Heredity and Eugenics 



formed by fertilization following reduction under unfavor- 

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

 is 2 N. Males arise only by parthenogenesis under unfavor- 

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

 occurs before development begins is partial only. A whole 

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

 only i or 2 chromosomes. Hence the male condition here 



is 2Ni 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 

 parthenogenesis, the female 

 is in the duplex (27V) condition, the male in the simplex 

 (TV) or partially duplex condition (2Ni, or 2N 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 known that, although the 

 male, like the female, develops only from a fertilized egg, 

 nevertheless the male possesses fewer chromosomes than 



FIG. 35. Diagram of sex determi- 

 nation when the female is homozygous, 

 the male heterozygous. 



