Scyllium Moore C95) finds twelve (the reduced number) ring-shaped chromosomes at the first division. These closely resemble tetrads ; but a resting stage follows, and the second division is likewise stated to be of the heterotypical form. Both divisions are stated to be equation-divisions — a conclusion well supported in case of the first, but so far from clear in the second that a careful reexamination of the matter is highly desirable.
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In mammals the first division is of the heterotypical form (Hermann, '89, Lenhossek, '98), though the rings are much smaller than in the salamander, recalling those seen in arthropods. No true tetrads are, however, formed, and the two divisions are separated by a resting period. The character of the second division is undetermined, though Lenhossek believes it to be heterotypical, like the first.
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(6) Plants. — It is in the flowering plants, where reduction likewise occurs, as a rule, without true tetrad-formation, that the contradiction of results reaches its climax ; and it must be said that until further research clears up the present confusion no definite result can be stated. The earlier work of Strasburger and Guignard indicated that no reducing division occurred, the numerical reduction being directly effected by a segmentation of the spireme-thread into half the somatic number of chromosomes. Thus these observers found in the male that the chromosomes suddenly appeared in the reduced number (twelve in the lily, eight in the onion) at the first division of the pollen-mother-cell, and in the female at the first division of the mother-cell of the embryo-sac. The subsequent phenomena differ in a very interesting way from those in animals, owing to the fact that the two maturation-divisions are followed in the female by one and in the male by two or more additional divisions, in both of which the reduced number of chromosomes persists. In the male the two maturation-divisions give rise to four pollen-grains, in the female to
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1 Cf. p. 269.
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the four primary cells of the embryo-sac (Fig. 132); and these two divisions undoubtedly correspond to the two maturation-divisions in animals. In the female, as in the animals, only one of the four resulting cells gives rise to the egg, the other three corresponding to the polar bodies in the animal egg, though they here continue to divide, and thus form a rudimentary prothallium. 1 The first-men
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Plg. 131. — General view of ihe malurat ion-divisions in flowering plants. [MOTHER.] A-C, in the male ; D-F, in Ihe female. A. The two secondary spermatocytes (pollen-mother. cells] just after the first division {Lilium). P. Final anaphase of second division [Podophyllum).
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C. Resulting telophase, which by division of Ihe cytoplasmic mass produces four pollen-grains.
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D. Embryo-sac after completion of Ihe first nuclear division {IJIium). E. The same after the second division. F. The upper four cells resulting from the third division (cf. Fig. 106) : 0, ovum ; /, upper polar cell; s, synergidai. (For further details, see Figs. 133, 134.)
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1 Of Ihese three cells one divides to form the " synergida?," the other two divide to form three "antipodal sells" (which like Ihe syncrgid.-c finally degenerate) and a "lower polar cell." The latter sooner or later conjugates with the " upper polar cell " (the sister-cell of the egg) to form the " secondary embryo-sac -nucleus," by the division of which Ihe endosperm-cells arise. Of the whole group of eight cells thus arising only the egg contribute*
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tioned cell, however, does not directly become the egg, but divides once, one of the products being the egg and the other the " upper polar cell" (Fig. 132, F), which contributes to the endosperm-formation (see footnote, and compare page 218).
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In the male the two maturation-divisions are in the angiosperms followed by two others, one of which separates a " vegetative " from a " generative " cell, while the second divides the generative nucleus into two definite germ-nuclei. In the gymnosperms more than two such additional divisions take place. In these later divisions, both in the male and in the female (with the exception noted in the footnote below), the reduced number persists, and the principal interest centres in the first two or maturation-divisions. Strasburger and Guignard found in Lilium that while both these divisions differed in many respects from the mitosis of ordinary vegetative cells, neither involved a transverse or reducing division, the chromosomes undergoing a longitudinal splitting for each of the maturation-divisions. Further investigations by Farmer ('93), Belajeff C94), Dixon ('96), Sargant ('96, '97), and others, showed that the first division is often of the heterotypical form, the daughter-chromosomes in the late-metaphase having the form of two V's united by their bases (<>). Despite the complication of these figures, due to torsion and other modifications, their resemblance to the ring-shaped bodies observed in the first maturation-division of so many animals is unmistakable, as was first clearly pointed out by Farmer and Moore ('95).
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Botanists have differed, and still differ, widely in their interpretation both of the origin and subsequent history of these bodies upon which the question of reduction turns. According to Strasburger's ('95) first account their origin has nothing in common with that of the tetrad-rings, since they were described as arising by a double longitudinal splitting of a primary rod, the halves then separating first from one end along one of the division-planes, and then from the other end along the other plane, meanwhile opening out to form a ring such as is shown in Fig. 133. (This process, somewhat difficult to understand from a description, will be understood from the diagram, Fig. 135, E-L) The four elements of the ring are then distributed without further division by the two ensuing maturation-divisions ; and the process, except for the peculiar opening out of the ring, is
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to the morphological formation of the embryo. It is a highly interesting fact that the number of chromosomes shown in the division of the lower of the two nuclei (i.e. the mothernucleus of the antipodal cells and lower polar-cell) formed at the first division of the embryo-sac-nucleus is inconstant, varying in the lily from 12, 16, 20, to 24 (Guignard, '91, 1), in which respect they contrast with the descendants (egg, synergidre) of the upper nucleus, which always show the reduced number (Mottier, '97, 1), i.e. in Lilium twelve. Thisexception only emphasizes the rule of the constancy of the chromosome-number in general; for these cells are destined to speedy degeneration.
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essentially in agreement with the facts described in Ascaris, and involves no reduction-division. Essentially the same result is reached by Guignard ('98) in his latest paper on Naias, and by Gregoire ('99) in the Liliaceas.
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Strasburger twice shifted ground in rapid succession. First ('97, 2), with Mottier ('97, 1 ), he somewhat doubtfully adopted a view agreeing
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\F, Strasbubgeb and MOT
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e of firs! division ; chromatined. Ft. Slightly later stage (split spireme) in the nucleus r prophase (pollen-mo I her-cel I. Podophyllum) t
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F. First maturation-spindle {Fritillaria, male). G. Dive
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essentially with the interpretation of Vom Rath, Riickert, etc. (p. 247). The primary rods split once, and bend into a V, the branches of which often come close together, and may be twisted on themselves, thus giving the appearance of the second longitudinal split described in Strasburger's paper of 1895. The two halves of the split U then separate, opening out from the apex, to form the o -figure. In the
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second division the limbs of the daughter-V's again come close together, remaining, however, united at one end, where they were believed finally to break apart during the second division. The latter was, therefore, regarded as a true reduction-division, the apparent longitudinal split being merely the plane along which the halves of the V come into contact (Fig. 134, C, D).
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The two accounts just given represent two extremes, the first agreeing essentially with Ascaris, the second with the copepods or insects. When we compare them with others, we encounter a truly bewildering confusion. Strasburger and Mottier ('97) themselves soon abandoned their acceptance of the reducing division, returning to the conclusion that in both sexes {Lilium, Podophyllum) both divisions involve a longitudinal splitting of the chromosomes (Figs. 133, 134). In the first division the longitudinally split spireme segments into twelve double rods, which bend at the middle to form double V's, with closely approximated halves. Becoming attached to the spindle by the apex, the limbs of each separate to form a -figure. At telophase the daughter-V's shorten, thicken, and join together to form a daughter-spireme consisting of a single contorted thread. This splits lengthwise throughout its whole extent, and then segments into double chromosomes, the halves of which separate at the second division (Fig. 135, L-M). The latter, therefore, like the first, involves no reducing division. This result agrees in substance with the slightly earlier work of Dixon C96) and of Miss Sargant ('96, '97), whose account of the origin of the -figure of the first division differs, however, in some interesting details. It is also in harmony with the general results of Farmer and Moore ('95), of Gr^goire ('99), and of Guignard ('98), who, however, describes the first division nearly in accordance with Strasburger's account of 1895, as stated above. On the other hand, Ishikawa (pollen-mother-cells of Allium, '97) and especially Belajeff (pollen-mother-cells of Iris, '98) conclude that the second division is a true transverse or reducing division. 1 Ishikawa described the first division as being nearly similar to the ring-formation in copepods, the four elements of the ring being often so condensed as nearly to resemble an actual tetrad. In the early anaphases the daughter-V's break at the apex ; and, although in the later anaphases the limbs reunite, Ishikawa is inclined to regard the transverse division as being a preparation for the second mitosis. Belajeffs earlier work ('94) on Lilium gave an indecisive result, though one on the whole favourable to a reducing division. In his latest paper, however ('98, 1), Belajeff takes more positive ground, stating that after the examination of a large number of forms he has found
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1 Schaffner ('97, 2) reaches exactly the reverse result in Lilium philadelphicum, i.e. the first division is transverse, the second longitudinal.
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in the pollen-mother-cells of Iris a much more favourable object of investigation than Lilitim, Frititlaria, and the other forms on which most of the work thus far has been done, and one in which the second division takes place with " admirable clearness " ; he also gives interesting additional details of the first division in this and other forms. In the first division the spireme splits lengthwise, and then breaks into chromosomes, which assume the shape of a V, Y, or X (Fig. 135, N—Q). The two limbs of these bodies do not, as might be
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ucleus of secondary spermatocyte (PodaphylliLm). B. Prophase of sec male) with longitudinally divided chromatin -threads. B. Corresponding stage /-'. Meiaphase of second division (Fodcphyllam, male). G. Initial anaphase (/. C. D. illustrate Motlier's earlier conclusions. C. Second division (Lilimm. male) imcs bent together so as to simulate a split. D. Slightly later stage (Fritillana, 1 siagc supposed to result from breaking apart of the limbs of the U at point of flexu
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supposed, represent sister-chromosomes (resulting from the longitudinal division of the spireme) attached by one end or at the middle, since each X, Y, or V is double, consisting of two similar superimposed halves. Belajeff, therefore, regards these figures as longitudinally divided bivalent chromosomes, having the value of tetrads, each limb being a longitudinally split single chromosome. The double V's, Y's, and X's take up a position with the apex (or one end of the X) attached to the spindle, and the longitudinal division in the equatorial plane. The halves then progressively diverge from the point of attachment, thus giving rise to -shaped, <>- -shaped, or xx -shaped figures, all of which in the end assume the -shape. This part of the process is in the main similar to that described by Strasburger and Mottier, and the daughter-V's diverge in the same way as these authors describe. The second division, however, differs radically from their account, since no splitting of the spireme-thread occurs. The chromosomes reappear in the V-, Y-, and X-forms, but are undivided^ and only half as thick as in the first division. Passing to the equator of the spindle, the V- and Y-forms break apart at the apex, while the X-forms separate into the two branches of the X, the daughter-chromosomes having the form of rods slightly bent at the outer end to form a J-figure (Fig. 135, R-T). This division is, accordingly, a transverse or reducing one, which " corresponds completely to the reduction-division in the animal organism " ('98, 2, p. 33.) Atkinson ('99) reaches the same general result in Trillium, stating very positively that no longitudinal division occurs in the second mitosis, and believing that the daughter-V's of the first (heterotypical) mitosis retain their individuality throughout the ensuing pause, and break apart at the apex (reducing division) in the second mitosis. This observer finds further that in Ariscema the heterotypical rings of the first mitosis condense into true tetrads, by one longitudinal and one transverse division, but believes that in this case it is the first division that effects the reduction, as in the insects.
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Such confusion in the results of the most competent observers of reduction in the flowering plants is itself a sufficient commentary on the very great difficulty and uncertainty of the subject ; and it would be obviously premature to draw any positive conclusions until further research shall have cleared up the matter. 1
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1 Strasburger's new book, entitled Uber Reduktiomtheilung, Spindelbildung, Centrosomen und Cilienbildner im Pflanzcnreich (Jena, 1900), is received while this work is in press, too late for analysis in the text. In this treatise the author gives an exhaustive review of the entire subject, contributing also many new and important observations on Lilium, Iris, Podophyllum, Tradescantia, Allium, Ijxrix, and several other forms. The general result of these renewed researches leads Strasburger to return, in the main, to his conclusions of 1895, w ' tn which agree, as stated above, the results of Guignard and Gregoire; and, in a careful critique of Belajeff's work, he shows how the results of this observer may be reconciled with his own. The essence of Strasburger's interpretation is as follows. In the prophases of the first division the chromosomes first undergo a longitudinal division, shorten to form double rods, and then again split lengthwise in a plane at right angles to the first. The following stages vary even in the same species (Lilium); and here lies the explanation of much of the divergence between the accounts of different observers. (1) In the typical case, the chromosomes are placed radially, with one end next the spindle ; and, during the metaphase, they open apart along the first division-plane, from the spindle outwards, to form h- -shaped figures. These figures meanwhile open apart from the free end inwards along the second division-plane. Thus arise the characteristic < > -shaped figures, the daughter-V's having separated along the first (equatorial) division-plane, while the two limbs of each V have resulted, not through bending, but from a second (axial) split (Fig. 135, E-I/). The
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- Diagrams illustrating diffcrem liiiivc mitoses (hi'terotypical forr tiurgers
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accounts of reduction in Ihe flowering plants. ) in Pica. [BEI.AJEFF.] ('95) and the later one of Guignard, of Ihe first
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in-division. E. Doubly split rod. F. Metaphtte, in profile. G. The ; showing ihe hctcrotvpc ring. H. I. Opening out and breaking apart of the ring.
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JT-.lt. 1-aliT iMMHiunl of Strasburger and Motlier {.if. tigs. 133. .34). J. longitudinally split. V-shaped rhmiimvmu^ of first division. A'. Opening out oi the ring, L. Prophase of second division, showing longitudinally split segmented spireme. M. Initial anaphase of second division,
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A'-(). First division. [Bki.ajf.ff.] A*. Lung itu din ally split chromosomes, viewed in the equatorial plane. 0. The same viewed in the axis of the spindle. /'. Separation of the daughterchromosomes. Q. Anaphase, all the chromosomes assuming the V-form.
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K-T. Second division in frit. [Hki.MKH-'.J A', l-^uaiorial plate, limbs of X's and V*s brraking apart (reducing division). S. Slightly later stage, with daughter-chromosomes si ill untied at one end. /: Anaphase.
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Rtsumt. In reduction without tetrad-formation the spireme segments into half the somatic number of chromosomes, which split lengthwise and open out to form rings for the first (heterotypical) mitosis. According to one set of observers, including Flemming, Meves, McGregor, Kingsbury, Moore, Klinckowstrom, Van der Stricht, Francotte, Griffin, Belajeff, Farmer, Dixon, Strasburger, Sargant, Mottier, Ishikawa, and Atkinson, the ring arises by a single longitudinal division. According to another group, including Carnoy, Le Brun, Guignard, and Gr^goire, the ring arises through a double longitudinal division, one representing the axial and the other the equatorial plane of the o -figure. The second group of observers regard both maturation-divisions as longitudinal. Among the first group, Flemming, Meves, McGregor, Kingsbury, Moore, Farmer, Dixon, Strasburger, Sargant, and Mottier likewise believe both divisions to be longitudinal, the daughter-V's or their products again splitting lengthwise for the second division ; while Klinckowstrom, Van der Stricht, Francotte, Griffin, Belajeff, Ishikawa, and Atkinson believe one of them to be transverse, the daughter-V's breaking apart at the apex, and thus giving the reducing division of Weismann. 1
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==D. Some Peculiarities of Reduction in the Insects==
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We may here briefly consider some interesting observations which show that in some cases the nuclear substance may be unequally distributed to the germ-nuclei. Henking ('90) discovered that in the second spermatocyte-division of Pyrrochoris one of the <% chromosomes " passes undivided into one of the daughter-cells (spermatids) which receives twelve chromatin-elements while its sister receives but eleven. (The number of chromosomes in the spermatogonia, and of rings in the first spermatocyte-division is twenty-four). This anomalous process is confirmed with interesting additional details by Paulmier ('99) in Anasa, and obviously related phenomena are described by Montgomery ('99, 1 ) in Pcntatoma, and by McClung ('99) in Xiphidium.
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breaking apart of the V's at the apex, as described by Belajeff, is, therefore, not a transverse division, but merely the completion of the second longitudinal division. (2) In a second and exceptional type, the chromosomes are placed tangentially to the spindle, and the halves separate from the middle, again producing O -shaped figures. These, however, are not of the same nature as those arising in the first case, since they are formed by a bending out of each daughter-chromosome at the middle to form the V, and not by the second longitudinal split. The effect of the latter is in this case to render each daughter-V in itself double, precisely as in the salamander. The difference between the two types results merely from the difference of position of the chromosome with respect to the spindle, and the final result is the same in both, i.e. two longitudinal divisions and no reducing one.
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This highly important work brings very strong evidence against the occurrence of transverse or reducing divisions in the higher plants, and seems to explain satisfactorily most of the differences of interpretation given by other olwervers. It will be interesting to see whether a similar interpretation is possible in the case of mollusks, annelids, and arthropods, where the early stages, in many cases, so strikingly resemble those occurring in the plants.
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1 Cf. footnote on page 269.
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In Pentatoma the number of chromosomes in the spermatocyte is fourteen. During the final anaphases of the last division, one of the fourteen daughter-chromosomes assumes a different staining-capacity from the others, and becomes a •* chromatin-nucleolus " which fragments into several smaller bodies during the ensuing resting-stage. During each of the succeeding spermatocyte-divisions appear seven chromosomes and a single small chromatin-nucleolus, and both of these kinds of bodies are halved at each division, so that each spermatid receives seven chromosomes and a single chromatin-nucleolus. 1 In Xiphidium a body called by McClung the accessory chromosome," and believed by him to correspond to the "chromatinnucleolus " of Pentatoma, appears in the early prophases of the last spermatogoniumdivision while the remaining chromatin still forms a reticulum. In the equatorial plate this lies outside the ring of chromosomes, but divides like the latter. The same body appears in the ensuing resting-stage, and during both of the spermatocytedivisions. In these it lies, as before, outside the chromosome-ring, and differs markedly from the other chromosomes, but divides like the latter, each of the halves passing into one of the spermatids, where it appears to form an important part of the sperm-nucleus.
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Despite the peculiarities described above, the chromatin, as a whole, seems to be equally distributed in both Pentatoma and Xiphidium. In Anasa, however, Paulmier's studies ('98, ^99), made in my laboratory, give a result agreeing with that of Henking, and suggest some very interesting further questions. The spermatogonianuclei contain two nucleolus-like bodies, aiid give rise to twenty-two chromosomes, of which two are smaller than the others (Fig. 126). In the first spermatocyte-division appear eleven tetrads. Ten of these arise from rings like those of Gryllotalpa, etc. The eleventh, which is much smaller than the others, seems to arise from a single nucleolus-like body of the spermatocyte-nucleus, and by a process differing considerably from the others. All of these bodies are halved to form dyads at the first division. In the second spermatocyte-division (Fig. 127) the larger dyads divide to form single chromosomes in the usual manner. The small dyad, however, fails to divide, passing over bodily into one of the spermatids. In this case, therefore, half of the spermatids receive ten single chromosomes, while the remainder receive in addition a small dvad.
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A comparison of the foregoing results indicates that the small tetrad (dyad) corresponds to the extra chromosome observed by Henking in Pyrrochoris, and perhaps also to the "accessory chromosome" of Xiphidium. Whether it corresponds to the " chromatin-nucleolus " of Pentatoma is not yet clear. The most remarkable of these strange phenomena is the formation of the small tetrad, which seems to be a non-essential element, since it does not contribute to all the spermatozoa. Paulmier is inclined to ascribe to it a vestigial significance, regarding it as a "degenerating" chromosome which has lost its functional value, though still undergoing in some measure its original morphological transformation ; in this connection it should be pointed out that the spermatocyte-nucleolus, from which it seems to be derived, is represented in the spermatogonia by two such nucleoli, just as the single small tetrad is represented by two small chromosomes in the spermatogonia-mitoses. The real meaning of the phenomenon is, however, wholly conjectural.
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==E. The Early History of the Germ-nuclei==
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There are many peculiarities in the early history of the germnuclei, both in plants and animals, that have a special interest in connection with the reduction-problem ; and some of these have raised some remarkable questions regarding the origin of reduction. A large number of observers are now agreed that during the growthperiod preceding the maturation-division (p. 236), in both sexes, the nucleus of the mother-cell (spermatogonium, oogonium), both in plants and in animals, passes through some of the changes preparatory to reduction at a very early period. Thus, in the egg the primary chroma tin-rods are often present in the very young ovarian eggs, and from their first appearance are already split longitudinally. 1 Hacker ('92, 2) made the interesting discovery that in some of the copepods {Cant/iocamptus, Cyclops) these double rods could be traced
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1 On this latter point Montgomery's observations do not seem quite decisive.
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Pig. 136. — Longitudinal section through the ovary of the copepod Canlhoiamptm. [Hiic
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eg. The youngest germ-cells or oogonia (dividing al eg. 1 ) : a. upper pan of the growlii-sone ; ot. oftcyte. or growing ovarian egg; if. fully formed egg. with double chroma tin-rods.
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back continuously to a double spireme-thread, following immediately upon the division of the last generation of oogonia, and that at no period is a true reticulum formed in the germinal vesicle (Fig. 136). In the following year Riickert ('93, 2) made a precisely similar discovery in the case of selachians. After division of the last generation of oogonia the daughter-chromosomes do not give rise to a reticulum, but split lengthwise, and persist in this condition throughout the entire growth-period of the egg. Riickert therefore concluded that the germinal vesicle of the selachians is to be regarded as a " daughter-spireme of the oogonium (Ur-ei) grown to enormous dimensions, the chromosomes of which are doubled and arranged in pairs." * In this case their number seems to be at first the somatic number (thirty-six), which is afterward halved by conjugation of the elements two and two (Riickert), as in Lutnbricus (Calkins). It is, however, certain that in many cases (insects, copepods) the double rods first appear in the reduced number, and the observations of Vom Rath C93) and Hacker ('95, 3) give some reason to believe that the reduced number may in some forms be present in the earlier progenitors of the germ-cells, the former author having found but half the normal number in some of the embryonic cells of the salamander, while Hacker ('95, 3) finds that in Cyclops brevicomis the reduced number of chromosomes (twelve) appears in the primordial germ-cells which are differentiated in the blastula-stage (Fig. 74). He adds the interesting discovery that in this form the somatic nuclei of the cleavagestages show the same number, and hence concludes that all the chromosomes of these stages are bivalent. As development proceeds, the germ-cells retain this character, while the somatic cells acquire the usual number (twenty-four) — a process which, if the conception of bivalent chromosomes be valid, must consist in the division of each bivalent rod into its two elements. We have here a wholly new light on the historical origin of reduction ; for the pseudo-reduction of the germ-nuclei seems to be in this case a persistence of the embryonic condition, and we may therefore hope for a future explanation of the process by which it has in other cases been deferred until the penultimate cell-generation, as is certainly the fact in Ascaris?
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This leads to the consideration of some very interesting recent discoveries regarding the relation of reduction to the alternation of generations in the higher plants. As already stated (p. 263), Strasburger, Guignard, and other observers have found that in the angiosperms the two maturation-divisions are in both sexes followed by one or more divisions in which the reduced number persists. The cells thus formed are generally recognized as belonging to the vestiges of the sexual generation (prothallium) of the higher cryptogams, the pollengrains (or their analogues in the female) corresponding to the asexual spores of the archegoniate cryptogams. We should, therefore, expect to find reduction in the latter forms occurring in the two corresponding divisions, by which the " tetrad " of spores is formed (as was first pointed out by Hartog, '91). Botanists were thus led to the surmise, first expressed by Overton in 1892, that the reduced number would be found to occur in the prothallium-cells derived from those spores.
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1 '92, 2, p. 51.
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2 It may be recalled that in Ascaris Boveri proved that the primordial germ-cells have the full number of chromosomes, and Ilertwig clearly showed that this number is retained up to the last division of the spermatogonia. Ishikawa ('97) finds that in Allium the reduced number (eight) appears in the mitosis of the " Urpollenzellen " preceding the pollen-mothcr-cclls. This is, however, contradicted by Motticr ('97, 2).
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This surmise quickly became a certainty. Overton himself discovered ('93 ) that the cells of the endosperm in the gymnosperm Ccratozamia divide with the reduced number, namely eight ; and Dixon observed the same fact in Pinus at the same time. In the following year Strasburger brought the matter to a definite conclusion in the case of a fern (Osmunda), showing that all the cells of the prothallium, from the original spore-mot her-cc 11 onwards to the formation of the germ-cells, have one-half the number of chromosomes found in the asexual generation, namely twelve instead of twentyfour ; in other words, the reduction takes place in the formation of the spore from which the sexual generation arises, many cell-generations before the germ-cells are formed, indeed before the formation of the body from which these cells arise. Similar facts were determined by Farmer in Pallavicinia, one of the Hepaticae, where all of the nuclei of the asexual generation (sporogonium) show eight chromosomes during division, those of the sexual generation (thallus) four. It now seems highly probable that this will be found a general rule.
