originally entered into the composition of the nucleus (Boveri’s Law of Chromosome-Constancy), it follows that, in the mitotic figures of the developing embryo, the chromosomes will be half maternal, half paternal in origin;[1] the germ nuclei thus necessarily possessing only half the number of chromosomes characteristic of the ordinary tissue cells of species, i.e. the somatic number.[2] The manner in which this “reduction” in the number of chromosomes in the germ-cells is brought about, and the significance to be attached to the process, constitute the most hotly debated questions in cytology. In all the metazoa the phenomenon of reduction is associated with the two last and, usually, rapidly succeeding “maturation” divisions by which the definitive germ-cells—ova or spermatozoa—are produced.[3]
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From Korschelt and Heider’s Lehrbuch d. vergl. Entwicklungsgeschichte d. wirbellosen Tiere, by permission of Gustav Fischer.
Fig. 10.—Maturation Divisions. a–d, Formation of the tetrads in Cyclops. (After Rückert.) e, 1st maturation division; separation of the bivalent sister chromosomes. f, 2nd maturation division; distribution of the univalent chromosomes.
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From Prof. E. B. Wilson’s The Cell in Development and Inheritance, by permission of the author and of the Macmillan Co., N. Y.
Fig. 11.—Maturation Divisions. Origin of the tetrads by ring formation in the spermatogenesis of the mole-cricket (Gryllotalpa) (vom Rath). a, Primary spermatocyte with six split, bivalent chromosomes. b and c, Split has opened out. d, Concentration of the chromatin has made visible the belated transverse division. e and f, Grouping of the completed tetrads in the equatorial plate of the first maturation division.
Assuming the persistent individuality of the chromosomes, then there are only three conceivable methods by which this numerical reduction can be brought about (Boveri, 1904, p. 60). (1) One-half the chromosomes degenerate. (2) The chromosomes are distributed entire, half to one daughter cell, half to the other (reducing division of Weismann, 1887). (3) The chromosomes fuse in pairs (Conjugation of the Chromosomes, Boveri, 1892). The first possibility—that of an actual degeneration of a part of the chromatin originally suggested by van Beneden and adopted by August Weismann, Boveri and others, has been long abandoned, and a steadily increasing bulk of evidence is tending to prove the general, if not universal, occurrence of the second method—the distribution between the daughter cells of undivided chromosomes. The occurrence of such a “reducing division” was postulated on theoretical grounds by Weismann (1887)[4] and by Boveri (1888); by the former as a result of his adoption of de Vries’s hypothesis of self-propagating and qualitatively varying units for the chromatin; by the latter in relation to his theory of chromosome individuality. The actual occurrence of this reducing division was first demonstrated by Henking (1891) for Pyrrhocoris, and afterwards by Häcker, vom Rath and many others, but especially by Rückert (1894) for Cyclops (fig. 10). In this latter type the chromatin of the oocyte, as this prepares for the first maturation division, resolves itself into 12 (instead of 24) longitudinally split chromosomes (fig. 10, a). As these continue to thicken and contract a transverse fission appears (fig. 10, c). This is to be regarded as a belated segmentation of the spireme thread, and shows that the reduction so far is only a “pseudo-reduction” (Rückert), the chromosomes being really all present but temporally united in pairs, i.e. “bivalent” (Häcker). A striking confirmation of this interpretation is provided by Korschelt’s description of reduction in the annelid Ophryotrocha. In this type the full somatic number of split chromosomes (here only four) appears, and these secondarily associate end to end in pairs, thus forming split “diads” (i.e. tetrads), in every way similar to those described by Rückert for Cyclops. In the latter type, at the first maturation division, the sister diads are separated from one another, an “equating” division thus taking place. At the second division the diads are resolved into their constituent parts, and the “univalent” chromosomes are distributed to the daughter cells (reducing division). A similar process has since been described for numerous other types (e.g. various arthropods, Häcker, 1895–1898; vom Rath, 1895; and by Sutton for Brachystola, 1902–1903). In Ophryotrocha, as in Pyrrhocoris (Henking), Anasa (Paulmeir), Peripatus (Montgomery), &c., reduction occurs at the first maturation division (“pre-reduction” of Korschelt and Heider, 1900), instead of at the second division (post-reduction) as in most Copepods and Orthoptera. In many cases the tetrads (i.e. split chromosomes associated in pairs) have the form of rings, the genesis of which was first clearly determined by vom Rath (1892) in the mole cricket Gryllotalpa (fig. 11). In this form the sister diads remain united by their ends but widely separate in the middle (fig. 11, b). As in Cyclops, the belated transverse segmentation appears as the condensation of the chromatin proceeds (fig. 11, d), but the symmetrical tetrads which this process here produces make it impossible to determine at which of the two divisions reduction is effected. An essentially similar ring formation occurs in
- ↑ Häcker, “Über die Selbstständigkeit der väterlichen und mütterlichen Kernbestandteile,” Arch. f. mikr. Anat. Bd. xlvi. (1896).
- ↑ First discovered by van Beneden (1883, 1887) for the egg of Ascaris.
- ↑ In the case of the egg the whole of the yolk stored by the “oocyte” (cell-generation immediately preceding the maturation divisions) is handed on to only one of the four resulting cells—an obvious economy. The three yolkless cells are necessarily functionless—abortive ova—and are known as the “polar bodies” (Hertwig). In spermatogenesis the maturation divisions, though bearing the same relation to reduction as in oogenesis (Platner, 1889; O. Hertwig, 1890), give rise to four functional germ-cells. The explanation of sexual differentiation given above, and that of polar body formation given here, render it needless to do more than mention the theories of Mimot (1877), van Beneden (1883) and others, by which “maturation” was regarded as removing the “male” element from the otherwise “hermaphrodite” egg.
- ↑ Weismann postulated a transverse division of the chromosomes, not a distribution of entire chromosomes; but the result as far as the reduction in the number of hereditary qualities goes is the same. The inability of the mitotic mechanism to effect the transverse division of unsplit chromosomes is pointed out by Boveri (1904).
