MECHANISM IN SEGREGATION 57 



sex chromosome has no mate, and therefore nothing to 

 fuse with. Its size, after the others have conjugated 

 (Fig. 27) shows that it remains single; while its failure 

 to divide twice, as do the other chromosomes, corrobo- 

 rates the view that having no mate of its own it never 

 combines with any other. At the other extreme, the two 

 very minute chromosomes in several of the DrosopMla 

 species must have united to form the smallest chromo- 

 some of the reduced series (Fig. 28, a-a', h-h'). In a 

 few cases the X and the Y are different in size. When 

 they fuse (in the male) the size of the fused mass is what 



\^ 6 sj 



Fig. 28. — Diploid and haploid chromosome groups of Drosophila busckii, a, a', and D. 



tnelanica, {neglecta) b, h'. (After Metz.) 



is expected, viz., the sum of the masses of X and Y, and 

 their subsequent separation into parts corresponding in 

 size to the fused bodies supports the view that conjuga- 

 tion amongst the chromosomes is a very definite process. 

 In the very exceptional case of a bug, Metapodius, there 

 is a pair of small chromosomes called m's. When the 

 other pairs enter the spindle the two m's come together, 

 touch, and then separate, to pass to opposite poles. 



Resume 



The evidence from studies of the maturation of eggs 

 and sperm shows that the paternal and maternal chromo- 



