. Cytology, with special reference to the metazoan nucleus. Cells; Cytology. NUMBER OF CHROMOSOMES 141 (Fig. 68). In this genus the type number is 35 for the male; the chromosomes are rod-shaped. In some individuals, however, the number is reduced to 34 or 33 (spermatogonia). The 34 type has 33 rod-shaped and one V chromosome, while the 33 type has 31 rods and two Vs. The former case is in- teresting as an example of homologous chromo- somes behaving dif- ferently, for since there is only one V instead of a pair of them, linkage must have taken place between one member of each of two pairs of
. Cytology, with special reference to the metazoan nucleus. Cells; Cytology. NUMBER OF CHROMOSOMES 141 (Fig. 68). In this genus the type number is 35 for the male; the chromosomes are rod-shaped. In some individuals, however, the number is reduced to 34 or 33 (spermatogonia). The 34 type has 33 rod-shaped and one V chromosome, while the 33 type has 31 rods and two Vs. The former case is in- teresting as an example of homologous chromo- somes behaving dif- ferently, for since there is only one V instead of a pair of them, linkage must have taken place between one member of each of two pairs of homologous chromo- somes, the other mem- ber remaining free. That is to say, the chromosome formula is AB+A+B+C + C+ . . Wliat happens in this case at syndesis ? While we do not know the details of this process, the result is clearly shown by the structure of the chromosomes of the first meiotic division. They consist, besides the sex chromosome, of fifteen ordinary biva- lents and one tetrapar- Fig. tite V of the type shown in the figure. As the. 9 mf r. '^# The chromosomes of Jamaicana subguttata (A-E) and /. tinicolor (F-G). (Woolsey, , 1915.) A, spermatogonium of an individual with the type number (35) of chromosomes. The chromosomes are numbered in pairs from the smallest to the largest. No. 18 is the sex SpermatOgOnial divisions chromosome. B-E, chromosomes of an individual in which one member _ _ _ of two homologous pairs (14 and 16) have become associated ; B, sper- Show, the original linkage matogonlum ; C, late prophase I. ; D, anaphase I.; E, another view of the bivalent 14-16 ; F, G, isolated bivalent and spermatogonia! group was between two chrOmO- trom an individual in which both members of the two homologous pairs are associated. somes of considerably different sizes, so that a V with unequal limbs is formed. The tetrapartite bivalent at meiosis is formed by the pairing of each limb of this V with its A B homologue, and may be represented thus : \ / (Fig. 68,
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