. Cytology, with special reference to the metazoan nucleus. Cells. 142 CYTOLOGY chap. chromosomes composing it, so that the V (AB) goes to one pole, and the two non-Unked chromosomes (A and B) to the other (Fig. 68, D). In the case of the individuals with 33 chromosomes— with two V's, or, in other words, in which linkage has occurred in both homologous couples—we find instead of the open tetrapartite V in metaphase I. a closed tetrapartite ring Hke the usual type of bivalent formed by syndesis of two V's (cf. Lepidosiren, Fig. 16). Other similar cases could be cited, , Notonecta (Brown
. Cytology, with special reference to the metazoan nucleus. Cells. 142 CYTOLOGY chap. chromosomes composing it, so that the V (AB) goes to one pole, and the two non-Unked chromosomes (A and B) to the other (Fig. 68, D). In the case of the individuals with 33 chromosomes— with two V's, or, in other words, in which linkage has occurred in both homologous couples—we find instead of the open tetrapartite V in metaphase I. a closed tetrapartite ring Hke the usual type of bivalent formed by syndesis of two V's (cf. Lepidosiren, Fig. 16). Other similar cases could be cited, , Notonecta (Browne, 1913). Here w = 13 or 14, the former number being produced by the linkage of two chromosomes which are separate in the latter. \ Fig. 69 illustrates five types of chromosome complexes found in various species of the genus Drosophila, with their possible relationships. Fig. 69. Five types of chromosome complex found in the genus Drosophila, showing their probable relationships. (After Metz, , 1914.) (Metz, 1914). The remaining four types are all derivable from type I. by (i) breaking across of one or both pairs of V's, and (2) disappearance, or attachment to another pair, of the pair of very small chromosomes. More recently (1916) Metz has added several other types, all, however, simply related to the above. So far we have dealt with linkage or fragmentation of chromosomes permanent for the individual or species. A fragmentation of the type chromosomes, leading to a variation of chromosome number in different tissues or cells of a given individual, has also often been observed or inferred. The classical case is that of A scar is megalocephala, in which the somatic chromosomes undergo fragmentation, so that somatic mitoses exhibit far smaller and more numerous chromosomes (about 60 in ^. m, univalens) than do those in the germ track (see Chapter III.). It is note- worthy that Payne (1913 h) found that subjecting eggs to radium emana- tion causes the chromosomes in th
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