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The striking point in these, as in Hacker's observations, is that the numerical reduction takes place so long before the fertilization for which it is the obvious preparation. Speculating on the meaning of this remarkable fact, Strasburger advances the hypothesis that the reduced number is the ancestral number inherited from the ancestral type. The normal, i.e. somatic, number arose through conjugation by which the chromosomes of two germ-cells were brought together. Strasburger does not hesitate to apply the same conception to animals, and suggests that the four cells arising by the division of the oogonium (egg plus three polar bodies) represent the remains of a separate generation, now a mere remnant included in the body in somewhat the same manner that the rudimentary prothallium of angiosperms is included in the embryo-sac. This may seem a highly improbable conclusion, but it must not be forgotten that so able a zoologist as Whitman expressed a nearly related thought, as long ago as 1878 : " I interpret the formation of polar globules as a relic of the primitive mode of asexual reproduction'* l Strasburger's view is exactly the reverse of this in identifying the polar bodies as the remains of a sexual generation ; and as Hacker has pointed out ('98, p. 102), it is difficult to reconcile with the fact that true reduction appears to occur already in the unicellular organisms (p. 277). The hypothesis is nevertheless highly suggestive and one which suggests a quite new point of view for the study not only of maturation but also of the whole problem of sexuality.
 +
 +
We may now return to the consideration of some details. In a considerable number of forms, though not in all, the early prophase is
 +
 +
1 '78, p. 262.
 +
 +
 +
characterized, especially in the male, by a more or less complete concentration of the chromatin-substance at one side of the nucleus. This stage, to which Moore has given the name synapsis (Fig. 120, A), sometimes occurs when the spireme thread is already split {Ascaris, Liliiim), sometimes before the division is visible (insects). In either case the chromatin-segments emerge from the synapsis stage longitudinally divided and in the reduced number, a fact which gives ground for the conclusion that the synapsis is in some way concerned with the rearrangement of the chromatin-substance involved in the numerical reduction. During the synapsis the nucleolus remains quite distinct from the chromatin, and in many cases it afterward persists beside the tetrads, in the formation of which it takes no part, to be cast out into the cytoplasm (Fig. 124) or to degenerate in situ during the first maturation-division.
 +
 +
A suggestive phenomena, described by several observers, 1 is the casting out of a large part of the nuclear reticulum of the germinal
 +
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 +
 +
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[CAK
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Fig.^.-Typesofmali spindle with tetrads, in lit OV and l.kliRUM.] C. First polar spindle of At
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pindles in the ft [HACKER].
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[FiJRST.]
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i polaj spindle
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vesicle at the time the polar bodies are formed (Figs. 97, 128). In these cases {Asterias, Polyehazrus, Thalassema, Nereis) only a small fraction of the chromatin-substance is preserved to form the chromosomes, the remainder degenerating in the cytoplasm. 2
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As a final point wc must briefly consider the varying accounts of the achromatic maturation-figures in the female already briefly referred to at page 85. In many forms {e.g. in turbellarians, nemertines, annelids, mollusks, cchinoderms) the polar amphiasters are of quite typical form, with large asters and distinct ccntrosomes nearly similar to those of the cleavage-fignres. In others, however (nematodes, arthropods, tunicates, vertebrates), the polar spindles differ markedly from those of the cleavage-figures, being described by many authors as entirely devoid of asters and even in some cases of centrosomes (Fig. 137).
 +
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ardiner ("98), Gtiffin ('99).
 +
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i-suhstanee in the clasmobta rich egg, p. 338.
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There can be no doubt that these polar spindles differ from the usual type, and that they approach those recently described in the mitosis of the higher plants, but it is doubtful whether the apparent absence of asters and centrosomes is normal. In Ascaris, the first polar spindle arising by a direct transformation of the germinal vesicle (Fig. 117) has a barrel-shape, with no trace of asters. At the poles of the spindle, however, are one or two deeply staining granules (Fig. 137), which have been identified as centrosomes by Hacker C94) and Erlanger ('97, 4), but by Fiirst ('98) are regarded as central granules, the whole spindle being conceived as an enlarged centrosome. 1 For the reasons stated at page 314, I believe the former to be the correct interpretation. 2 Spindles without centrosomes have been described in the eggs of tunicates (Julin, Hill, Crampton), in Amphioxus (SoboXXz), in some species of copepods (Hacker), and in some vertebrates (Diemyctylus, Jordan ; mouse, Sobotta). In Amphioxus (Sobotta) and Triton (Carnoy and LeBrun) complete asters are not. formed, but fibrillae apparently corresponding to astral rays and converging to the spindle-poles are found outside the limits of the spindle (Fig. 137). In the guinea-pig, according to Montgomery ('98), centrosomes and asters are present in the first polar spindle, but absent in the second. The evidence is on the whole rather strong that the achromatic figure in these cases approaches in form that seen in the higher plants ; but it is an open question whether the appearances described may not be a result of imperfect fixation.
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==F. Reduction in Unicellular Forms==
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Although the one-celled and other lower forms have not yet been sufficiently investigated, we have already good ground for the conclusion that a process analogous to the reduction of higher types regularly recurs in them. In the conjugation of Infusoria, as already described (p. 223), the original nucleus divides several times before union, and only one of the resulting nuclei becomes the conjugating germ-nucleus, while the others perish, like the polar bodies. The numerical correspondence between the rejected nuclei or "corpuscules de rebut" has already been pointed out (p. 227). Hertwig could not count the chromosomes with absolute certainty, yet he states ('89) that in Paramcecium caudatum, during the final division, the number of spindle-fibres and of the corresponding chromatic elements is but 4-6, while in the
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1 Cf. p. 312.
 +
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2 Sala C94) and Fiirst have shown that occasionally the polar spindles of Ascaris are provided with large typical asters, and thus resemble those of annelids or mollusks. Sala believed this to be an effect of lowered temperature, but Fiirst's observations are unfavourable to this conclusion.
 +
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earlier divisions the number is approximately double this(8-9). This observation makes it nearly certain that a numerical reduction of chromosomes occurs in the Protozoa in a manner similar to that of the higher forms ; but the reduction here appears to be deferred until
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Fig. 138. — Conjugation and formation of the polar bodies in Attinipkrys. [SCHMIDINN.] A. Union of tbe gametes: first polar spindle. B. Fusion of the cell-bodies; a single polar
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r the periphery of each. C Fusion of them
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the final division. In the gregarines Wolters ('91) has observed the formation of an actual polar body as a small cell segmented off from each of the two conjugating animals soon after their union ; but the number of chromosomes was not determined. Sc haudinn ('96, 2) has observed a like process in Actinephtys, each of the gametes segmenting off a single polar body, after which the germnuclei fuse (Fig. 138). It is possible, as R. Hertwig ('98) points out, that in both these forms a second polar body may have been overlooked, owing perhaps to its rapid disintegration. In Actinosphtzrittm, according to R. Hertwig ('98), the nucleus of each gamete divides twice in rapid succession to form two polar bodies (nuclei), which degenerate, after
 +
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which the germ-nuclei unite (Fig. 139). Whether a reduction in the number of chromosomes occurs in these cases was not determined.'
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A. Soon after union, f< distinct. C. Fusion of the F. Second cleavage. G. 1
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Conjugation of Cfosttrivm. [Ki.h
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hroTnatopliorcs. li. C hro mat oph ores reduced lo two, nuclei ii. D. First cleavage of the lygole. E. Resulting a-cell stage.
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ing stage, each cell bi-nucleate. H. St-paration of the cells; one o: the nuclei in eacn enlarging to form the permanent nucleus, the other (probably representing a polar body) degenerating.
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1 AiHnospharium forms one of the most extreme known cases of in-breeding; for the gametes are sitlcr-cills which immediately reunite after forming the polar bodies. The general facta are as follows: The mother animal, containing very numerous nuclei, become* encysted, and a very large number of the nuclei degenerate. The body then segments into
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Adelea (one of the Coccidiae) is a very interesting case, for according to Siedlecki C99) polar bodies or their analogues are formed in both sexes. The gametes are here of very unequal size. Upon their union the smaller male cell divides twice to form apparently equivalent spermatozoids, of which, however, only one enters the ovum, while three degenerate as polar bodies. These two divisions are of different type ; the first resembles true mitosis, while the second is of simpler character and is believed by Siedlecki to effect a reduction in the number of chromosomes. In the meantime the nucleus of the macrogamete moves to the surface and there expels a portion of its chromatin, after which union of the nuclei takes place. Interesting facts have been observed in unicellular plants which indicate that the reduction may here occur either before (diatoms) or after (desmids) fusion of the conjugating nuclei. In the former (Rhopalodind) Klebahn ('96) finds that each nucleus divides twice, as in many Infusoria, giving rise to two large and two small nuclei. Each of the conjugates then divides, each daughter-cell receiving one large and one small nucleus. The four resulting individuals then conjugate, two and two, the large nuclei fusing while the small (polar bodies) degenerate. The comparison of this case with that of the Infusoria is highly interesting. In the desmids on the other hand {Closterium and Cosmarium, Fig. 140), according to Klebahn ('92), the nuclei first unite to form a cleavagenucleus, after which the zygote divides into two. Each of the new nuclei now divides, one of the products persisting as the permanent nucleus, while the other degenerates and disappears. Chmielewski asserts that a similar process occurs in Spirogyra. Although the numerical relations of the chromosomes have not been determined in these cases, it appears probable that the elimination of a nucleus in each cell is a process of reduction occurring after fertilization.
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==G. Maturation of Parthenogenetic Eggs==
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The maturation of eggs that develop without fertilization is a subject of special interest, partly because of its bearing on the general theory of fertilization, partly because it is here, as I believe, that one of the strongest supports is found for the hypothesis of the individuality of chromosomes. In an early article by Minot {'77) on the
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a number (five to twelve) of "primary cysts," each containing one of the remaining nuclei. Each primary cyst divides by mitosis to form two gametes ("secondary cysts "), which, after forming the polar bodies, reunite, their nuclei fusing to form a single one. The resulting cell soon creeps out of the cyst-wall and assumes the active life, its nucleus meanwhile multiplying to produce the multinuclear condition characteristic of the adult animal. What is here the physiological motive for the formation of the polar bodies, and how shall it be explained under the Weismann hypothesis?
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theoretical meaning of maturation, the suggestion is made that parthenogenesis may be due to failure on the part of the egg to form the polar bodies, the egg-nucleus thus remaining hermaphrodite, and hence capable of development without fertilization. This suggestion forms the germ of all later theories of parthenogenesis. Balfour ('8o) suggested that the function of forming polar cells has been acquired by the ovum for the express purpose of preventing parthenogenesis, and a nearly similar view was afterward maintained by Van Beneden. 1 These authors assumed accordingly that in parthenogenetic eggs no polar bodies are formed. Weismann ('86) soon discovered, however, that the parthenogenetic eggs of Polyphemus (one of the Daphnidae) produce a single polar body. This observation was quickly followed by the still more significant discovery by Blochmann ('88) that in Aphis the parthenogenetic eggs produce a single polar body, while the fertilized eggs produce two, Weismann was able to determine the same fact in ostracodes and Rotifera, and was thus led to the view 2 which later researches have entirely confirmed, that it is the second polar body that is of special significance in parthenogenesis. Blochmann observed that in insects the polar bodies were not actually thrown out of the egg, but remained embedded in its substance near the periphery. At the same time Boveri ('87, 1) discovered that in Ascaris the second polar body might in exceptional cases remain in the egg and there give rise to a resting-nucleus indistinguishable from the egg-nucleus or sperm-nucleus. He was thus led to the interesting suggestion that parthenogenesis might be due to the retention of the second polar body in the egg and its union with the egg-nucleus. "The second polar body would thus, in a certain sense, assume the role of the spermatozoon, and it might not without reason be said : " Parthenogenesis is the result of fertilisation by the second polar body." 3
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This conclusion received a brilliant confirmation through the observations of Brauer ('93) on the parthenogenetic egg of Artetnia, though it appeared that Boveri arrived at only a part of the truth. Blochmann ('88-89) had found that in the parthenogenetic eggs of the honey-bee two polar bodies are formed, and Platner discovered the same fact in the butterfly Liparis ('89) — a fact which seemed to contradict Boveri's hypothesis. Brauer's beautiful researches resolved the contradiction by showing that there are two types of parthenogenesis which may occur in the same animal. In the one case Boveri's conception is exactly realized, while the other is easily brought into relation with it.
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1 '83, p. 622. a Essay VI., p. 359. • /.<■., p. 73.
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(a) In both modes typical tetrads are formed in the germ-nucleus to the number of eighty-four. In the first and more frequent case (Fig. 141) but one polar body is formed, which removes eighty-four dyads, leaving eighty-four in the egg. There may be an abortive attempt to form a second polar spindle, but no division results, and the eighty-four dyads give rise to a reticular cleavage-nucleus. From
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Pig. 141. — First type of m
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aluralion
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in ihe parthenojjtnc
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A. The nrsl polar spindle- 1 first polur hoily . H4 dyads rema F. Appearance of Ihe egg-uenl of Ihe cleavage-figure; ihe equa
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lie equal
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in in the
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■ml 'lister, fs. G. 1
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'<• consists of 84 apf
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o the CRg-nnclcus. showi
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this arise eighty-four thread-like chromosomes, and the same number appears in later cleavage-stages,
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(A) It is the second and rarer mode that realizes Bovcri's conception (Fig. 142). Hoth polar bodies are formed, the first removing eighty-four dyads and leaving the same number in the egg. In the formation of the second, the eighty-four dyads are halved to form two daughter-groups, each containing eighty-four single chromosomes, Both these groups remain in the egg, and each gives rise to a single reticular nucleus, as described by Bovcri in Ascaris. These two nuclei place themselves side by side in the cleavage-figttre, and give rise each to eighty-four chromosomes, precisely like two germ-nuclei in ordinary fertilization. The one hundred and sixty-eight chromosomes split
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Fig. 14a. — Second type of maluralii . Formation of second polar body
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il plate
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1 two groups of 84 each.
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lengthwise, and are distributed in the usual manner, and reappear in the same number in later stages. In other words, the second polar body here plays the part of a sperm-nucleus precisely as maintained by Boveri.
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In all individuals arising from eggs of the first type, therefore, the somatic number of chromosomes is eighty-four; in all those arising from eggs of the second type, it is one hundred and sixty-eight This difference is clearly due to the fact that in the latter case the chromosomes are single or univalent, while in the former they are bivalent (actually arising from dyads or double chromosomes). The remarkable feature, on which too much emphasis cannot be laid, is that the numerical difference should persist despite the fact that the mass, and, as far as we can see, the quality, of the chromatin is the same in both cases. In this fact we must recognize a strong support, not only of Hacker's and Vom Rath's conception of bivalent chromosomes, but also of the more general hypothesis of the individuality of chromosomes (Chapter VI.).
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===1. Accessory Cells of the Testis===
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It is necessary to touch here on the nature of the so-called " Sertoli-cells." or supporting cells of the testis in mammals, partly because of the theoretical significance attached to them by Minot, partly because of their relations to the question of amitosis in the testis. In the seminiferous tubules of the mammalian testis, the parentcells of the spermatozoa develop from the periphery inwards toward the lumen, where the spermatozoa are finally formed and set free. At the periphery is a layer of cells next the basement-membrane, having flat, oval nuclei. Within this, the cells are arranged in columns alternating more or less regularly with long, clear cells, containing large nuclei. The latter are the Sertoli-cells \ or supporting cells ; they extend nearly through from the basement-membrane to the lumen, and to their inner ends the young spermatozoa are attached by their heads, and there complete their growth. The spermatozoa are developed from cells which lie in columns between the Sertolicells, and which undoubtedly represent spermatogonia, spermatocytes, and spermatids, though their precise relationship is, to some extent, in doubt. The innermost of these cells, next the iumen, are spermatids, which, after their formation, are found attached to the Sertoli-cells, and are there converted into spermatozoa without further division. The deeper cells from which they arise are spermatocytes, and the spermatogonia lie deeper still, being probably represented by the large, rounded cells.
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Two entirely different interpretations of the Sertoli-cells were advanced as long ago as 1 87 1, and both views still have their adherents. Von Ebner ('71) at first regarded the Sertoli-cell as the parent-cell of the group of spermatozoa attached to it. and the same view was afterward especially advocated by Biondi ('85 ) and by Mi not (92), the latter of whom regarded the nucleus of the Sertoli-cell as the physiological analogue of the polar bodies, i.e. as containing the female nuclear substance C92, p. 77). According to the opposing view, first suggested by Merkel C71). the Sertolicell is not the parent-cell, but a nurse-cell, the spermatozoa developing from the columns of rounded cells, and becoming secondarily attached to the Sertoli-cell, which serves merely as a support and a means of conveying nourishment to the growing spermatozoa. This view was advocated by Brown ('85), and especially by Benda C87). In the following year ('88), von Ebner himself abandoned his early hypothesis and strongly advocated Benda's views, adding the very significant result XhaXfour spermatids arise from each spermatocyte, precisely as was afterward shown to be the case in Ascaris, etc. The very careful and thorough work of Benda and von Ebner, confirmed by that of Lcnhosse'k ('98, 2), leaves no doubt that mammalian spermatogenesis conforms, in its main outlines, with that of Ascaris, the salamander, and other forms, and that Biondi's account is untenable. Minors theoretical interpretation of the Sertoli-cell, as the physiological equivalent of the polar bodies, therefore collapses.
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===2. A mitosis in the Early Sex-cells===
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Whether the progenitors of the germ-cells ever divide amitotically is a question of high theoretical interest. Numerous observers have described amitotic division in testis-cells. and a few also in those of the ovary. The recent observations of Meves ('91 ). Vom Rath C93), and others leave no doubt whatever that such divisions occur in the testis of manv animals. Vom Rath maintains, after an extended investigation. that all cells so dividing do not belong in the cycle of development of the germ-cells ('93, p. 164) : that amitosis occurs only in the supporting or nutritive cells (Sertoli-cells, etc.), or in such as are destined to degenerate, like the "residual bodies" of Van Beneden. Meves has, however, produced strong evidence C94) that in the salamander the spermatogonia may, in the autumn, divide by amitosis, and in the ensuing spring may again resume the process of mitotic division, and give rise to functional spermatozoa. On the strength of these observations Flemming C93) himself now admits the possibility that amitosis may form part of a normal cycle of development. 1
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==H. Summary and Conclusion==
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The one fact of maturation that stands out with perfect clearness and certainty amid all the controversies surrounding it is a reduction of the number of chromosomes in the ultimate germ-cells to one-half the number characteristic of the somatic cells. It is equally clear that this reduction is a preparation of the germ-cells for their subsequent union, and a means by which the number of chromosomes is held constant in the species. With a few exceptions the first indication of the numerical reduction appears through the segmentation of the spiremethread, or the resolution of the nuclear reticulum, into a number of masses one-half that of the somatic chromosomes. In nearly all higher animals this process first takes place two cell-generations before the formation of the definitive germ-cells, and the process of reduction is completed by two rapidly succeeding " maturation-divisions," giving rise to four cells, all of which become functional in the male, while in the female only one becomes the cgg 9 while the other three — the polar bodies or their analogues — are cast aside. During these two divisions each of the original chromatin-masses gives rise to four chromosomes, of which each of the four daughter-cells receives one ; hence, each of the latter receives one-half the somatic number of chromosomes. In the higher plants, however, the two maturationdivisions are followed by a number of others, in which the reduced number of chromosomes persists, a process most strikingly shown in the pteridophytes, where a separate sexual generation (prothallium) thus arises, all the cells of which show the reduced number.
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1 For more recent literature on this subject see Meves, Zelltheilung, in Merkel and Bonnet's Ergebnisse, VIII., 1898.
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Two general types of maturation may be distinguished according to the manner in which the primary chromatin-masses divide. In one, typically represented by Ascaris and the arthropods, each of these masses divides into four to form a tetrad, thus preparing at once for two rapidly succeeding divisions, which are not separated by a reconstruction of the daughter-nuclei during an intervening resting period. In the other, examples of which are given by the flowering plants and the spermatogenesis of the Amphibia, no true tetrads are formed, the primary chromatin-masses dividing separately for each of the maturation-divisions, which are separated by a period in which the nuclei regress toward the resting state, though often not completely returning to the reticular condition. These two types differ, however, only in degree, and with few exceptions they agree in the fact that during the prophases of the first division the chromatin-bodies assume the form of rings, the mitosis thus being of the heterotypical form, and each ring having the prospective value of four chromosomes.
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Thus far the phenomena present no difficulty, and they give us a clear view of the process by which the numerical reduction of the chromosomes is effected. The confusion of the subject arises, on the one hand, from its complication with theories regarding the individuality of the chromosomes and the functions of chromatin in inheritance, on the other through conflicting results of observation on the mode of tetrad-formation and the character of the maturation-divisions. Regarding the latter question nearly all observers are now agreed that one of these divisions, usually the first, is a longitudinal or equationdivision, essentially like that occurring in ordinary mitosis. The main question turns upon the other division, which has been shown in some cases to be transverse and not longitudinal, and thus separates what were originally different regions of the spireme-thread or nuclear substance. The evidence in favour of such a division seems at present well-nigh demonstrative in the case of insects and copepods, and hardly less convincing in the turbellarians, annelids, and mollusks. On the other hand, both divisions are regarded as longitudinal by most of those who have investigated the phenomena in Ascaris and in the vertebrates, and by some of the most competent investigators of the flowering plants.
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The evidence as it stands is so evenly balanced that the subject is hardly yet ripe for discussion. The principle for which Weismann contended in his theory of reducing division has received strong support in fact ; yet should it be finally established that numerical reduction may be effected either with or without transverse division, as now seems probable, not only will that theory have to be abandoned or wholly remodelled, but we shall have to seek a new basis for the interpretation of mitosis in general. Weismann's theory is no doubt of a highly artificial character; but this should not close our eyes to the great interest of the problem that it attempted to solve.
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The existing contradiction of results has led to the opinion, expressed by a number of recent writers, that the difference between longitudinal or transverse division is of minor importance, and that the entire question of reduction is a barren one. This opinion fails to reckon with the facts on which rests the hypothesis of the individuality of chromosomes (Chap. VI.); but these facts cannot be left out of account. We must find a common basis of interpretation for them and for the phenomena of reduction ; yet how shall we reconcile them with reduction by longitudinal division only ? I cannot, therefore, share the opinion that we are dealing with a barren problem. The peculiarities of the maturation-mitoses are obviously correlated in some way with the numerical reduction, and the fact that they differ in so many ways from the characters of ordinary mitosis gives ground to hope that their exhaustive study will throw further light not only on the reduction-problem itself but also on mitosis in general and on still wider problems relating to the individuality of the chromosomes and the morphological organization of the nucleus. It is indeed very probable that Weismann's theory is but a rude attempt to attack the problem, and one that may prove to have been futile. The problem itself cannot be ignored, nor can it be dissociated from the series of kindred problems of which it forms a part
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==Literature==
 +
* See also Literature, IV., p. 231.
 +
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Van Beneden, E. — Recherches sur la maturation de Toeuf, la fecondation et la division
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cellulaire: Arch. Biol. % IV. 1883. Boveri, Th. — Zellenstudien, I., III. Jena, 1887-90. See also " Befruchtung "
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(List IV.). Brauer, A. — Zur Kenntniss der Spermatogenese von Ascaris megalocephala : Arch.
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mik. Anat., XLIL 1893. Id. — Zur Kenntniss der Reifung der parthenogenetisch sich entwickelnden Eies von
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Artemia Salina: Arch. mik. Aflat., XLI1I. 1894. Guignard, L. — Le deVeloppement du pollen et la reduction chromatique dans le
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Naias: Arch. Anat. Mic. % II. 1899. (Full literature on reduction in plants.) Griffin, B. B. — See Literature, IV. Hacker, V. — Die Vorstadien der Eireifung (General Review) : Arch. mik. Anat. t
 +
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XLV. 2. 1895. Id. — Ober weitere Obereinstimmungen zwischen den Fertpflanzungsvorgangen der
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Thiere und Pflanzen : Biol. Centralb.s XVII. 1897. Id. — Cber vorbereitende Theilungsvorgange bei Thieren und Pflanzen : Verh.
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deutsch. Zool. Ges. % VIII. 1898. Id. — Die Reifungserscheinungen : Merkel und Bonnets Ergebnisse. VIII. 1898. Hertwig, 0. — Vergleich der Ei- und Samenbildung bei Nematoden. Eine Grund lage flir cellulare Streitfragen : Arch. mik. Anat., XXX V I. 1890. Mark, E. L. — (See List IV.) Peter, K. — Die Bedeutung der Nahrzellen im Hoden : Arch. mik. Anat., LI II. 1898.
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Platner, 6. — Uber die Bedeutung der Richtungskorperchen : Biol. Centralb., VIII. 1889.
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Vom Rath, 0. — Zur Kenntniss der Spermatogenese von Gryllotalpa vulgaris: Arch, mik. Anat.s XL. 1892.
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Id. — Neue Beitrage zur Frage der Chromatinreduktion in der Samen- und Eireife : Arch. mik. A fiat., XLVI. 1895.
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Riickert, J. — Die Chromatinreduktion der Chromosomenzahl im Entwicklungsgang der Organismen : Ergebn. d. Anal. u. Entwick.* III. 1893 (1894).
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Strasburger, E. — (Jber periodische Reduktion der Chromosomenzahl im Entwicklungsgang der Organismen : Biol. Centralb., XIV. 1894.
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Id. — Reduktionstheilung, Spindelbildung, etc. : Jena, Fischer, 1900.
  
