184 



PHYSICAL BASIS OF HEEEDITY 



chromosome in the polar body has not yet been observed 

 in the group. 



In both phylloxerans and aphids there are two classes 

 of sperm produced in the males as in other insects, one 

 with X, one without it. The latter degenerates, and only 

 the X or female-producing sperm remains functional. A 

 few stages in the spermatogenesis of the bearberry aphid 



Fig. 84. — First and second spermatocyte division in the bearberry aphid with the formatian 

 of one rudimentary cell. 



are shown in Fig. 84, a-g. In h, the chromosomes have 

 divided and moved to opposite poles while the sex-chromo- 

 some is drawn out but has not moved yet to either pole. 

 In c, the sex-chromosome has been drawn into the larger 

 of the two cells that is produced. In d, the division into a 

 larger and a smaller cell is completed. In e, preparations 

 for another division are taking place in the larger cell, and 

 in / and g this is completed. The smaller cell does not 

 divide, and later degenerates. The two spermatozoa from 



