7° 



Heredity and Eugenics 



formed by fertilization following reduction under unfavor- 

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

 is 2 X, 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, X, of chromosomes is not eliminated in maturation, but 

 only I or 2 chromosomes. Hence the male condition here 



is 2N—1 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 X—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 {2N) condition, the male in the simplex 



(T) or partially duplex condition (2.Y— i, or 2X — 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. 