  

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

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

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

" Es kuromt also in der Generationenreihe der Keimzelle irgendwo zu einer Reduktion der ursprunglich vorhandenen Chromosomenzahl auf die Halfte, and diese ZaMtn-reduktion ist demnach nicht etwa nur ein theoretisches Postulat, sondern eine Thatsache."

Boveri.

  • Zcllensiudien, III., p. 62. * Cf. Figs. 97, 116.

Van Beneden's epoch-making discovery that the nuclei of the conjugating germ-cells contain each one-half the number of chromosomes characteristic of the body-cells has now been extended to so many plants and animals that it may probably be regarded as a universal law of development. The process by which the reduction in number is effected, forms the most essential part of the phenomena of maturation by which the germ-cells are prepared for their union. No phenomena of cell-life possess a higher theoretical interest than these. For, on the one hand, nowhere in the history of the cell do we find so unmistakable and striking an adaptation of means to ends or one of so marked a prophetic character, since maturation looks not to the present but to the future of the germ-cells. On the other hand, the chromatin-reduction suggests questions relating to the morphological constitution of nucleus and chromatin, which have an important bearing on all theories of the ultimate structure of living matter and now stand in the foreground of scientific discussion among the most debatable and interesting of biological problems.

Two fundamentally different views have been held of the manner in which the reduction is effected. The earlier and simpler view, which was suggested by Van Beneden and adopted in the earlier works of Weismann, Boveri, and others, assumed an actual degeneration or casting out of half of the chromosomes during the growth of the germ-cells — a simple and easily intelligible process. Later researches conclusively showed, however, that this view cannot be sustained, and that reduction is effected by a rearrangement and redistribution of the nuclear substance without loss of any of its essential constituents. It is true that a large amount of chromatin is lost during the growth of the egg. 2 It is nevertheless certain that this loss is not directly connected with the process of reduction ; for, as Hertwig and others have shown, no such loss occurs during spermatogenesis, and even in the oogenesis the evidence is clear that an explanation must be sought in another direction. The attempts to find such an explanation have led to some of the most interesting researches of modern cytology ; and though only partially successful, they have raised many new questions which promise to give in the end a deeper insight into some of the fundamental questions of cell-morphology. For this reason they deserve careful consideration, despite the fact that taken as a whole the subject still remains an unsolved riddle in the face of which we can only return again and again to Boveri's remark that whatever be its theoretical interpretation the numerical reduction of the chromosomes is itself not a theory but a fact.



Pig. 114. — Formation of the polar bodies before entrance of the spermatozoon, as seen in the living ovarian egg of the sea-urchin, Toxopneustes (X 365).

A. Preliminary change of form in the germinal vesicle. B. The first polar body formed, the second forming. C. The ripe egg, ready for fertilization, after formation of the two polar bodies (p. b. 1, 2) ; e. the egg-nucleus. In this animal the first polar body fails to divide. For its division see Fig. 89.


A. General Outline

The general phenomena of maturation fall under two heads : viz. oogenesis, which includes the formation and maturation of the ovum, and spermatogenesis, comprising the corresponding phenomena in case of the spermatozoon. Recent research has shown that maturation conforms to the same type in both sexes, which show as close a parallel in this regard as in the later history of the germ-nuclei. Stated in the most general terms, this parallel is as follows : l In both sexes the final reduction in the number of chromosomes is effected in the course of the last two cell-divisions, or maturation-divisions, by which the definitive germ-cells arise, each of the four cells thus formed having but half the usual number of chromosomes. In the female but one of the four cells forms the " ovum " proper, while the other three, known as the polar bodies, are minute, rudimentary, and incapable of development (Figs. 89, 97, 114). In the male, on the other hand, all four of the cells become functional spermatozoa. This difference between the two sexes is probably due to the physiological division of labour between the germ-cells, the spermatozoa being motile and very small, while the egg contains a large amount of protoplasm and yolk, out of which the main mass of the embryonic body is formed. In the male, therefore, all of the four cells may become functional ; in the female the functions of development have become restricted to but one


1 The parallel was first clearly pointed out by Platner in 1889, and was brilliantly demonstrated by Oscar Hertwig in the following year.



Fig- 115. — Diagram showing the genesis of the egg. [After BOVERI.]

of the four, while the others have become rudimentary (cf. p. 124). The polar bodies are therefore not only rudimentary cells (Giard, '76), but may further be regarded as abortive eggs — a view first put forward by Mark in 1881, and ultimately adopted by nearly all investigators. 1 The evidence is steadily accumulating that reduction is accomplished by two maturation-divisions throughout the animal kingdom, even in the unicellular forms ; though in certain Infusoria an additional division occurs, while in some other Protozoa only one maturation-division has thus far been made out. Among plants, also, two maturation


1 A beautiful confirmation of this view is given by Francottes's ('97) observations on a turbellarian, Prostheceraus, The first polar body is here often abnormally large, all gradations having been observed from the normal size up to cells nearly as large as the egg itself. Such polar bodies are occasionally fertilized and develop into small gastrulas, first forming a single polar body like the second polar body of the egg. Here, therefore, two of the four cells are exceptionally capable of development. It may be added that Fol long ago observed the penetration of the small polar bodies by spermatozoa in the echinoderms; and this has been more recently observed by Kostanecki in mollusks.


236 REDUCTION OF THE CHROMOSOMES

divisions occur in all the higher forms (Muscineae, pteridophytes, and phanerogams), and in some, at least, of the lower ones. Here, however, the phenomena are complicated by the fact that the two divisions do not as a rule give rise directly to the four sexual germ-cells, but to four asexual spores which undergo additional divisions before the definitive germ-cells are produced. In the flowering plants there are only a few such divisions, which give rise to structures within the pollen-tube or embryo-sac. In the archegoniate cryptogams, on the other hand, each spore gives rise, by repeated divisions, to a "sexual generation " (prothallium, etc.) that intervenes between the process of reduction and that of fertilization. The following account deals primarily with reduction in animals, the plants being afterward considered.

I. Reduction in the Female. Formation of tfie Polar Bodies

As described in Chapter III., the egg arises by the division of cells descended from the primordial egg-cells of the maternal organism, and these may be differentiated from the somatic cells at a very early period, sometimes even in the cleavage-stages. As development proceeds, each primordial cell gives rise, by division of the usual mitotic type, to a number of descendants known as oogonia (Fig. 115), which are the immediate predecessors of the ovarian egg. At a certain period these cease to divide. Each of them then grows to form an ovarian egg, its nucleus enlarging to form the germinal vesicle, its cytoplasm becoming more or less laden with food-matters (yolk or deutoplasm), while egg-membranes may be formed around it. The ovum may now be termed the oocyte (Boveri) or ovarian egg.

