Heredity and Eugenics 



onl\- I or 2 chromosomes. 



9 



formed by fertilization following reduction under unfavor- 

 able conditions, just as in rotifers. In both cases the female 

 is 2 X. jN Iale s arise only by parthenogenesis under unfavor- 

 able conditions, just as in rotifers, but the reduction wMch 

 occurs before develoi)ment begins is partial onlv^ A whole 

 set, X, of chromosomes is not eliminated in maturation, but 



Hence the male condition here 

 is 2X— I or —2. The condi- 

 tion of the gametes formed, 

 however, is N in both sexes. 

 In spermatogenesis, division 

 of the germ cells takes place 

 into N and N~i daughter 

 cells, but the latter degene- 

 rate (like the non-nucleated 

 cells of the bee and wasp), 

 and only the former produce 

 spermatozoa. Hence in ferti- 

 lization only 2N zygotes are 

 produced, which are invari- 

 ably female. 



Summarizing the three 

 categories described, we may 

 say that in all known cases of 

 parthen ogenesis, th e f ema 1 e 

 is in the__du|>lex {2N) conditioii, the male _in_ the simjjlex 

 (N) oTpartially duplex condition (2iV — i, or 2iV— 2). The 

 female in all cases has the greater chromatin content. 



In a great many insects and other arthropods, which 

 are not parthenogenetic, it is k nown that, althoug h_Jji£. 

 ni ale, like the fe m_aLe,__d£ve lops onh' fr om a fertilized egg , 

 ne\"ertheless the male possesses fewer chromosomes than 



5. — Diagram of sex determi- 

 nation when the female is hom(iz},'gous, 

 the male heterozyf,'ous. 



