274 GENETICS 



parthenogenic eggs no whole ^-chromosome is extruded 

 into the single polar cell given off and consequently the 

 egg, retaining 2n -\- xx chromosomes ( six in number) , 

 develops into a female. 



In the spermatogenesis of these forms it has been 

 found that only one secondary spermatocyte develops 

 from each primary spermatocyte, namely, the one 

 which receives the J7-chromosome. Thus, only two in- 

 stead of four spermatids result from a primary sper- 

 matocyte and these two form female-determining sper- 

 matozoa. The "winter eggs" of these insects have two 

 maturation divisions reducing the chromosomes to the 

 haploid condition. The female diploid number is re- 

 stored upon fertilization. 



It would seem, therefore, that in the phylloxerans 

 and aphids at least, maturation is actually controlled 

 by the size and composition of the egg. 



B. ROTIFERS AND DAPHNIDS 



It is unfortunate that the rotifers and daphnids, 

 which lend themselves so favorably to breeding experi- 

 ments, are not as favorable cytological material as the 

 homopterons, for it is not at all unlikely that their 

 sex-determination rests upon a similar basis to that 

 above described. 



In rotifers and daphnids, as in homopterons, fer- 

 tilized eggs give rise to females, whereas during par- 

 thenogenesis both females and males may arise, the 

 latter coming from smaller eggs than the former. 

 These facts are all the more interesting for the reason 



