. Cytology, with special reference to the metazoan nucleus. Cells. V HOMOLOGOUS CHROMOSOMES 125 called homologous chromosomes, pair together to form the bivalents. The nucleus of the primary oocyte or spermatocyte can therefore be written : AA+BB+CC+ ... At the reduction division the homologous chromosomes are separated, as described in Chapter II., so that each gamete nucleus has the formula A+B+C+ . . The diploid nucleus of the first formula is of course reconstituted at syngamy. Thus it follows that one member of each pair of homologous chromo- •v.*. V 6 4. Fig. 55. Illustrating the tendenc
. Cytology, with special reference to the metazoan nucleus. Cells. V HOMOLOGOUS CHROMOSOMES 125 called homologous chromosomes, pair together to form the bivalents. The nucleus of the primary oocyte or spermatocyte can therefore be written : AA+BB+CC+ ... At the reduction division the homologous chromosomes are separated, as described in Chapter II., so that each gamete nucleus has the formula A+B+C+ . . The diploid nucleus of the first formula is of course reconstituted at syngamy. Thus it follows that one member of each pair of homologous chromo- •v.*. V 6 4. Fig. 55. Illustrating the tendency of homologous chromosomes to lie near each other in somatic nuclei. (Miiller, , 1912.) AC, polar views of equatorial plates; D, prophase. A, Eucomis bicolor; B, Albuca Jastigiata ; C, Galtonia candicans ; D, Dahlia coronata. Some of the pairs are numbered. somes in a diploid nucleus has been derived from the male and one from the female parent. This morphological fact, together with its theoretical consequences for heredity (to be discussed in the next chapter) was first pointed out clearly by Sutton in the case of the insect Brachystola magna. The degree to which the chromosomes of a single nucleus differ from one another in length varies greatly, and indeed in some species no certain differences are detectable. Such species are of course of negative value as evidence in this respect, the generahzation being founded on those numerous other forms in which the chromosomes exhibit marked size differences. The seriation of the chromosomes according to size is often facihtated by a tendency on the part of homologous chromosomes to He near or next to one another on the equatorial plate. This tendency also varies in different species, in some indeed apparently not existing, while in others {, Yucca', Muller, 1912) it is pronounced (Fig. 55).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - colora
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