In this condition the egg-cell remains until near the time of fertilization, when the process of maturation proper — i.e. the formation of the polar bodies — takes place. In some cases, e.g. in the sea-urchin, the polar bodies are formed before fertilization, while the Ggg is still in the ovary. More commonly, as in annelids, gasteropods, nematodes, they are not formed until after the spermatozoon has made its entrance ; while in a few cases one polar body may be formed before fertilization and one afterward, as in the lamprey-eel, the frog, and Amphioxus} In all these cases the essential phenomena are the same. Two minute cells are formed, one after the other, near the upper or animal pole of the ovum (Figs. 97, 1 16); and in many cases the first of these divides into two as the second is formed (Fig. 89).

A group of four cells thus arises, namely, the mature egg, which gives rise to the embryo, and three small cells or polar bodies which take no part in the further development, are discarded, and soon die

1 Cf. p. 189.


GENERAL OUTLINE


237


without further change. The egg-nucleus is now ready for union with the sperm-nucleus.




, ' r-: \



D



E H

Fig. xx6. — Diagrams showing the essential facts in the maturation of the egg. The somatic number of chromosomes is supposed to be four.

A. Initial phase; two tetrads have been formed in the germinal vesicle. B. The two tetrads have been drawn up about the spindle to form the equatorial plate of the first polar mitotic figure. C. The mitotic figure has rotated into position, leaving the remains of the germinal vesicle at g.v. D. Formation of the first polar body; each tetrad divides into two dyads. E. First polar body formed; two dyads in it and in the egg. F. Preparation for the second division. G. Second polar body forming and the first dividing; each dyad divides into two single chromosomes. H. Final result; three polar bodies and the egg-nucleus (9), each containing two single chromosomes (half the somatic number) ; c. the egg-centrosome which now degenerates and is lost


238 REDUCTION OF THE CHROMOSOMES

A study of the nucleus during these changes brings out the following facts. During the multiplication of the oogonia the number of chromosomes is the same as that occurring in the division of the somatic cells, and the same number enters into the formation of the chromatic reticulum of the germinal vesicle. During the formation of the polar bodies this number becomes reduced to one-half, the nucleus of each polar body and the egg-nucleus receiving the reduced number. In some manner, therefore, the formation of the polar bodies is connected with the process by which the reduction is effected. The precise nature of this process is, however, a matter which has been certainly determined in only a few cases.

We need not here consider the history of opinion on this subject further than to point out that the early observers, such as Purkinje, Von Baer, Bischoff, had no real understanding of the process and believed the germinal vesicle to disappear at the time of fertilization. To Butschli ('76), Hertwig, and Giard ('76, '77) we owe the discovery that the formation of the polar bodies is through mitotic division, the chromosomes of the equatorial plate being derived from the chromatin of the germinal vesicle. 1 In the formation of the first polar body the group of chromosomes splits into two daughter-groups, and this process is immediately repeated in the formation of the second without an intervening reticular resting stage. The egg-nucleus therefore receives, like each of the polar bodies, one-fourth of the mass of chromatin derived from the germinal vesicle.

But although the formation of the polar bodies was thus shown to be a process of true cell-division, the history of the chromosomes was found to differ in some very important particulars from that of the tissue-cells. The essential facts, which were first carefully studied in Ascaris by Van Beneden ('83, '87), and especially by Boveri ('87, 1 ), are in a typical case as follows (Figs. 116, 117): As the egg prepares for the formation of the first polar body, the chromatin of the germinal vesicle groups itself in a number of masses, each of which splits up into a group of four bodies united by linin-threads to form a " quadruple group" or tetrad (Vierergruppe). The number of tetrads is always one-half the usual number of chromosomes. Thus in Ascaris {mcgaloccphala, bivalcns) the germinal vesicle gives rise to two tetrads, the normal number of chromosomes in the earlier divisions being four ; in the mole-cricket there are six tetrads, the somatic number of chromosomes being twelve ; in Cyclops the respective numbers are twelve and twenty-four (one of the most frequent cases); while in Artemia there are eighty-four tetrads and one hundred and sixty 1 The early accounts asserting the disappearance of the germinal vesicle were based on the fact that in many cases only a small fraction of the chromatic network gives rise to chromosomes, the remainder disintegrating and being scattered through the yolk.


GENERAL OUTLINE


eight somatic chromosomes — the highest number thus far accurately counted. As the first polar body forms, each of the tetrads is halved to form two double groups, or dyads, one of which remains in the egg



Fig. 117 A. The egg with the spermaloioiin just e shaped tetrads (only one clearly shown), the been (bur. B. The tetrads seen in profile.


C. First polar body formed, contain) single chromosomes, completing the 1


dyads. H. I. The dyads rotatii lg. A'. Each dyad has divided ir er stages see Fig. 90.)


240


REDUCTION OF THE CHROMOSOMES


while the other passes into the polar body. Both the egg and the first polar body therefore receive each a number of dyads equal to one-half the usual number of chromosomes. The egg now proceeds at once to the formation of the second polar body without previous reconstruction of the nucleus. Each dyad is halved to form two single chromosomes, one of which, again, remains in the egg while its sister passes into the polar body. Both the egg and the second polar body accordingly receive two single chromosomes (one-half the usual number), each of which is one-fourth of an original tetrad group. From the two remaining in the egg a reticular nucleus, much smaller than the original germinal vesicle, is now formed. 1

Primordial germ-cell.


Spermatogonia.


Primary spermatocyte.

Secondary spermatocytes.

Spermatids. Spermatozoa.



Division-period (the number of divisions is much greater).


• Growth-period.


M aturation-period.


Fig. zz8. — Diagram showing the genesis of the spermatozoon. [After Boveri.]

Essentially similar facts have now been determined in a considerable number of animals, though, as we shall presently see, tetradformation is not of universal occurrence, nor is it always of the same type. For the moment we need only point out that the numerical reduction of chromatm-masses takes place before the polar bodies are actually formed, through processes which determine the number of tetrads within the germinal vesicle. The numerical reduction is therefore determined in the grandmother-cell of the egg. The actual divisions by which the polar bodies are formed merely distribute the elements of the tetrads.


1 It is nearly certain that the division of the first polar body (which, however, may be omitted) is analogous to that by which the second is formed, i.e. each of the dyads is similarly halved. Cf. Griffin, '99.


GENERAL OUTLINE 24 1

2. Reduction in the Male. Spermatogettesis

The researches of Platner ('89), Boveri, and especially of Oscar Hertwig ('90, 1) have demonstrated that reduction takes place in the male in a manner almost precisely parallel to that occurring in the female. Platner first suggested ('89) that the formation of the polar bodies is directly comparable to the last two divisions of the sperm mother-cells (spermatocytes). In the following year Boveri reached the same result in Ascaris, stating his conclusion that reduction in the male must take place in the " grandmother-cell of the spermatozoon, just as in the female it takes place in the grandmother-cell of the egg," and that the egg-formation and sperm-formation really agree down to the smallest detail ('90, p. 64). Later in the same year appeared Oscar Hcrtwig's splendid work on the spermatogenesis of Ascaris, which established this conclusion in the most striking manner. Like the ova, the spermatozoa are descended from primordial germ-cells which by mitotic division give rise to the spermatogonia from which the spermatozoa are ultimately formed (Fig. 118). Like the oogonia, the spermatogonia continue for a time to divide with the usual (somatic) number of chromosomes, i.e. four in Ascaris mcgalocephala bivalens. Ceasing for a time to divide, they now enlarge considerably to form spermatocytes, each of which is morphologically equivalent to an unripe ovarian ovum, or oocyte. Each spermatocyte finally divides twice in rapid succession, giving rise first to two daughter-spermatocytes and then to four spermatids, each of which is directly converted into a single spermatozoon. The history of the chromatin in these tzvo divisions is exactly parallel to that in the formation of the polar bodies (Figs. 119, 120). From the chromatin of the spermatocyte are formed a number of tetrads equal to one-half the usual number of chromosomes. Each tetrad is halved at the first division to form two dyads which pass into the respective daughter-spermatocytes. At the ensuing division, which occurs without the previous formation of a resting reticular nucleus, each dyad is halved to form two single chromosomes which enter the respective spermatids (ultimately spermatozoa). From each spermatocyte, therefore, arise four spermatozoa, and each sperm-nucleus receives half the usual number of single chromosomes. The parallel with the egg-reduction is complete.

These facts leave no doubt that the spermatocyte is the morphological equivalent of the oocyte or immature ovarian egg, and that the group of four spermatozoa to which it gives rise is equivalent to the ripe egg plus the three polar bodies. Hertwig was thus led to the following beautifully clear and simple conclusion : " The polar bodies are abortive eggs which are formed by a final process of


242


REDUCTION OF THE CHROMOSOMES


division from the egg-mother-cell (oocyte) in the same manner as the spermatozoa are formed from the sperm-mother-cell (spermatocyte). But while in the latter case the products of the division are all used as functional spermatozoa, in the former case one of the products



Pig. 119. — Diagrams showing the essential facts of reduction in the male. The somatic number of chromosomes is supposed to be four.

A. Ii. Division of one of the spermatogonia, showing the full number (four) of chromosomes. C. Primary spermatocyte preparing for division ; the chromatin forms two tetrads. D. E. F. First division to form two secondary spermatocytes each of which receives two dyads. G. H. Division of the two secondary spermatocytes to form four spermatids. Each of the latter receives two single chromosomes and a centrosome which passes into the middle-piece of the spermatozoon.


1

of the egg-mother-cell tecomes the egg, appropriating to itself the entire mass of the yolk at the cost of the others which persist in rudimentary form as the polar bodies." l

1 *9o> i> P* I2 & '


GENERAL OUTLINE 243

3. Weismanris Interpretation of Reduction

Up to this point the facts are clear and intelligible. Before coming to closer quarters with them it will be useful to make a digression in order to consider some of the theoretical aspects of reduction ; though the reader must be warned that this will lead us into very uncertain ground traversed by a labyrinth of conflicting hypotheses from which no exit has yet been discovered.

The process of reduction is very obviously a provision to hold constant the number of chromosomes characteristic of the species ; for if it did not occur, the number would be doubled in each succeeding generation through union of the germ-cells. 1 A number of writers have contented themselves with this simple interpretation, Oscar Hertwig, for example, regarding reduction as " merely a process to prevent a summation through fertilization of the nuclear mass and of the chromatic elements." 2 A moment's reflection reveals the entire inadequacy of such an explanation. As far as the chromatin-mass is concerned, it does not agree with the facts ; for in reduction with tetrad-formation the chromatin-mass is reduced not to one-half, but to one-fourth. That reduction must mean more than mere mass-reduction is moreover proved by the fact that the bulk of the nucleus may enormously increase or decrease at different periods in the same cell, irrespective of the number of chromosomes. The real problem is why the number of chromosomes should be held constant. The

1 Of the many earlier attempts to interpret the meaning of the polar bodies, we need only consider at this point the very interesting suggestion of Minot ('77), afterward adopted by Van Beneden ('83), that the ordinary cell is hermaphrodite, and that maturation is for the purpose of producing a unisexual germ-cell by dividing the mother-cell into its sexual constituents, or "genoblasts." Thus, the male element is removed from the egg in the polar bodies, leaving the mature egg a female. In like manner he believed the female element to be cast out during spermatogenesis (in the " Sertoli cells "), thus rendering the spermatozoa male. By the union of the germ-cells in fertilization, the male and female elements are brought together so that the fertilized egg or oosperm is again hermaphrodite or neuter. This ingenious view was independently advocated by Van Beneden in his great work on Ascaris ('83). A fatal objection to it, on which both Strasburger and Weismann have insisted, lies in the fact that male as well as female qualities are transmitted by the egg-cell, while the sperm-cell also transmits female qualities. The germ-cells are therefore non-sexual. The researches of many observers show, moreover, that all of the four spermatids derived from a spermatocyte become functional spermatozoa. Minot's hypothesis must, therefore, in my opinion, be abandoned.

Balfour doubtless approximated more nearly to the truth when he said, " In the formation of the polar cells part of the constituents of the germinal vesicle, which are requisite for its functions as a complete and independent nucleus, is removed to make room for the supply of the necessary parts to it again by the spermatic nucleus" ('80, p. 62). He fell, however, into the same error as Minot and Van Beneden in characterizing the germ-nuclei as " male " and "female"; and, as shown at pages 194, 353, it has been found that a single germnucleus is able to carry out development of an embryo without union with another.

2 '90, I, p. 1 12. Cf. Hartog, '91, p. 57.


„, REDUCTION OF THE CHROMOSOMES

o. k ,. :icaiiin^ of the phenomena was first seriously considered by »c-«a.i.i;i in his essays of 1885 and 1887 ; and, although his conclun-'o* *«•« 01 j. highly speculative character, they nevertheless gave so



ffg. i»o. — Reduction in the spermatogenesis of Aicaril mtgafocephiila.vai.6ii aitn

A-tl. Successive stages in the division of the primary sperm atocyte. The onxin.n: telicul umkrxocs a very early division of the chromatin -pa nulrs which then form a doubly split spim HuviUl, /'. This shortens ((.'),. ind breaks in two to form (he two tetrads (D in profile. £ viev endwise). F. G. H. First division to form two secondary spermatocytes, each rrcciring t«o dya /. Secondary spermatocyte. J, K. The same dividing. L. Two resulting spermatids, each « 


great a stimulus to the study of the entire problem that his views deserve special attention. Weismann's interpretation was based on a remarkable paper published by Wilhelm Roux in 1883,' in which are

1 For division of the spermatogonia see Fig. 55; for the corresponding phf univalent see Fig. 148.

Ubtr Jit BiJeutung Jer Kirnthtilungsfigurin.


GENERAL OUTLINE 245

developed certain ideas which afterward formed the foundation of Weismann's whole theory of inheritance and development. Roux argued that the facts of mitosis are only explicable under the assumption that chromatin is not a uniform and homogeneous substance, but differs qualitatively in differerit regions of the nucleus ; that the collection of the chromatin into a thread and its accurate division into two halves is meaningless unless the chromatin in different regions of the thread represents different qualities which are to be divided and distributed to the daughter-cells according to some definite law. He urged that if the chromatin were qualitatively the same throughout the nucleus, direct division would be as efficacious as indirect, and the complicated apparatus of mitosis would be superfluous. Roux and Weismann, each in his own way, subsequently elaborated this conception to a complete theory of inheritance and development, but at this point we may confine our attention to the views of Weismann. The starting-point of his theory is the hypothesis of De Vries that the chromatin is a congeries or colony of invisible self-propagating vital units or biophorcs somewhat like Darwin's "gemmules" (p. 12), each of which has the power of determining the development of a particular quality. Weismann conceives these units as aggregated to form units of a higher order known as " determinants," which in turn are grouped to form " ids," each of which, for reasons that need not here be specified, 1 is assumed to possess the complete architecture of the germ-plasm characteristic of the species. The "ids" finally, which are identified with the visible chromatin-granules, are arranged in linear series to form " idants " or chromosomes. It is assumed further that the " ids " differ slightly in a manner corresponding with the individual variations of the species, each chromosome therefore being a particular group of slightly different germ-plasms and differing qualitatively from all the others.

We come now to the essence of Weismann's interpretation. The end of fertilization is to produce new combinations of variations by the mixture of different ids. Since, however, their number, like that of the chromosomes which they form, is doubled by the union of two germ-nuclei, an infinite complexity of the chromatin would soon arise did not a periodic reduction occur. Assuming, then, that the " ancestral germ-plasms" (ids) are arranged in a linear series in the spiremethread or the chromosomes derived from it, Weismann ventured the prediction ('87) that two kinds of mitosis would be found to occur. The first of these is characterized by a longitudinal splitting of the thread, as in ordinary cell-division, " by means of which all the ancestral germ-plasms are equally distributed in each of the daughter-nuclei after having been divided into halves." This form of division, which

1 Cf. the Germ-plasm, p. 60.


246 REDUCTION OF THE CHROMOSOMES

he called equal division (Aequationstheilung), was then a known fact. The second form, at that time a purely theoretical postulate, he assumed to be of such a character that each daughter-nucleus should receive only half the number of ancestral germ-plasms possessed by the mother-nucleus. This he termed a reducing division (Reduktionstheilung), and suggested that this might be effected either by a transverse division of the chromosomes, or by the elimination of entire chromosomes without division. 1 By either method the number of " ids M would be reduced ; and Weismann argued that such reducing divisions must be involved in the formation of the polar bodies, and in the parallel phenomena of spermatogenesis.

The fulfilment of Weismann's prediction is one of the most interesting results of recent cytological research. It has been demonstrated, in a manner which seems to be incontrovertible, that the reducing divisions postulated by Weismann actually occur, though not precisely in the manner conceived by him. Unfortunately for the general theory, however, transverse divisions have been certainly determined in only a few types, while in others, of which Ascaris is the best-known example, the facts thus far known seem clearly opposed to the assumption. On the whole, the evidence of reducing divisions, i.e. such as involve a transverse and not a longitudinal division of the chromatin-thread, has steadily increased ; but it remains quite an open question whether they have the significance attributed to them by Weismann.

B. Origin of the Tetrads

I. General Sketch

In considering the origin of the tetrads or their equivalents, it should be borne in mind that true tetrad-formation, as described above, has only been certainly observed in a few groups (most clearly in the nematodes and arthropods). But even in cases where the chromatin does not condense into actual tetrads these bodies are represented by chromosomes in the form of rings, crosses, and the like, which are closely similar, and doubtless equivalent, to those from which actual tetrads arise, and present us with the same problems. With a few apparent exceptions, described hereafter, the tetrads of their equivalents always arise by a double division of a single primary chromatin-rod or mass. Nearly all observers agree further that the number of primary rods at their first appearance in the germinal vesicle or in the spermatocyte-nucleus is one-half the usual number of chromosomes, and that this numerical reduction is due to the fact that the spireme-thread segments into one-half the

1 Essay VI., p. 375.


ORIGIN OF THE TETRADS


247


/



5 6

7

a



usual number of pieces. Apparently, however, there are two radically different types of tetrad-formation as follows.

