Heredity and Sex ji 



the f emal e. 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 z>-gotes of two sorts result, those which are 

 2N, female, and those which are 2N — 1 or 2N — 2, male. 

 Thus in the sciimshhug, Anasa Irisiis, according to Wilson, 

 tlie_ mature egg contains 11 cln-omosomes^ the '^pprmatozoa , 

 e ither 10 or 11 chro mosomgs, th£_tw o sorts being equ ally 

 numerous! ' 



Egs ii+si)erm 11 produces a z3-p;ote 22 (27V), a female 

 Egg ii+sperm 10 produces a zygote 21 {2N-1), a male 



N in this species = ii; 2N = 22, the female; 2yA^ — 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 INIendelian 

 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 



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 

 2.^ but the female 2N-\-. 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 wa s 

 the same, one of the chromosomes was larger than it s mate, i 

 having an extra or odd element attached to it. IrTsuch a ^ 



