Heredity and Sex 71 



the female. In such cases the female forms, as in cases of 

 parthenogenesis, only N gametes, but the male forms 

 gametes of two sorts, N and N i or N 2 (Fig. 35). 

 In consequence zygotes of two sorts result, those which are 

 2N, female, and those which are 2Ni or 2N2, male. 

 Thus in the squash bug, Anasa tristis, according to Wilson, 

 the mature egg contains n chromosomes, the spermatozoa 

 either 10 or n chromosomes, the two sorts being equally 

 numerous. 



Egg n+sperm n produces a zygote 22 (2^V), a female 

 Egg n+sperm 10 produces a zygote 21 (zN-i}, a male 



N in this species = n; 2N=22, the female; 2N 1 = 21, 

 the male. Males and females are therefore approximately 

 equal in number, as in most animals where the two sexes 

 are not subject to unequal mortality. In the Mendelian 

 sense the female is in such cases a homozygote, the male a 

 heterozygote. The sex of an individual in such cases 

 depends upon which sort of a sperm chances to enter the 



egg- 



But the experimental evidence indicates that both as 

 regards sex and as regards heritable characters correlated 

 with sex, these relations may in some cases be reversed, the 

 female being heterozygous, the male homozygous. In such 

 cases there is reason to think that structurally the male is 

 2N but the female iN-}-. That is, the female is still the 

 equivalent of the male plus some additional element and 

 function. A structural basis in the chromosomes for such a 

 condition has been described by Baltzer in the case of the 

 sea-urchin. He found the regular duplex number of chromo- 

 somes in the male; but in the female, while the number was 

 the same, one of the chromosomes was larger than its mate, 

 having an extra or odd element attached to it. In such a 