In the first type the tetrad arises by one longitudinal and one transverse division of each primary ckromatin-rod, the latter effecting the reduction demanded by _

Weismann , shypothesis(Fig. /\ £j Ls U

I2i,l). To give the usual graphic representation, let us, for the sake of discussion, assume the somatic number of chromosomes to be four, designating the spireme-thread as a b c d, each letter representing a chromosome, each of which we may in turn assume to consist of a series of four granulesor "ids "(Fig. 121). In ordinary mitosis the spireme would segment into a — b — c — d t which then would divide lengthwise to form pairs of identical sister

, abed chromosomes — •

abed To form the tetrad, on the other hand, the spireme first segments into two rods ab and cd, each of which, in view of its subsequent history, may be regarded as

bivalent, representing tWO Pig. 121. — Diagrams of tetrad-formation; I, with

chromosomes United end to one transverse an d one longitudinal division (copepod

, __ .. type); II. with two longitudinal divisions (Ascaris type).

end (VOm Rath, RUckert, A _ D succes sive stages; chromatin-granules num Hacker). Eagh of these beredfrom 1 to 8. The two types diverge at C. In D

divides once longitudinally, giving the identical pairs or

dyads — , and once transversely, giving the tetrads

ab cd a


U


8



ab ab


ab ab


the granules of each constituent of the tetrad fuse to form a homogeneous sphere.


a


b b


d m d


Inspection of Fig. 121, 1, shows that through the second or transverse division, each member of the tetrad receives only half the number of ids contained in the original segment. This number, four, is the same as that assumed for a single chromosome ; and, since each of the two tetrads contributes one chromosome to the germ-cell, the latter receives


248 REDUCTION OF THE CHROMOSOMES

but half the usual number both of chromosomes and of ids. This mode of tetrad-formation has been most clearly demonstrated in insects and copepods, and an equivalent process occurs also in mollusks, annelids, turbellarians, and some other animals, as described beyond. In the second type, illustrated especially by Ascaris, the tetfad is apparently formed by two longitudinal divisions of each primary chromatin-rod, and no reducing division occurs. If, therefore, we adopt the same terminology as before, we have first ab and cd, then

ab cd

ab cd


ab cd j r n ab — , and finally


cd

— , by two longitudinal divisions. In

cd


ab cd ab

this case, according to Brauer's careful studies, each chromatin-granule (" id") divides at each longitudinal division of the primary rod. The four chromosomes of the tetrad are therefore exactly equivalent, being derived from the same region of the spireme-thread, and containing the undiminished number of " ids " (Fig. 121, II).

The contradiction may be stated in a different way. In the first type of tetrad formation, the number both of granules and of chromosomes is first doubled (i.e. in the assumed case, through the formation of two tetrads, each consisting of four chromosomes, or eight in all), and then reduced to half that number by the two successive maturation-divisions. In the second type, on the other hand, the number of chromosomes is likewise doubled, but that of the granules is quadrupled, so that, although in both types the two maturation-divisions reduce the number of chromosomes to one-half, only in the first type do they reduce the number of granules or " ids," as Weismann's hypothesis demands. We must therefore distinguish sharply between the reduction of the chromosomes and that of the "ids." The former is primarily effected by the segmentation of the primary spiremethread, or the resolution of the nuclear reticulum, into one-half the usual number of segments (i.e. the "pseudo-reduction" of Riickert); and here the real secret of the reduction of the chromosomes lies. The reduction of the "ids," if they have any real existence, is a distinct, and as yet unsolved, question.

2. Detailed Evidence

We may now consider some of the phenomena in detail, though the limits of this work will only allow the consideration of a few typical cases.

(a) Tetradfonnation with one Longitudinal and one Transverse Division. — In many of the cases of this type the tetrads arise from ring-shaped bodies which are analogous to the ring-shaped chromosomes occurring in heterotypical mitosis (p. 86). First observed by HenkingCgi) in Pyrrhocoris, tetrad-origin of this type has since been found in other insects by Vom Rath, Toyama, Paulmier, and others,


ORIGIN OF THE TETRADS


249


in copepods by Riickert, Hacker, and Vom Rath, in pteridophytes by Calkins and Osterhout, in the onion, Allium, by Ishikawa, and in various other forms where their history has been less clearly made out. The genesis of the ring was first determined by Vom Rath in the mole cricket {Gryllotalpa, '92), and has been thoroughly elucidated by the later work of Riickert ('94), Hacker ('95, 1), and Paulmier ('99). All these observers have reached the same conclusion ; namely, that the ring arises by the longitudinal splitting of a primary chro matin-rod, the two halves remaining united by their ends, and opening out to form a ring. The ring-formation is, in fact, a form of



ontogenesis of the mole-cricket


Pig. 132. — Origin of the tetrads by ting- formation Gryllotalpa. [VOM RATH.]

somes united end to end anrl longitudinally split cxct-pl at Ihc free ends. B. C. Opening out of (he double rods to form rings. I), Concentration of the rings. E. The rings broken up into letrads. /'. First division -figure established.

heterotypical mitosis (p. 86). The breaking of the ring into four parts involves, first, the separation of these two halves (corresponding with the original longitudinal split), and second, the transverse division of each half, the latter being the reducing division of Weismann. The number of primary rods, from which the rings arise, is one-half the somatic number. Hence each of them is conceived by Vom Rath, Hacker, and Riickert as bivalent or double ; i.e. as representing two chromosomes united end to end. This appears with the greatest clearness in the spermatogenesis of Gryllotalpa (Fig. 122). Here the spireme-thread splits lengthwise before its segmentation into rods. It then divides transversely to form six double rods (half the usual number of chromosomes), which open out to form six closed rings. These become small and thick, break each into four parts, and thus



Fif. 113.— Formation of the tetrads and polar bodies in Cyclops, slightly si full number of tetrads is no! shown.) [Rt'c:KKKT.]

A. Germinal vesicle containing eight longitudinally split chromatin-rods (half the number). II. Shortening of the rods; transverse division (to furm the tetrads) in C. Position of the tetrads in the first polar spindle, the longitudinal split horizontal, phase: longitudinal divisions of the tetrads. E. The first polar liody formed; seco spindle with the eight dyads in position fur the ensuing division, which will be a trait


9. Ana


give rise to six typical tetrads. An essentially similar account of the ring-formation is given by Vom Rath in Euchceta and Calanus, and by Riickert in Hetcrocopc and Diaptomus.

That the foregoing interpretation of the rings is correct, is beautifully demonstrated by the observations of Hacker, and especially of


Ruckert, on a number of other copepods {Cyclops, Canthocamptus\ in which rings are not formed, since the splitting of the primary chromatin-rods is complete. The origin of the tetrads has here been traced with especial care in Cyclops strcnuus y by Ruckert C94), whose observations, confirmed by Hacker, are quite as convincing as those



Pig. 134. — Diagrams of various modes of tetrad-formation. [HACKER.]

a. Common starting-point, a double spireme-thread in the germinal vesicle ; d. common result, the typical tetrads ; b. c. intermediate stages : at the left the ring-formation (as in Diaptomus, Gryllotalpa, Heterocopt) ; middle series, complete splitting of the rods (as in Cyclops according to Ruckert, and in Canthocamptus) ; at the right by breaking of the V-shaped rods (as in Cyclops strenuus, according to Hacker.


of Brauer on Ascaris, though they led to a diametrically opposite result.

The normal number of chromosomes is here twenty-two. In the germinal vesicle arise eleven threads, which split lengthwise (Fig. 123), and finally shorten to form double rods, manifestly equivalent to the closed rings of Diaptomus. Each of these now segments transversely to form a tetrad group, and the eleven tetrads then place themselves in the equator of the spindle for the first polar body (Fig. 123, C), in such a manner that the longitudinal split is transverse to the axis of the spindle. As the polar body is formed, the longitudinal halves of the tetrad separate, and the formation of the first polar body is thus demonstrated to be an "equal division" in Weismann's sense. The eleven dyads remaining in the eggs now rotate (as in Ascaris),



B. Later stage of the same, condensation and segmentation of the rings. [ROcKERT.]

C. " fyefops tlnauus," illustrating Hacker's account of the tetrad-formation from elongate double rods; a group of " accessory nucleoli." [H ACKER.]

/). Germinal vesicle of an annelid (Ophryotrocka} showing nucleolus and four chromosomes. [K.ORSCHKLT.]

so that the transverse division lies in the equatorial plane, and are halved during the formation of the second polar body. The division is accordingly a " reducing division," which leaves eleven single chromosomes in the egg. Paulmier's work on Anasa and other Hemiptera ('99) gives the same result as the above in regard to the origin of the tetrads (Figs. 126, 127). The process is, however, slightly complicated by the fact that no continuous spi rem e-th read is formed, while the rings are often bent or twisted and never open out to a circular form. They finally condense into true tetrads which are successively divided into dyads and monads by the two divisions; but it is an interesting fact that the order of division occurring in the copepods appears here to be reversed, the first division being the transverse and the second the longitudinal one — a result agreeing with Henking's earlier conclusion in the case of Pyrrochoris. Osterhout C97) and Calkins C97) independently discovered tetrads in the vascular cryptogams (Equisctutn, Pteris), and the last-named observer finds that in Pteris they may arise either from rings, as in Gryllotalpa or Heterocope, or from double rods as in Cyclops, the halves in the latter case being either parallel or forming a cross. This longitudinal split, occurring in the spireme, is followed by a transverse division by which the tetrad is formed. Tetrads having an essentially similar mode of origin are also described by Atkinson ('99) in Ariscema, and tetrad-formation is nearly approached in Allium according to Ishikawa C99). 1 These cases are considered at page 263.

Re'sume'. In all the foregoing cases the tetrads arise from a spireme which splits lengthwise, segments into one-half the somatic number of rods (each longitudinally divided) and each of the latter divides transversely to form the tetrad. When the ends of the daughter-chromosomes resulting from the longitudinal split remain united (as in insects) ring-forms result, and the earlier phases of tetradformation are thus identical with those of heterotypical mitosis. When the split is complete, so that the ends remain free, double rods result; while, if the daughter-chromosomes remain temporarily united at the middle or at the end, X-, Y-, and V-shaped figures may arise. In all these forms tetrad-formation is completed by the complete separation of the daughter-rods, the transverse division of each in the middle, and the condensation of the four resulting bodies into a quadruple mass. As will be shown in Section C (p. 258) the transverse division is in many forms delayed until after separation of the longitudinal halves. In such cases no actual tetrads are formed, though the result is the same.

(b) Second Type. Tetrad-formation zuit/t two Longitudinal Divisions. — The only accurately known case of this type is Ascaris y the object in which tetrads were first discovered by Van Beneden in 1883. Carnoy ('86, 2) reached the conclusion that the tetrads in some other nematodes (Op//iostomum t Ascaris clavata, A. lumbricoides) arose by a double longitudinal splitting of the primary chromatin-rods.

1 Vom Rath ('93, '59) has endeavoured to show that a process involving the formation of true tetrads occurs in the salamander and the frog, but the later and more accurate studies of Meves ('96) seem to leave little doubt that this was an error, and that the tetrads observed in these forms are not of normal occurrence, as Hemming ('87) had earlier concluded. €/. p. 259.



In the first of his classical celt-studies Boveri ('87, 1) reached the same result through a careful study of Ascaris megalocephala, showing that each tetrad appears in the germinal vesicle in the form of four parallel rods, each consisting of a row of chrom at in -granules (Fig. 117, A-C). He believed these rods to arise by the double longitudinal splitting of a single primary chromatin- rod, each cleavage being a



A. Resting spermatogonium with single plasmosome ; irial plate of dividing s]>ermatogonium : twenty large a [jcrmatogonium- division. /'-/. Prophases of first malt; ingle chromalin-ilucleolus. /'. Segmented split spireme. /. /. Concentration of the rings to form tetrads.


nam. [PAULMIER.]

1 two chromatin-nucleoli. B. Equatwo small chromosomes. C. Final lion-division. D. £. Synapsis, with C. H. Formation of the tetrad-rings.


preparation for one of the polar bodies. In his opinion, therefore, the formation of the polar bodies differs from ordinary mitosis only in the fact that the chromosomes split very early, and not once, but twice, in preparation for two rapidly succeeding divisions without an intervening resting period. He supported this view by further observations in 1890 on the polar bodies of Sagitta and several gasteropods, in which he again determined, as he believed, that the tetrads arose by double longitudinal splitting. An essentially similar view of the tetrads was taken by Hertwig in 1890, in the spermatogenesis of Ascaris, though he couid not support this conclusion by very convincing evidence. In 1893, finally, Brauer made a most thorough and apparently exhaustive study of their origin in the spermatogenesis of Ascaris, which seemed to leave no doubt of the correctness of Boveri's result. Every step in the origin of the tetrads from the reticulum of the resting spermatocytes was traced with the most painstaking care. In the early prophases of the first division the nuclear reticulum breaks up more or Jess completely into granules, which


Fig. 137. — Maturat ion-divisions in an insect, Anaia. [Paui.mier.] A. Primary spermatocyte in metaphase. B. Equatorial plate, showing ten large tetrads and one small one ; " odd chromosome " al o, C. Separation of the dyads. D. Telophase, which is also a prophase of the second division. E. Secondary spermatocyte; division of the dyads; small dyad shown undivided. F. Final anaphase ; small dyad near the lower chromosome-group. - (The figures are numbered from left to right. For later stales, see Fig. 8a.)

become in part aggregated in a mass at one side of the nucleus {"synapsis," p. 276), from which delicate threads extend through the remaining nuclear space (Fig. 120, A). Even at this period the granules of the threads are divided into four parts. As the process proceeds the chromatin resolves itself into a single spireme-thread, consisting of four parallel rows of granules, which break in two to form the two tetrads (var. diva/ens), or is directly converted into a single tetrad (var. utiivalens) (Fig. 120). From these observations Brauer concludes that each tetrad arises from a rod, doubly split lengthwise by a process initiated at a very early period through the double fission of the chromatin-granules. If this be correct, there can be no reduction in Wcismann's sense ; for the four products of each primary chromatin-granule are equally distributed among the four daughter-cells. A similar conclusion, based on much more incomplete evidence, was reached by Brauer ('92) in the phyllopod Branchipus.

Brauer's evidently conscientious figures very strongly sustain his conclusion, which, reinforced by the earlier work of Hertwig and Boveri, has until now seemed to rest upon an unassailable basis. The recent work of Sabaschnikoff ('97) nevertheless raises the possibility of a different interpretation. Brauer himself justly urges that the essence of the process lies in the double fission of the chromatingranules to which the formation of chromosomes is secondary. 1 Everything, therefore, turns on the manner in which the quadruple granules arise ; and Sabaschnikoff's work gives some ground for the view that they may arise, not by a double fission, but in some other way.

According to this author there is a period (in the oogenesis) at which the nuclear threads wholly disappear, the entire chromatin being broken up into granules. From this state the granules emerge in quadruple form to arrange themselves in the doubly split spireme exactly as Brauer describes : and a few observations are given (regarding the size and arrangement of the granules) which suggest the possibility that the quadruple granules may arise by the conjugation either of four separate granules or of two pairs of double granules. Since there is ground for the view that tetrads may arise by the conjugation of chromosomes (see following section), there is no a priori objection to such a conclusion. Could it be sustained, the maturationdivisions of Ascaris would in fact involve a true reduction in Weismann\s sense ; for despite the fact that the chromosomes are only longitudinally divided, the four longitudinal constituents of each tetrad would not be equivalent with respect to the granules, and it is the reduction of the latter (" ids ") that forms the essence of Weismann's hypothesis (p. 245). Another consideration, suggested to me by Professor T. H. Morgan, opens still another possibility, which seems well worthy of test by further research. As already stated (p. 88). the long chromosomes of Ascaris are plurivalcnt, since in all but the germ-cells each breaks up into a much larger number of smaller chromosomes (Fig. 73, p. 148). If, therefore, the latter correspond to the chromosomes of other forms in which tetrads occur (c.%. Cyclops or Ar/emia), the so-called "tetrad" of Ascaris is a compound body: and the true process of reduction must be sought in the origin of the smaller elements of which it is composed, which are, perhaps, directly comparable with Sabaschnikoff's i; granules." Until the questions thus opened have been further studied, the case for Ascaris must remain open : and it is perhaps worth suggesting that a new point of view may here be found for further study also of reduction in the vertebrates. 2

1 Cf. p. 113.

2 Bodies closely resembling tetrads arc sometimes formed in mitosis, where no reduction should occur. Thus, R. Hertwig ('95) has observed tetrads in the first cleavage-spindle of echinoderm-eggs after treatment with dilute poisons (p. 306). Klinckowstr8m figures them in the second polar spindle of Prostheccraus eggs, while Moore ('95) describes in the elasmobranchs small ring-shaped chromosomes, not only in the first but also in the second spermatocyte- divisions, concluding that no reduction occurs in either division.


(c) The Formation of Tetrads by Conjugation. — A considerable number of observers have maintained that reduction may be effected by the union or conjugation of chromosomes that were previously separate. This view agrees in principle with that of Riickert, Hacker, and Vom Rath ; for the bivalent chromosomes assumed by these authors may be conceived as two conjugated chromosomes. It seems to be confirmed by the observations of Born and Fick on Amphibia and those of Riickert on selachians {Pristiurus) ; for in all these cases the number of chromatin-masses at the time the first polar body is formed is but half the number observed in younger stages of the germinal vesicle. In Pristiurus there are at first thirtysix double segments in the germinal vesicle. At a later period these give rise to a close spireme, which then becomes more open, and is found to form a double thread segmented into eighteen double segments ; i.e. the reduced number. In this case, therefore, the preliminary pseudo-reduction is almost certainly effected by the union of the original thirty-six double chromosomes, two by two. The most specific accounts of such a mode of origin have, however, been given by Calkins (earthworm) and Wilcox (grasshopper). The latter author asserts ('95) that in Caloptcnus the spireme of the first spermatocyte gives rise without longitudinal division to twenty-four chromosomes (double the somatic number). These then become associated in pairs, and still later the twelve pairs conjugate two and two to form six tetrads. There is, therefore, no longitudinal splitting of the chromosomes. The a priori improbability of such a conclusion is increased by the studies of Paulmier on the Hemiptera, which demonstrate the occurrence of a longitudinal division in a number of these forms and confirm the original studies of Vom Rath on Gryllotalpa}

The second case, which is perhaps better founded, is that of the earthworm (Lumbricus terrcstris), as described by Calkins ('95, 2), whose work was done under my own direction. Calkins finds that the spireme splits longitudinally and then divides transversely into 32 double segments. These then unite, two by two, to form 16 tetrads. The 32 primary double segments therefore represent chromosomes of the normal number that have split longitudinally,

i.e. — -p etc., and the formula for a tetrad is - , or — -. Such a b a o a\x

a tetrad, therefore, agrees as to its composition with the formulas

of Hacker, Vom Rath, and Riickert, and agrees in mode of origin

with the process described by Riickert in the eggs of Pristiurus.

While these observations are not absolutely conclusive, they never 1 Montgomery, who has denied the occurrence of a longitudinal division in Pentatoma ('98, i), has subsequently found such a division in the nearly related if not identical genus Euchistis ('99).

theless rest on strong evidence, and they do not stand in actual contradiction of what is known in the copepods and vertebrates. The possibility of such a mode of origin in other forms must, I think, be held open.

