MECHANISM IN SEGREaATION 55 



development (cell-divisions) its two X's and its Y. There 

 is evidence for this, obtained by Bridges, both from 

 observation of the cells themselves and from the genetic 

 behavior of such an individual. 



In certain crosses between moths with different num- 

 bers and sizes of chromosomes, Federley, and Harrison, 

 and Doncaster have shown that the cell of the hybrid 

 contains half the number of each species, even with 

 their characteristic size differences (Fig. 23). In crosses 

 between different species of fish, where the size differ- 

 ences are quite conspicuous, it has been shown by Moenk- 

 haus, Morris and Pinney (Fig. 24) that the embryonic 

 cells may continue through their divisions to retain 

 the characteristic chromosomes of both species. These 

 hybrid cases are particularly significant ; for the chromo- 

 somes derived from the father are in the foreign 

 medium of the protoplasm of the other species. Never- 

 theless, in some cases they retain their own peculiarities, 

 through successive cell generations. 



EvmENCE That Homologous Chromosomes Mate with 



Each Other 



That the mating of the chromosomes in pairs is not a 

 haphazard process, but that each paternal chromosome 





O<^o 



Fig. 25. — Female and male chromosome groups of Prolenar. (After Wilson.) 



mates with a definite maternal chromosome, has been 

 established by evidence from several sources. In many 

 species the chromosomes are of different sizes, and some- 

 times certain ones are markedly different in size from the 

 others. In the bug Protenor the two sex-chromosomes 



