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+ . . . 

 What 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- 

 tite V of the type shown 

 in the figure. As the 

 divisions 



FIG. 



The chromosomes of Jamaicana subguttata (A-E) and /. unicolor 

 (F-G). (Woolsey, B.B., 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 

 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 matogonium ; C, late prophase I. ; D, anaphase I. ; E, another view of 



the bivalent 14-16 ; F, G, isolated bivalent and spermatogonial group 



Was between tWO chrOmO- from 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, C). The 



A B 



composite bivalent divides at the points of junction of the homologous 