Under the same category must be placed Korschelt's unique results in the egg-reduction of the annelid Ophryotrocha ('95), which are very difficult to reconcile with anything known in other forms. The typical somatic number of chromosomes is here four. The same number of chromosomes appear in the germinal vesicle (Fig. 125, D). They are at first single, then double by a longitudinal split, but afterward single again by a reunion of the halves. The four chromosomes group themselves in a single tetrad, two passing into the first polar body, while two remain in the egg, but meanwhile each of them again splits into two. Of the four chromosomes thus left in the egg two are passed out into the second polar body, while the two remaining in the egg give rise to the germ-nucleus. From this it follows that the formation of the first polar body is a reducing division — a result which agrees with the earlier conclusions of Henking on Pyrrochoris, and with those of Paulmier on the Hemiptera.


C. Reduction without Tetrad-formation

As already stated (p. 246), the formation of actual tetrads is of relatively rare occurrence, being thus far certainly known only in the arthropods, nematodes, and some annelids. In the greater number of cases the two divisions of the primary chromatin-masses {i.e. of the primary oocyte or spermatocyte) are separated by a considerable interval, during which the first maturation cell-division takes place or is initiated, and hence no actual tetrads are formed. This obviously differs only in degree from tetrad-formation, the latter occurring only when the two divisions are simultaneous or occur in rapid succession.

In the cases now to be considered the length of the pause between the maturation-divisions varies considerably, and in some forms (vertebrates, flowering plants) it is so prolonged that the nucleus is partially reconstructed. In all, or nearly all, these cases the first maturationdivision is of the hetcrotypical form, the chromosomes having the form of rings and arising by a process that agrees in most of its features with that leading to tetrad-formation. There is here, however, exactly the same contradiction of results as in the case of tetrad-formation described at page 247, and a bewildering confusion of the subject still exists. In brief, it may be stated that most observers of reduction of this type in the lower animals (flat-worms, annelids, mollusks) have found one transverse and one longitudinal, division ; most of those who have studied the vertebrates find two longitudinal divisions; while opinion regarding the plants is still divided.

{a) Animals. — In the gephyrean Thalassema and the mollusk Zirp/uea (Figs. 128-130) Griffin ('99) finds that the rings, arising as described above, place themselves in the equator of the spindle with the longitudinal division in the equatorial plane. They are then drawn out toward the spindle-poles from the middle point, first assuming the form of a double cross, then of elongated ellipses, and finally break into two daughter-U's or -Vs. The first division is therefore longitudinal. During the late anaphase the V's break at the apex, the two limbs come close together, so as to give the decep


Fig. 128. — Diagrams of reduction in the types represented by Thalassema (A) and Solamandra (/?). In both the first division is heterotypical. The second division (6) is transverse in the first and longitudinal in the second.

tive appearance of a longitudinal split, and are separated by the second division (following immediately upon the first without intervening resting stage). The latter is therefore a transverse division (Fig. 130). An essentially similar result, though less completely worked out, is independently reached by Bolles Lee (*97) in Helix ; by Klinckowstrom ('97) in the turbellarian Prosthccerceus ; and by Francotte('97)and Van der Stricht ('98, 1) in Thysanzoon. Klinckowstrom shows that there is much variation in the way in which the rings open out and break apart, though the result is the same in all. In case of the vertebrates, Flemming ('87) long since described and figured typical tetrads in the salamander, but regarded them as "anomalies." Vom Rath's later conclusion ('93, '95) that they are normal tetrads has not been sustained by the still more recent work of Mevcs ('96), whose careful studies, together with those of Moore, Lenhossek, and others, thus far give no evidence of tetrad-formation, and seem opposed to the occurrence of reducing divisions in the vertebrates. Meves's work in the main confirms the earlier results of Flemming, except that he shows that, as in so many other animals, only two generations of spermatocytes exist. At the first division the nuclear reticulum resolves itself into twelve (the reduced number) segments, which split lengthwise, the halves remaining united to form elongated rings (Figs. 27, 37). These do not, however, contain dense into tetrads, but break apart during the first division at the points corresponding with the ends of the united halves. The first div'^ionSs th ere fore an equation -division. As the V-shaped halves separate theya&ain split lengthwise (Fig. 131), each of the secondary spermatocytes receiving twelve double V's or dyads. In the telophases and ensuing renting stage, however, all traces of this splitting are lost, the nuclei partially returning to the resting stage, but retaining traces of a spireme-like arrangement (Fig. 131). In the second division twelve double V's reappear, showing a longitudinal division which Flemming and Meves believe to be directly related to that seen during the foregoing anaphases. There is therefore no evidence of a transverse division. McGregor ('99) describes a nearly similar process in Amphiuma, where the longitudinal division of the

A. A few moments after entrance of the spcrmatoii forming. B. Early prophase of first polar mitosis with c wall. D, Cenrralspindleeslablished; elimination of nuclei laler stage viewed from above. F. First polar spindle est





Pig- >30. — Maturalion in Ihe lamellibranch Zitfhaa 4-E, Zirfkaa; F-I, Thalaiitma.


in Thalaiitma. [GUI


A. Unfcrtiliied egg. ring-shaped and cross-shaped chromosomes. B, Prophase of first polar mitosis. C. First polar spindle ; double crosses. D. Slightly later stage. E. The double crosses have broken apart (equation-division). G. Ensuing stage; daughter-V's broken apart at the apex. H. Telophase of first, early prophase of second, division; limbs of the v's separate bul closely opposed. F. Later prophase of second division. /. Second polar spindle in metaphase.

daughter-V's is seen with the greatest clearness throughout the anaphases.

The weak point in both the foregoing cases is the fact that all traces of the second longitudinal division are lost during the ensuing resting period ; and I do not think that even the observations of Flemming ('97), who has published the fullest evidence in the case, completely establish the occurrence of a subsequent longitudinal divi


ng. 131. — {Compare Fig. 17). Maturation-division! in Salamc the others from Meves.]

A. First division in metap base, showing heterotype rings, B. Anaphase; longiim ting of the daughter-loops. C. Telophase. D. Ensuing pause. E. Early prophase division with longitudinally divided segmented spireme. F. Later prophase. G. Mel second division.


[/; from Flrmminc,


sion of the chromosomes in the second mitosis. In Desmognathns, however, where the resting stage is less complete, Kingsbury ('99) finds the longitudinal split in the persistent chromosomes of the pause following the first division; and he believes this to be the same division as that seen during the anaphase. Carnoy and Le Brun ('99) reach the same result in the formation of the polar bodies in Triton, though their general account of the heterotypical mitosis differs very considerably from that of other authors, the rings being stated to arise by a double instead of a single longitudinal split These observers describe the rings of the early anaphase as having almost exactly the same double cross-form as those in Thalassema or Zirphcea (Griffin, '99), but believe them to arise in a manner nearly in accordance with Strasburger's abandoned view of 1895, 1 and with Guignard's ('98, 2) and Gr^goire's ('99) latest results on the flowering plants, the ring being stated to arise by a double longitudinal splitting, as explained at page 265.

In the elasmobranch Scyllium Moore C95) finds twelve (the reduced number) ring-shaped chromosomes at the first division. These closely resemble tetrads ; but a resting stage follows, and the second division is likewise stated to be of the heterotypical form. Both divisions are stated to be equation-divisions — a conclusion well supported in case of the first, but so far from clear in the second that a careful reexamination of the matter is highly desirable.

In mammals the first division is of the heterotypical form (Hermann, '89, Lenhossek, '98), though the rings are much smaller than in the salamander, recalling those seen in arthropods. No true tetrads are, however, formed, and the two divisions are separated by a resting period. The character of the second division is undetermined, though Lenhossek believes it to be heterotypical, like the first.

(6) Plants. — It is in the flowering plants, where reduction likewise occurs, as a rule, without true tetrad-formation, that the contradiction of results reaches its climax ; and it must be said that until further research clears up the present confusion no definite result can be stated. The earlier work of Strasburger and Guignard indicated that no reducing division occurred, the numerical reduction being directly effected by a segmentation of the spireme-thread into half the somatic number of chromosomes. Thus these observers found in the male that the chromosomes suddenly appeared in the reduced number (twelve in the lily, eight in the onion) at the first division of the pollen-mother-cell, and in the female at the first division of the mother-cell of the embryo-sac. The subsequent phenomena differ in a very interesting way from those in animals, owing to the fact that the two maturation-divisions are followed in the female by one and in the male by two or more additional divisions, in both of which the reduced number of chromosomes persists. In the male the two maturation-divisions give rise to four pollen-grains, in the female to

1 Cf. p. 269.


the four primary cells of the embryo-sac (Fig. 132); and these two divisions undoubtedly correspond to the two maturation-divisions in animals. In the female, as in the animals, only one of the four resulting cells gives rise to the egg, the other three corresponding to the polar bodies in the animal egg, though they here continue to divide, and thus form a rudimentary prothallium. 1 The first-men


Plg. 131. — General view of ihe malurat ion-divisions in flowering plants. [MOTHER.] A-C, in the male ; D-F, in Ihe female. A. The two secondary spermatocytes (pollen-mother. cells] just after the first division {Lilium). P. Final anaphase of second division [Podophyllum).

C. Resulting telophase, which by division of Ihe cytoplasmic mass produces four pollen-grains.

D. Embryo-sac after completion of Ihe first nuclear division {IJIium). E. The same after the second division. F. The upper four cells resulting from the third division (cf. Fig. 106) : 0, ovum ; /, upper polar cell; s, synergidai. (For further details, see Figs. 133, 134.)


1 Of Ihese three cells one divides to form the " synergida?," the other two divide to form three "antipodal sells" (which like Ihe syncrgid.-c finally degenerate) and a "lower polar cell." The latter sooner or later conjugates with the " upper polar cell " (the sister-cell of the egg) to form the " secondary embryo-sac -nucleus," by the division of which Ihe endosperm-cells arise. Of the whole group of eight cells thus arising only the egg contribute*


tioned cell, however, does not directly become the egg, but divides once, one of the products being the egg and the other the " upper polar cell" (Fig. 132, F), which contributes to the endosperm-formation (see footnote, and compare page 218).

In the male the two maturation-divisions are in the angiosperms followed by two others, one of which separates a " vegetative " from a " generative " cell, while the second divides the generative nucleus into two definite germ-nuclei. In the gymnosperms more than two such additional divisions take place. In these later divisions, both in the male and in the female (with the exception noted in the footnote below), the reduced number persists, and the principal interest centres in the first two or maturation-divisions. Strasburger and Guignard found in Lilium that while both these divisions differed in many respects from the mitosis of ordinary vegetative cells, neither involved a transverse or reducing division, the chromosomes undergoing a longitudinal splitting for each of the maturation-divisions. Further investigations by Farmer ('93), Belajeff C94), Dixon ('96), Sargant ('96, '97), and others, showed that the first division is often of the heterotypical form, the daughter-chromosomes in the late-metaphase having the form of two V's united by their bases (<>). Despite the complication of these figures, due to torsion and other modifications, their resemblance to the ring-shaped bodies observed in the first maturation-division of so many animals is unmistakable, as was first clearly pointed out by Farmer and Moore ('95).

Botanists have differed, and still differ, widely in their interpretation both of the origin and subsequent history of these bodies upon which the question of reduction turns. According to Strasburger's ('95) first account their origin has nothing in common with that of the tetrad-rings, since they were described as arising by a double longitudinal splitting of a primary rod, the halves then separating first from one end along one of the division-planes, and then from the other end along the other plane, meanwhile opening out to form a ring such as is shown in Fig. 133. (This process, somewhat difficult to understand from a description, will be understood from the diagram, Fig. 135, E-L) The four elements of the ring are then distributed without further division by the two ensuing maturation-divisions ; and the process, except for the peculiar opening out of the ring, is

to the morphological formation of the embryo. It is a highly interesting fact that the number of chromosomes shown in the division of the lower of the two nuclei (i.e. the mothernucleus of the antipodal cells and lower polar-cell) formed at the first division of the embryo-sac-nucleus is inconstant, varying in the lily from 12, 16, 20, to 24 (Guignard, '91, 1), in which respect they contrast with the descendants (egg, synergidre) of the upper nucleus, which always show the reduced number (Mottier, '97, 1), i.e. in Lilium twelve. Thisexception only emphasizes the rule of the constancy of the chromosome-number in general; for these cells are destined to speedy degeneration.


essentially in agreement with the facts described in Ascaris, and involves no reduction-division. Essentially the same result is reached by Guignard ('98) in his latest paper on Naias, and by Gregoire ('99) in the Liliaceas.

Strasburger twice shifted ground in rapid succession. First ('97, 2), with Mottier ('97, 1 ), he somewhat doubtfully adopted a view agreeing




\F, Strasbubgeb and MOT


e of firs! division ; chromatined. Ft. Slightly later stage (split spireme) in the nucleus r prophase (pollen-mo I her-cel I. Podophyllum) t

F. First maturation-spindle {Fritillaria, male). G. Dive



essentially with the interpretation of Vom Rath, Riickert, etc. (p. 247). The primary rods split once, and bend into a V, the branches of which often come close together, and may be twisted on themselves, thus giving the appearance of the second longitudinal split described in Strasburger's paper of 1895. The two halves of the split U then separate, opening out from the apex, to form the o -figure. In the


second division the limbs of the daughter-V's again come close together, remaining, however, united at one end, where they were believed finally to break apart during the second division. The latter was, therefore, regarded as a true reduction-division, the apparent longitudinal split being merely the plane along which the halves of the V come into contact (Fig. 134, C, D).

The two accounts just given represent two extremes, the first agreeing essentially with Ascaris, the second with the copepods or insects. When we compare them with others, we encounter a truly bewildering confusion. Strasburger and Mottier ('97) themselves soon abandoned their acceptance of the reducing division, returning to the conclusion that in both sexes {Lilium, Podophyllum) both divisions involve a longitudinal splitting of the chromosomes (Figs. 133, 134). In the first division the longitudinally split spireme segments into twelve double rods, which bend at the middle to form double V's, with closely approximated halves. Becoming attached to the spindle by the apex, the limbs of each separate to form a -figure. At telophase the daughter-V's shorten, thicken, and join together to form a daughter-spireme consisting of a single contorted thread. This splits lengthwise throughout its whole extent, and then segments into double chromosomes, the halves of which separate at the second division (Fig. 135, L-M). The latter, therefore, like the first, involves no reducing division. This result agrees in substance with the slightly earlier work of Dixon C96) and of Miss Sargant ('96, '97), whose account of the origin of the -figure of the first division differs, however, in some interesting details. It is also in harmony with the general results of Farmer and Moore ('95), of Gr^goire ('99), and of Guignard ('98), who, however, describes the first division nearly in accordance with Strasburger's account of 1895, as stated above. On the other hand, Ishikawa (pollen-mother-cells of Allium, '97) and especially Belajeff (pollen-mother-cells of Iris, '98) conclude that the second division is a true transverse or reducing division. 1 Ishikawa described the first division as being nearly similar to the ring-formation in copepods, the four elements of the ring being often so condensed as nearly to resemble an actual tetrad. In the early anaphases the daughter-V's break at the apex ; and, although in the later anaphases the limbs reunite, Ishikawa is inclined to regard the transverse division as being a preparation for the second mitosis. Belajeffs earlier work ('94) on Lilium gave an indecisive result, though one on the whole favourable to a reducing division. In his latest paper, however ('98, 1), Belajeff takes more positive ground, stating that after the examination of a large number of forms he has found

1 Schaffner ('97, 2) reaches exactly the reverse result in Lilium philadelphicum, i.e. the first division is transverse, the second longitudinal.


in the pollen-mother-cells of Iris a much more favourable object of investigation than Lilitim, Frititlaria, and the other forms on which most of the work thus far has been done, and one in which the second division takes place with " admirable clearness " ; he also gives interesting additional details of the first division in this and other forms. In the first division the spireme splits lengthwise, and then breaks into chromosomes, which assume the shape of a V, Y, or X (Fig. 135, N—Q). The two limbs of these bodies do not, as might be



ucleus of secondary spermatocyte (PodaphylliLm). B. Prophase of sec male) with longitudinally divided chromatin -threads. B. Corresponding stage /-'. Meiaphase of second division (Fodcphyllam, male). G. Initial anaphase (/. C. D. illustrate Motlier's earlier conclusions. C. Second division (Lilimm. male) imcs bent together so as to simulate a split. D. Slightly later stage (Fritillana, 1 siagc supposed to result from breaking apart of the limbs of the U at point of flexu


supposed, represent sister-chromosomes (resulting from the longitudinal division of the spireme) attached by one end or at the middle, since each X, Y, or V is double, consisting of two similar superimposed halves. Belajeff, therefore, regards these figures as longitudinally divided bivalent chromosomes, having the value of tetrads, each limb being a longitudinally split single chromosome. The double V's, Y's, and X's take up a position with the apex (or one end of the X) attached to the spindle, and the longitudinal division in the equatorial plane. The halves then progressively diverge from the point of attachment, thus giving rise to -shaped, <>- -shaped, or xx -shaped figures, all of which in the end assume the -shape. This part of the process is in the main similar to that described by Strasburger and Mottier, and the daughter-V's diverge in the same way as these authors describe. The second division, however, differs radically from their account, since no splitting of the spireme-thread occurs. The chromosomes reappear in the V-, Y-, and X-forms, but are undivided^ and only half as thick as in the first division. Passing to the equator of the spindle, the V- and Y-forms break apart at the apex, while the X-forms separate into the two branches of the X, the daughter-chromosomes having the form of rods slightly bent at the outer end to form a J-figure (Fig. 135, R-T). This division is, accordingly, a transverse or reducing one, which " corresponds completely to the reduction-division in the animal organism " ('98, 2, p. 33.) Atkinson ('99) reaches the same general result in Trillium, stating very positively that no longitudinal division occurs in the second mitosis, and believing that the daughter-V's of the first (heterotypical) mitosis retain their individuality throughout the ensuing pause, and break apart at the apex (reducing division) in the second mitosis. This observer finds further that in Ariscema the heterotypical rings of the first mitosis condense into true tetrads, by one longitudinal and one transverse division, but believes that in this case it is the first division that effects the reduction, as in the insects.

Such confusion in the results of the most competent observers of reduction in the flowering plants is itself a sufficient commentary on the very great difficulty and uncertainty of the subject ; and it would be obviously premature to draw any positive conclusions until further research shall have cleared up the matter. 1

1 Strasburger's new book, entitled Uber Reduktiomtheilung, Spindelbildung, Centrosomen und Cilienbildner im Pflanzcnreich (Jena, 1900), is received while this work is in press, too late for analysis in the text. In this treatise the author gives an exhaustive review of the entire subject, contributing also many new and important observations on Lilium, Iris, Podophyllum, Tradescantia, Allium, Ijxrix, and several other forms. The general result of these renewed researches leads Strasburger to return, in the main, to his conclusions of 1895, w ' tn which agree, as stated above, the results of Guignard and Gregoire; and, in a careful critique of Belajeff's work, he shows how the results of this observer may be reconciled with his own. The essence of Strasburger's interpretation is as follows. In the prophases of the first division the chromosomes first undergo a longitudinal division, shorten to form double rods, and then again split lengthwise in a plane at right angles to the first. The following stages vary even in the same species (Lilium); and here lies the explanation of much of the divergence between the accounts of different observers. (1) In the typical case, the chromosomes are placed radially, with one end next the spindle ; and, during the metaphase, they open apart along the first division-plane, from the spindle outwards, to form h- -shaped figures. These figures meanwhile open apart from the free end inwards along the second division-plane. Thus arise the characteristic < > -shaped figures, the daughter-V's having separated along the first (equatorial) division-plane, while the two limbs of each V have resulted, not through bending, but from a second (axial) split (Fig. 135, E-I/). The


- Diagrams illustrating diffcrem liiiivc mitoses (hi'terotypical forr tiurgers


accounts of reduction in Ihe flowering plants. ) in Pica. [BEI.AJEFF.] ('95) and the later one of Guignard, of Ihe first


in-division. E. Doubly split rod. F. Metaphtte, in profile. G. The ; showing ihe hctcrotvpc ring. H. I. Opening out and breaking apart of the ring.

