Page:EB1911 - Volume 07.djvu/741

 Enchaeta and Calanus (vom Rath), and in the Copepods Heterocope and Diaptomus (Rückert), and in other types.



From O. Hertwig, Allgemeine Biologie, by permission of Gustav Fischer.

—Heterotypical Mitosis. (Schematic, after Flemming.) All the above cases, in which the reduction is effected by the distribution of entire chromosomes at one or other of the maturation divisions, may be grouped together as “pseudomitotic” (Häcker, and Korschelt & Heider). In sharp contrast to the pseudomitotic method is the “Eumitotic” method, in which the chromosomes are longitudinally divided at both divisions. Such a method not only robs the process of any “reducing” value in Weismann’s sense, but is in serious conflict with the chromosome-individuality hypothesis. Nevertheless it is in this sense that Boveri (1881) and van Beneden (1883–1887) described the maturation of the egg, and at a later period Brauer (1893) that of the spermatozoon, in Ascaris. In each case the tetrads are formed by the double longitudinal splitting of the chromosomes, the latter appearing in the prophase in the reduced number. Not only was the eumitotic method of Ascaris the first method to be described, but the descriptions are fully equal in point of clearness to that of Hertwig for the pseudomitotic maturation of Cyclops. A similar eumitotic maturation has been described for other types also, e.g. Sagitta and the Heteropods, but nowhere more frequently than in the Vertebrates among animals and the Phanerogams among plants. In these two latter groups the chromosomes of the reducing division only rarely have a ring form comparable to that seen in Gryllotalpa, &c. When such rings do occur their genesis is very obscure, and at no time do they present the appearance of “tetrads.” It is the characteristic appearance these looped chromosomes give to the first maturation division in many Vertebrates, and especially in the Amphibia (fig. 12), that originally led Flemming (1887) to term this type of mitosis “heterotypical”; the second division, lacking this peculiar appearance, being distinguished as “homotypical.” Until quite recently these looped chromosomes of the heterotypical mitosis of Vertebrates (and plants) were described as arising by the opening out of longitudinally split chromosomes, exactly as this occurs in the early prophase of the maturation divisions in such types as Gryllotalpa, Diaptomus, &c. In the heterotype mitosis, however, no transverse segmentation appears, and the halves of the rings, as they separate in the first division, show an obvious longitudinal split in preparation for the second division. Both divisions were thus interpreted as equating divisions. The more recent works of Farmer and Moore (1903–1905), Montgomery (1903, Amphibia), and (for plants) Strasburger (1903–1904) have shown, however, that even for the higher plants and animals, a reducing division in Weismann’s sense occurs in an essentially similar manner to that so convincingly described by Rückert, vom Rath and others, for Invertebrate types. For the chromosomes of the heterotype mitosis arise by the looping round, not opening out, of the bivalent chromosomes. The first division is thus a reducing division, while the split appearing in the anaphase of the heterotype and presumably reappearing in the prophase of the homotype is the original split of the spireme thread.

The widespread, if not universal, formation of tetrads, i.e. the temporary union in pairs of split chromosomes, in reduction, and the relation this latter process always bears to two rapidly succeeding maturation divisions—those completing the gametogenic cycle in animals and terminating the sporophytic generation in plants,—has received a suggestive explanation at the hands of Boveri (1904). The growth of the chromatin is an indispensable prelude to its reproduction (Boveri’s Law of Proportional Growth). The chromatin is therefore incapable of undergoing reproductive fission in two successive mitotic divisions when these are not separated by a resting (i.e. growth) period. In addition to this, the “bipolar” condition of the adult chromosomes, which determines its mode of attachment to mantle fibres from both poles of the spindle, is not possessed by the unripe chromatin. The undivided, i.e. unripe, chromosomes are therefore incapable of utilizing the mitotic mechanism for such a transverse fission as Weismann originally postulated. The difficulty is, however, at once overcome if the unripe chromosomes are associated in pairs in the equatorial plate, for the bivalent chromosomes so produced are bipolar just as are the adult (i.e. split) chromosomes in the ordinary and homotype mitosis.

Synopsis (, to fuse together).—During the prophase of the reducing or heterotype divisions the whole of the chromatin becomes temporarily massed together at one pole of the nucleus (Moore, 1896, for Elasmobranchs). Montgomery (1901) has suggested that this is to facilitate the temporary union in pairs, or “conjugation” of homologous paternal and maternal chromosomes. In Ascaris megalocephala var. univalens, where the somatic number is only two, the association must necessarily be between homologous chromosomes. The assumption that this “selective pairing” of equivalent chromosomes is universal is supported by the behaviour of the “Heterochromosomes” (Montgomery) of the Hemiptera. These chromosomes, distinguished by their size, are paired before, and single after, the “pseudo-reduction” has taken place. Even more convincing is Sutton’s account of reduction in Brachystola already referred to. Boveri (1904) has suggested that this temporary association of the chromosomes—presumably facilitated by the synapsis—has a much deeper meaning than to ensure their correct distribution between the daughter nuclei in the heterotype mitosis; the associated chromosomes exchanging material in a manner analogous to conjugation in Paramoecium.

Present Position of the Cell-theory.—Since the time of Schleiden and Schwann a wealth of evidence has accumulated in support of the “cell-theory”—the theory which regards the cell as the unit of organic structure. “The organism consists