JT-.lt. 1-aliT iMMHiunl of Strasburger and Motlier {.if. tigs. 133. .34). J. longitudinally split. V-shaped rhmiimvmu^ of first division. A'. Opening out oi the ring, L. Prophase of second division, showing longitudinally split segmented spireme. M. Initial anaphase of second division,

A'-(). First division. [Bki.ajf.ff.] A*. Lung itu din ally split chromosomes, viewed in the equatorial plane. 0. The same viewed in the axis of the spindle. /'. Separation of the daughterchromosomes. Q. Anaphase, all the chromosomes assuming the V-form.

K-T. Second division in frit. [Hki.MKH-'.J A', l-^uaiorial plate, limbs of X's and V*s brraking apart (reducing division). S. Slightly later stage, with daughter-chromosomes si ill untied at one end. /: Anaphase.


Rtsumt. In reduction without tetrad-formation the spireme segments into half the somatic number of chromosomes, which split lengthwise and open out to form rings for the first (heterotypical) mitosis. According to one set of observers, including Flemming, Meves, McGregor, Kingsbury, Moore, Klinckowstrom, Van der Stricht, Francotte, Griffin, Belajeff, Farmer, Dixon, Strasburger, Sargant, Mottier, Ishikawa, and Atkinson, the ring arises by a single longitudinal division. According to another group, including Carnoy, Le Brun, Guignard, and Gr^goire, the ring arises through a double longitudinal division, one representing the axial and the other the equatorial plane of the o -figure. The second group of observers regard both maturation-divisions as longitudinal. Among the first group, Flemming, Meves, McGregor, Kingsbury, Moore, Farmer, Dixon, Strasburger, Sargant, and Mottier likewise believe both divisions to be longitudinal, the daughter-V's or their products again splitting lengthwise for the second division ; while Klinckowstrom, Van der Stricht, Francotte, Griffin, Belajeff, Ishikawa, and Atkinson believe one of them to be transverse, the daughter-V's breaking apart at the apex, and thus giving the reducing division of Weismann. 1


D. Some Peculiarities of Reduction in the Insects

We may here briefly consider some interesting observations which show that in some cases the nuclear substance may be unequally distributed to the germ-nuclei. Henking ('90) discovered that in the second spermatocyte-division of Pyrrochoris one of the <% chromosomes " passes undivided into one of the daughter-cells (spermatids) which receives twelve chromatin-elements while its sister receives but eleven. (The number of chromosomes in the spermatogonia, and of rings in the first spermatocyte-division is twenty-four). This anomalous process is confirmed with interesting additional details by Paulmier ('99) in Anasa, and obviously related phenomena are described by Montgomery ('99, 1 ) in Pcntatoma, and by McClung ('99) in Xiphidium.

breaking apart of the V's at the apex, as described by Belajeff, is, therefore, not a transverse division, but merely the completion of the second longitudinal division. (2) In a second and exceptional type, the chromosomes are placed tangentially to the spindle, and the halves separate from the middle, again producing O -shaped figures. These, however, are not of the same nature as those arising in the first case, since they are formed by a bending out of each daughter-chromosome at the middle to form the V, and not by the second longitudinal split. The effect of the latter is in this case to render each daughter-V in itself double, precisely as in the salamander. The difference between the two types results merely from the difference of position of the chromosome with respect to the spindle, and the final result is the same in both, i.e. two longitudinal divisions and no reducing one.

This highly important work brings very strong evidence against the occurrence of transverse or reducing divisions in the higher plants, and seems to explain satisfactorily most of the differences of interpretation given by other olwervers. It will be interesting to see whether a similar interpretation is possible in the case of mollusks, annelids, and arthropods, where the early stages, in many cases, so strikingly resemble those occurring in the plants.

1 Cf. footnote on page 269.


In Pentatoma the number of chromosomes in the spermatocyte is fourteen. During the final anaphases of the last division, one of the fourteen daughter-chromosomes assumes a different staining-capacity from the others, and becomes a •* chromatin-nucleolus " which fragments into several smaller bodies during the ensuing resting-stage. During each of the succeeding spermatocyte-divisions appear seven chromosomes and a single small chromatin-nucleolus, and both of these kinds of bodies are halved at each division, so that each spermatid receives seven chromosomes and a single chromatin-nucleolus. 1 In Xiphidium a body called by McClung the accessory chromosome," and believed by him to correspond to the "chromatinnucleolus " of Pentatoma, appears in the early prophases of the last spermatogoniumdivision while the remaining chromatin still forms a reticulum. In the equatorial plate this lies outside the ring of chromosomes, but divides like the latter. The same body appears in the ensuing resting-stage, and during both of the spermatocytedivisions. In these it lies, as before, outside the chromosome-ring, and differs markedly from the other chromosomes, but divides like the latter, each of the halves passing into one of the spermatids, where it appears to form an important part of the sperm-nucleus.

Despite the peculiarities described above, the chromatin, as a whole, seems to be equally distributed in both Pentatoma and Xiphidium. In Anasa, however, Paulmier's studies ('98, ^99), made in my laboratory, give a result agreeing with that of Henking, and suggest some very interesting further questions. The spermatogonianuclei contain two nucleolus-like bodies, aiid give rise to twenty-two chromosomes, of which two are smaller than the others (Fig. 126). In the first spermatocyte-division appear eleven tetrads. Ten of these arise from rings like those of Gryllotalpa, etc. The eleventh, which is much smaller than the others, seems to arise from a single nucleolus-like body of the spermatocyte-nucleus, and by a process differing considerably from the others. All of these bodies are halved to form dyads at the first division. In the second spermatocyte-division (Fig. 127) the larger dyads divide to form single chromosomes in the usual manner. The small dyad, however, fails to divide, passing over bodily into one of the spermatids. In this case, therefore, half of the spermatids receive ten single chromosomes, while the remainder receive in addition a small dvad.

A comparison of the foregoing results indicates that the small tetrad (dyad) corresponds to the extra chromosome observed by Henking in Pyrrochoris, and perhaps also to the "accessory chromosome" of Xiphidium. Whether it corresponds to the " chromatin-nucleolus " of Pentatoma is not yet clear. The most remarkable of these strange phenomena is the formation of the small tetrad, which seems to be a non-essential element, since it does not contribute to all the spermatozoa. Paulmier is inclined to ascribe to it a vestigial significance, regarding it as a "degenerating" chromosome which has lost its functional value, though still undergoing in some measure its original morphological transformation ; in this connection it should be pointed out that the spermatocyte-nucleolus, from which it seems to be derived, is represented in the spermatogonia by two such nucleoli, just as the single small tetrad is represented by two small chromosomes in the spermatogonia-mitoses. The real meaning of the phenomenon is, however, wholly conjectural.


E. The Early History of the Germ-nuclei

There are many peculiarities in the early history of the germnuclei, both in plants and animals, that have a special interest in connection with the reduction-problem ; and some of these have raised some remarkable questions regarding the origin of reduction. A large number of observers are now agreed that during the growthperiod preceding the maturation-division (p. 236), in both sexes, the nucleus of the mother-cell (spermatogonium, oogonium), both in plants and in animals, passes through some of the changes preparatory to reduction at a very early period. Thus, in the egg the primary chroma tin-rods are often present in the very young ovarian eggs, and from their first appearance are already split longitudinally. 1 Hacker ('92, 2) made the interesting discovery that in some of the copepods {Cant/iocamptus, Cyclops) these double rods could be traced


1 On this latter point Montgomery's observations do not seem quite decisive.




Pig. 136. — Longitudinal section through the ovary of the copepod Canlhoiamptm. [Hiic

eg. The youngest germ-cells or oogonia (dividing al eg. 1 ) : a. upper pan of the growlii-sone ; ot. oftcyte. or growing ovarian egg; if. fully formed egg. with double chroma tin-rods.

back continuously to a double spireme-thread, following immediately upon the division of the last generation of oogonia, and that at no period is a true reticulum formed in the germinal vesicle (Fig. 136). In the following year Riickert ('93, 2) made a precisely similar discovery in the case of selachians. After division of the last generation of oogonia the daughter-chromosomes do not give rise to a reticulum, but split lengthwise, and persist in this condition throughout the entire growth-period of the egg. Riickert therefore concluded that the germinal vesicle of the selachians is to be regarded as a " daughter-spireme of the oogonium (Ur-ei) grown to enormous dimensions, the chromosomes of which are doubled and arranged in pairs." * In this case their number seems to be at first the somatic number (thirty-six), which is afterward halved by conjugation of the elements two and two (Riickert), as in Lutnbricus (Calkins). It is, however, certain that in many cases (insects, copepods) the double rods first appear in the reduced number, and the observations of Vom Rath C93) and Hacker ('95, 3) give some reason to believe that the reduced number may in some forms be present in the earlier progenitors of the germ-cells, the former author having found but half the normal number in some of the embryonic cells of the salamander, while Hacker ('95, 3) finds that in Cyclops brevicomis the reduced number of chromosomes (twelve) appears in the primordial germ-cells which are differentiated in the blastula-stage (Fig. 74). He adds the interesting discovery that in this form the somatic nuclei of the cleavagestages show the same number, and hence concludes that all the chromosomes of these stages are bivalent. As development proceeds, the germ-cells retain this character, while the somatic cells acquire the usual number (twenty-four) — a process which, if the conception of bivalent chromosomes be valid, must consist in the division of each bivalent rod into its two elements. We have here a wholly new light on the historical origin of reduction ; for the pseudo-reduction of the germ-nuclei seems to be in this case a persistence of the embryonic condition, and we may therefore hope for a future explanation of the process by which it has in other cases been deferred until the penultimate cell-generation, as is certainly the fact in Ascaris?

This leads to the consideration of some very interesting recent discoveries regarding the relation of reduction to the alternation of generations in the higher plants. As already stated (p. 263), Strasburger, Guignard, and other observers have found that in the angiosperms the two maturation-divisions are in both sexes followed by one or more divisions in which the reduced number persists. The cells thus formed are generally recognized as belonging to the vestiges of the sexual generation (prothallium) of the higher cryptogams, the pollengrains (or their analogues in the female) corresponding to the asexual spores of the archegoniate cryptogams. We should, therefore, expect to find reduction in the latter forms occurring in the two corresponding divisions, by which the " tetrad " of spores is formed (as was first pointed out by Hartog, '91). Botanists were thus led to the surmise, first expressed by Overton in 1892, that the reduced number would be found to occur in the prothallium-cells derived from those spores.

1 '92, 2, p. 51.

2 It may be recalled that in Ascaris Boveri proved that the primordial germ-cells have the full number of chromosomes, and Ilertwig clearly showed that this number is retained up to the last division of the spermatogonia. Ishikawa ('97) finds that in Allium the reduced number (eight) appears in the mitosis of the " Urpollenzellen " preceding the pollen-mothcr-cclls. This is, however, contradicted by Motticr ('97, 2).


This surmise quickly became a certainty. Overton himself discovered ('93 ) that the cells of the endosperm in the gymnosperm Ccratozamia divide with the reduced number, namely eight ; and Dixon observed the same fact in Pinus at the same time. In the following year Strasburger brought the matter to a definite conclusion in the case of a fern (Osmunda), showing that all the cells of the prothallium, from the original spore-mot her-cc 11 onwards to the formation of the germ-cells, have one-half the number of chromosomes found in the asexual generation, namely twelve instead of twentyfour ; in other words, the reduction takes place in the formation of the spore from which the sexual generation arises, many cell-generations before the germ-cells are formed, indeed before the formation of the body from which these cells arise. Similar facts were determined by Farmer in Pallavicinia, one of the Hepaticae, where all of the nuclei of the asexual generation (sporogonium) show eight chromosomes during division, those of the sexual generation (thallus) four. It now seems highly probable that this will be found a general rule.

The striking point in these, as in Hacker's observations, is that the numerical reduction takes place so long before the fertilization for which it is the obvious preparation. Speculating on the meaning of this remarkable fact, Strasburger advances the hypothesis that the reduced number is the ancestral number inherited from the ancestral type. The normal, i.e. somatic, number arose through conjugation by which the chromosomes of two germ-cells were brought together. Strasburger does not hesitate to apply the same conception to animals, and suggests that the four cells arising by the division of the oogonium (egg plus three polar bodies) represent the remains of a separate generation, now a mere remnant included in the body in somewhat the same manner that the rudimentary prothallium of angiosperms is included in the embryo-sac. This may seem a highly improbable conclusion, but it must not be forgotten that so able a zoologist as Whitman expressed a nearly related thought, as long ago as 1878 : " I interpret the formation of polar globules as a relic of the primitive mode of asexual reproduction'* l Strasburger's view is exactly the reverse of this in identifying the polar bodies as the remains of a sexual generation ; and as Hacker has pointed out ('98, p. 102), it is difficult to reconcile with the fact that true reduction appears to occur already in the unicellular organisms (p. 277). The hypothesis is nevertheless highly suggestive and one which suggests a quite new point of view for the study not only of maturation but also of the whole problem of sexuality.

We may now return to the consideration of some details. In a considerable number of forms, though not in all, the early prophase is

1 '78, p. 262.


characterized, especially in the male, by a more or less complete concentration of the chromatin-substance at one side of the nucleus. This stage, to which Moore has given the name synapsis (Fig. 120, A), sometimes occurs when the spireme thread is already split {Ascaris, Liliiim), sometimes before the division is visible (insects). In either case the chromatin-segments emerge from the synapsis stage longitudinally divided and in the reduced number, a fact which gives ground for the conclusion that the synapsis is in some way concerned with the rearrangement of the chromatin-substance involved in the numerical reduction. During the synapsis the nucleolus remains quite distinct from the chromatin, and in many cases it afterward persists beside the tetrads, in the formation of which it takes no part, to be cast out into the cytoplasm (Fig. 124) or to degenerate in situ during the first maturation-division.

A suggestive phenomena, described by several observers, 1 is the casting out of a large part of the nuclear reticulum of the germinal



[CAK


Fig.^.-Typesofmali spindle with tetrads, in lit OV and l.kliRUM.] C. First polar spindle of At


pindles in the ft [HACKER].


[FiJRST.]


i polaj spindle


vesicle at the time the polar bodies are formed (Figs. 97, 128). In these cases {Asterias, Polyehazrus, Thalassema, Nereis) only a small fraction of the chromatin-substance is preserved to form the chromosomes, the remainder degenerating in the cytoplasm. 2

As a final point wc must briefly consider the varying accounts of the achromatic maturation-figures in the female already briefly referred to at page 85. In many forms {e.g. in turbellarians, nemertines, annelids, mollusks, cchinoderms) the polar amphiasters are of quite typical form, with large asters and distinct ccntrosomes nearly similar to those of the cleavage-fignres. In others, however (nematodes, arthropods, tunicates, vertebrates), the polar spindles differ markedly from those of the cleavage-figures, being described by many authors as entirely devoid of asters and even in some cases of centrosomes (Fig. 137).

ardiner ("98), Gtiffin ('99).

i-suhstanee in the clasmobta rich egg, p. 338.


There can be no doubt that these polar spindles differ from the usual type, and that they approach those recently described in the mitosis of the higher plants, but it is doubtful whether the apparent absence of asters and centrosomes is normal. In Ascaris, the first polar spindle arising by a direct transformation of the germinal vesicle (Fig. 117) has a barrel-shape, with no trace of asters. At the poles of the spindle, however, are one or two deeply staining granules (Fig. 137), which have been identified as centrosomes by Hacker C94) and Erlanger ('97, 4), but by Fiirst ('98) are regarded as central granules, the whole spindle being conceived as an enlarged centrosome. 1 For the reasons stated at page 314, I believe the former to be the correct interpretation. 2 Spindles without centrosomes have been described in the eggs of tunicates (Julin, Hill, Crampton), in Amphioxus (SoboXXz), in some species of copepods (Hacker), and in some vertebrates (Diemyctylus, Jordan ; mouse, Sobotta). In Amphioxus (Sobotta) and Triton (Carnoy and LeBrun) complete asters are not. formed, but fibrillae apparently corresponding to astral rays and converging to the spindle-poles are found outside the limits of the spindle (Fig. 137). In the guinea-pig, according to Montgomery ('98), centrosomes and asters are present in the first polar spindle, but absent in the second. The evidence is on the whole rather strong that the achromatic figure in these cases approaches in form that seen in the higher plants ; but it is an open question whether the appearances described may not be a result of imperfect fixation.

F. Reduction in Unicellular Forms

Although the one-celled and other lower forms have not yet been sufficiently investigated, we have already good ground for the conclusion that a process analogous to the reduction of higher types regularly recurs in them. In the conjugation of Infusoria, as already described (p. 223), the original nucleus divides several times before union, and only one of the resulting nuclei becomes the conjugating germ-nucleus, while the others perish, like the polar bodies. The numerical correspondence between the rejected nuclei or "corpuscules de rebut" has already been pointed out (p. 227). Hertwig could not count the chromosomes with absolute certainty, yet he states ('89) that in Paramcecium caudatum, during the final division, the number of spindle-fibres and of the corresponding chromatic elements is but 4-6, while in the

1 Cf. p. 312.

2 Sala C94) and Fiirst have shown that occasionally the polar spindles of Ascaris are provided with large typical asters, and thus resemble those of annelids or mollusks. Sala believed this to be an effect of lowered temperature, but Fiirst's observations are unfavourable to this conclusion.


earlier divisions the number is approximately double this(8-9). This observation makes it nearly certain that a numerical reduction of chromosomes occurs in the Protozoa in a manner similar to that of the higher forms ; but the reduction here appears to be deferred until


Fig. 138. — Conjugation and formation of the polar bodies in Attinipkrys. [SCHMIDINN.] A. Union of tbe gametes: first polar spindle. B. Fusion of the cell-bodies; a single polar



r the periphery of each. C Fusion of them


the final division. In the gregarines Wolters ('91) has observed the formation of an actual polar body as a small cell segmented off from each of the two conjugating animals soon after their union ; but the number of chromosomes was not determined. Sc haudinn ('96, 2) has observed a like process in Actinephtys, each of the gametes segmenting off a single polar body, after which the germnuclei fuse (Fig. 138). It is possible, as R. Hertwig ('98) points out, that in both these forms a second polar body may have been overlooked, owing perhaps to its rapid disintegration. In Actinosphtzrittm, according to R. Hertwig ('98), the nucleus of each gamete divides twice in rapid succession to form two polar bodies (nuclei), which degenerate, after


which the germ-nuclei unite (Fig. 139). Whether a reduction in the number of chromosomes occurs in these cases was not determined.'



A. Soon after union, f< distinct. C. Fusion of the F. Second cleavage. G. 1


Conjugation of Cfosttrivm. [Ki.h

hroTnatopliorcs. li. C hro mat oph ores reduced lo two, nuclei ii. D. First cleavage of the lygole. E. Resulting a-cell stage.

ing stage, each cell bi-nucleate. H. St-paration of the cells; one o: the nuclei in eacn enlarging to form the permanent nucleus, the other (probably representing a polar body) degenerating.

1 AiHnospharium forms one of the most extreme known cases of in-breeding; for the gametes are sitlcr-cills which immediately reunite after forming the polar bodies. The general facta are as follows: The mother animal, containing very numerous nuclei, become* encysted, and a very large number of the nuclei degenerate. The body then segments into


Adelea (one of the Coccidiae) is a very interesting case, for according to Siedlecki C99) polar bodies or their analogues are formed in both sexes. The gametes are here of very unequal size. Upon their union the smaller male cell divides twice to form apparently equivalent spermatozoids, of which, however, only one enters the ovum, while three degenerate as polar bodies. These two divisions are of different type ; the first resembles true mitosis, while the second is of simpler character and is believed by Siedlecki to effect a reduction in the number of chromosomes. In the meantime the nucleus of the macrogamete moves to the surface and there expels a portion of its chromatin, after which union of the nuclei takes place. Interesting facts have been observed in unicellular plants which indicate that the reduction may here occur either before (diatoms) or after (desmids) fusion of the conjugating nuclei. In the former (Rhopalodind) Klebahn ('96) finds that each nucleus divides twice, as in many Infusoria, giving rise to two large and two small nuclei. Each of the conjugates then divides, each daughter-cell receiving one large and one small nucleus. The four resulting individuals then conjugate, two and two, the large nuclei fusing while the small (polar bodies) degenerate. The comparison of this case with that of the Infusoria is highly interesting. In the desmids on the other hand {Closterium and Cosmarium, Fig. 140), according to Klebahn ('92), the nuclei first unite to form a cleavagenucleus, after which the zygote divides into two. Each of the new nuclei now divides, one of the products persisting as the permanent nucleus, while the other degenerates and disappears. Chmielewski asserts that a similar process occurs in Spirogyra. Although the numerical relations of the chromosomes have not been determined in these cases, it appears probable that the elimination of a nucleus in each cell is a process of reduction occurring after fertilization.


G. Maturation of Parthenogenetic Eggs

The maturation of eggs that develop without fertilization is a subject of special interest, partly because of its bearing on the general theory of fertilization, partly because it is here, as I believe, that one of the strongest supports is found for the hypothesis of the individuality of chromosomes. In an early article by Minot {'77) on the

a number (five to twelve) of "primary cysts," each containing one of the remaining nuclei. Each primary cyst divides by mitosis to form two gametes ("secondary cysts "), which, after forming the polar bodies, reunite, their nuclei fusing to form a single one. The resulting cell soon creeps out of the cyst-wall and assumes the active life, its nucleus meanwhile multiplying to produce the multinuclear condition characteristic of the adult animal. What is here the physiological motive for the formation of the polar bodies, and how shall it be explained under the Weismann hypothesis?


theoretical meaning of maturation, the suggestion is made that parthenogenesis may be due to failure on the part of the egg to form the polar bodies, the egg-nucleus thus remaining hermaphrodite, and hence capable of development without fertilization. This suggestion forms the germ of all later theories of parthenogenesis. Balfour ('8o) suggested that the function of forming polar cells has been acquired by the ovum for the express purpose of preventing parthenogenesis, and a nearly similar view was afterward maintained by Van Beneden. 1 These authors assumed accordingly that in parthenogenetic eggs no polar bodies are formed. Weismann ('86) soon discovered, however, that the parthenogenetic eggs of Polyphemus (one of the Daphnidae) produce a single polar body. This observation was quickly followed by the still more significant discovery by Blochmann ('88) that in Aphis the parthenogenetic eggs produce a single polar body, while the fertilized eggs produce two, Weismann was able to determine the same fact in ostracodes and Rotifera, and was thus led to the view 2 which later researches have entirely confirmed, that it is the second polar body that is of special significance in parthenogenesis. Blochmann observed that in insects the polar bodies were not actually thrown out of the egg, but remained embedded in its substance near the periphery. At the same time Boveri ('87, 1) discovered that in Ascaris the second polar body might in exceptional cases remain in the egg and there give rise to a resting-nucleus indistinguishable from the egg-nucleus or sperm-nucleus. He was thus led to the interesting suggestion that parthenogenesis might be due to the retention of the second polar body in the egg and its union with the egg-nucleus. "The second polar body would thus, in a certain sense, assume the role of the spermatozoon, and it might not without reason be said : " Parthenogenesis is the result of fertilisation by the second polar body." 3

This conclusion received a brilliant confirmation through the observations of Brauer ('93) on the parthenogenetic egg of Artetnia, though it appeared that Boveri arrived at only a part of the truth. Blochmann ('88-89) had found that in the parthenogenetic eggs of the honey-bee two polar bodies are formed, and Platner discovered the same fact in the butterfly Liparis ('89) — a fact which seemed to contradict Boveri's hypothesis. Brauer's beautiful researches resolved the contradiction by showing that there are two types of parthenogenesis which may occur in the same animal. In the one case Boveri's conception is exactly realized, while the other is easily brought into relation with it.


1 '83, p. 622. a Essay VI., p. 359. • /.<■., p. 73.

(a) In both modes typical tetrads are formed in the germ-nucleus to the number of eighty-four. In the first and more frequent case (Fig. 141) but one polar body is formed, which removes eighty-four dyads, leaving eighty-four in the egg. There may be an abortive attempt to form a second polar spindle, but no division results, and the eighty-four dyads give rise to a reticular cleavage-nucleus. From



Pig. 141. — First type of m


aluralion


in ihe parthenojjtnc


A. The nrsl polar spindle- 1 first polur hoily . H4 dyads rema F. Appearance of Ihe egg-uenl of Ihe cleavage-figure; ihe equa


lie equal

in in the


■ml 'lister, fs. G. 1

'<• consists of 84 apf


o the CRg-nnclcus. showi


this arise eighty-four thread-like chromosomes, and the same number appears in later cleavage-stages,

(A) It is the second and rarer mode that realizes Bovcri's conception (Fig. 142). Hoth polar bodies are formed, the first removing eighty-four dyads and leaving the same number in the egg. In the formation of the second, the eighty-four dyads are halved to form two daughter-groups, each containing eighty-four single chromosomes, Both these groups remain in the egg, and each gives rise to a single reticular nucleus, as described by Bovcri in Ascaris. These two nuclei place themselves side by side in the cleavage-figttre, and give rise each to eighty-four chromosomes, precisely like two germ-nuclei in ordinary fertilization. The one hundred and sixty-eight chromosomes split



Fig. 14a. — Second type of maluralii . Formation of second polar body


il plate


1 two groups of 84 each.


lengthwise, and are distributed in the usual manner, and reappear in the same number in later stages. In other words, the second polar body here plays the part of a sperm-nucleus precisely as maintained by Boveri.

In all individuals arising from eggs of the first type, therefore, the somatic number of chromosomes is eighty-four; in all those arising from eggs of the second type, it is one hundred and sixty-eight This difference is clearly due to the fact that in the latter case the chromosomes are single or univalent, while in the former they are bivalent (actually arising from dyads or double chromosomes). The remarkable feature, on which too much emphasis cannot be laid, is that the numerical difference should persist despite the fact that the mass, and, as far as we can see, the quality, of the chromatin is the same in both cases. In this fact we must recognize a strong support, not only of Hacker's and Vom Rath's conception of bivalent chromosomes, but also of the more general hypothesis of the individuality of chromosomes (Chapter VI.).

1. Accessory Cells of the Testis

It is necessary to touch here on the nature of the so-called " Sertoli-cells." or supporting cells of the testis in mammals, partly because of the theoretical significance attached to them by Minot, partly because of their relations to the question of amitosis in the testis. In the seminiferous tubules of the mammalian testis, the parentcells of the spermatozoa develop from the periphery inwards toward the lumen, where the spermatozoa are finally formed and set free. At the periphery is a layer of cells next the basement-membrane, having flat, oval nuclei. Within this, the cells are arranged in columns alternating more or less regularly with long, clear cells, containing large nuclei. The latter are the Sertoli-cells \ or supporting cells ; they extend nearly through from the basement-membrane to the lumen, and to their inner ends the young spermatozoa are attached by their heads, and there complete their growth. The spermatozoa are developed from cells which lie in columns between the Sertolicells, and which undoubtedly represent spermatogonia, spermatocytes, and spermatids, though their precise relationship is, to some extent, in doubt. The innermost of these cells, next the iumen, are spermatids, which, after their formation, are found attached to the Sertoli-cells, and are there converted into spermatozoa without further division. The deeper cells from which they arise are spermatocytes, and the spermatogonia lie deeper still, being probably represented by the large, rounded cells.

Two entirely different interpretations of the Sertoli-cells were advanced as long ago as 1 87 1, and both views still have their adherents. Von Ebner ('71) at first regarded the Sertoli-cell as the parent-cell of the group of spermatozoa attached to it. and the same view was afterward especially advocated by Biondi ('85 ) and by Mi not (92), the latter of whom regarded the nucleus of the Sertoli-cell as the physiological analogue of the polar bodies, i.e. as containing the female nuclear substance C92, p. 77). According to the opposing view, first suggested by Merkel C71). the Sertolicell is not the parent-cell, but a nurse-cell, the spermatozoa developing from the columns of rounded cells, and becoming secondarily attached to the Sertoli-cell, which serves merely as a support and a means of conveying nourishment to the growing spermatozoa. This view was advocated by Brown ('85), and especially by Benda C87). In the following year ('88), von Ebner himself abandoned his early hypothesis and strongly advocated Benda's views, adding the very significant result XhaXfour spermatids arise from each spermatocyte, precisely as was afterward shown to be the case in Ascaris, etc. The very careful and thorough work of Benda and von Ebner, confirmed by that of Lcnhosse'k ('98, 2), leaves no doubt that mammalian spermatogenesis conforms, in its main outlines, with that of Ascaris, the salamander, and other forms, and that Biondi's account is untenable. Minors theoretical interpretation of the Sertoli-cell, as the physiological equivalent of the polar bodies, therefore collapses.


2. A mitosis in the Early Sex-cells

Whether the progenitors of the germ-cells ever divide amitotically is a question of high theoretical interest. Numerous observers have described amitotic division in testis-cells. and a few also in those of the ovary. The recent observations of Meves ('91 ). Vom Rath C93), and others leave no doubt whatever that such divisions occur in the testis of manv animals. Vom Rath maintains, after an extended investigation. that all cells so dividing do not belong in the cycle of development of the germ-cells ('93, p. 164) : that amitosis occurs only in the supporting or nutritive cells (Sertoli-cells, etc.), or in such as are destined to degenerate, like the "residual bodies" of Van Beneden. Meves has, however, produced strong evidence C94) that in the salamander the spermatogonia may, in the autumn, divide by amitosis, and in the ensuing spring may again resume the process of mitotic division, and give rise to functional spermatozoa. On the strength of these observations Flemming C93) himself now admits the possibility that amitosis may form part of a normal cycle of development. 1

H. Summary and Conclusion

The one fact of maturation that stands out with perfect clearness and certainty amid all the controversies surrounding it is a reduction of the number of chromosomes in the ultimate germ-cells to one-half the number characteristic of the somatic cells. It is equally clear that this reduction is a preparation of the germ-cells for their subsequent union, and a means by which the number of chromosomes is held constant in the species. With a few exceptions the first indication of the numerical reduction appears through the segmentation of the spiremethread, or the resolution of the nuclear reticulum, into a number of masses one-half that of the somatic chromosomes. In nearly all higher animals this process first takes place two cell-generations before the formation of the definitive germ-cells, and the process of reduction is completed by two rapidly succeeding " maturation-divisions," giving rise to four cells, all of which become functional in the male, while in the female only one becomes the cgg 9 while the other three — the polar bodies or their analogues — are cast aside. During these two divisions each of the original chromatin-masses gives rise to four chromosomes, of which each of the four daughter-cells receives one ; hence, each of the latter receives one-half the somatic number of chromosomes. In the higher plants, however, the two maturationdivisions are followed by a number of others, in which the reduced number of chromosomes persists, a process most strikingly shown in the pteridophytes, where a separate sexual generation (prothallium) thus arises, all the cells of which show the reduced number.


1 For more recent literature on this subject see Meves, Zelltheilung, in Merkel and Bonnet's Ergebnisse, VIII., 1898.


Two general types of maturation may be distinguished according to the manner in which the primary chromatin-masses divide. In one, typically represented by Ascaris and the arthropods, each of these masses divides into four to form a tetrad, thus preparing at once for two rapidly succeeding divisions, which are not separated by a reconstruction of the daughter-nuclei during an intervening resting period. In the other, examples of which are given by the flowering plants and the spermatogenesis of the Amphibia, no true tetrads are formed, the primary chromatin-masses dividing separately for each of the maturation-divisions, which are separated by a period in which the nuclei regress toward the resting state, though often not completely returning to the reticular condition. These two types differ, however, only in degree, and with few exceptions they agree in the fact that during the prophases of the first division the chromatin-bodies assume the form of rings, the mitosis thus being of the heterotypical form, and each ring having the prospective value of four chromosomes.

Thus far the phenomena present no difficulty, and they give us a clear view of the process by which the numerical reduction of the chromosomes is effected. The confusion of the subject arises, on the one hand, from its complication with theories regarding the individuality of the chromosomes and the functions of chromatin in inheritance, on the other through conflicting results of observation on the mode of tetrad-formation and the character of the maturation-divisions. Regarding the latter question nearly all observers are now agreed that one of these divisions, usually the first, is a longitudinal or equationdivision, essentially like that occurring in ordinary mitosis. The main question turns upon the other division, which has been shown in some cases to be transverse and not longitudinal, and thus separates what were originally different regions of the spireme-thread or nuclear substance. The evidence in favour of such a division seems at present well-nigh demonstrative in the case of insects and copepods, and hardly less convincing in the turbellarians, annelids, and mollusks. On the other hand, both divisions are regarded as longitudinal by most of those who have investigated the phenomena in Ascaris and in the vertebrates, and by some of the most competent investigators of the flowering plants.

The evidence as it stands is so evenly balanced that the subject is hardly yet ripe for discussion. The principle for which Weismann contended in his theory of reducing division has received strong support in fact ; yet should it be finally established that numerical reduction may be effected either with or without transverse division, as now seems probable, not only will that theory have to be abandoned or wholly remodelled, but we shall have to seek a new basis for the interpretation of mitosis in general. Weismann's theory is no doubt of a highly artificial character; but this should not close our eyes to the great interest of the problem that it attempted to solve.


The existing contradiction of results has led to the opinion, expressed by a number of recent writers, that the difference between longitudinal or transverse division is of minor importance, and that the entire question of reduction is a barren one. This opinion fails to reckon with the facts on which rests the hypothesis of the individuality of chromosomes (Chap. VI.); but these facts cannot be left out of account. We must find a common basis of interpretation for them and for the phenomena of reduction ; yet how shall we reconcile them with reduction by longitudinal division only ? I cannot, therefore, share the opinion that we are dealing with a barren problem. The peculiarities of the maturation-mitoses are obviously correlated in some way with the numerical reduction, and the fact that they differ in so many ways from the characters of ordinary mitosis gives ground to hope that their exhaustive study will throw further light not only on the reduction-problem itself but also on mitosis in general and on still wider problems relating to the individuality of the chromosomes and the morphological organization of the nucleus. It is indeed very probable that Weismann's theory is but a rude attempt to attack the problem, and one that may prove to have been futile. The problem itself cannot be ignored, nor can it be dissociated from the series of kindred problems of which it forms a part


Literature

  • See also Literature, IV., p. 231.

Van Beneden, E. — Recherches sur la maturation de Toeuf, la fecondation et la division

cellulaire: Arch. Biol. % IV. 1883. Boveri, Th. — Zellenstudien, I., III. Jena, 1887-90. See also " Befruchtung "

(List IV.). Brauer, A. — Zur Kenntniss der Spermatogenese von Ascaris megalocephala : Arch.

mik. Anat., XLIL 1893. Id. — Zur Kenntniss der Reifung der parthenogenetisch sich entwickelnden Eies von

Artemia Salina: Arch. mik. Aflat., XLI1I. 1894. Guignard, L. — Le deVeloppement du pollen et la reduction chromatique dans le

Naias: Arch. Anat. Mic. % II. 1899. (Full literature on reduction in plants.) Griffin, B. B. — See Literature, IV. Hacker, V. — Die Vorstadien der Eireifung (General Review) : Arch. mik. Anat. t

XLV. 2. 1895. Id. — Ober weitere Obereinstimmungen zwischen den Fertpflanzungsvorgangen der

Thiere und Pflanzen : Biol. Centralb.s XVII. 1897. Id. — Cber vorbereitende Theilungsvorgange bei Thieren und Pflanzen : Verh.

deutsch. Zool. Ges. % VIII. 1898. Id. — Die Reifungserscheinungen : Merkel und Bonnets Ergebnisse. VIII. 1898. Hertwig, 0. — Vergleich der Ei- und Samenbildung bei Nematoden. Eine Grund lage flir cellulare Streitfragen : Arch. mik. Anat., XXX V I. 1890. Mark, E. L. — (See List IV.) Peter, K. — Die Bedeutung der Nahrzellen im Hoden : Arch. mik. Anat., LI II. 1898.

Platner, 6. — Uber die Bedeutung der Richtungskorperchen : Biol. Centralb., VIII. 1889.

Vom Rath, 0. — Zur Kenntniss der Spermatogenese von Gryllotalpa vulgaris: Arch, mik. Anat.s XL. 1892.

Id. — Neue Beitrage zur Frage der Chromatinreduktion in der Samen- und Eireife : Arch. mik. A fiat., XLVI. 1895.

Riickert, J. — Die Chromatinreduktion der Chromosomenzahl im Entwicklungsgang der Organismen : Ergebn. d. Anal. u. Entwick.* III. 1893 (1894).

Strasburger, E. — (Jber periodische Reduktion der Chromosomenzahl im Entwicklungsgang der Organismen : Biol. Centralb., XIV. 1894.

Id. — Reduktionstheilung, Spindelbildung, etc. : Jena, Fischer, 1900.




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

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


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