Heredity and Sex , 71 



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



parthenogenesis, only .Y gametes, but the male forms 



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



In consequence zygotes of two sorts result, those which are 



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



Thus in the sqxidishhng, Anasa tristis, according to Wilson, 



the mature egg contains 11 chromosomes, the spermatozoa 



either 10 or 11 chromosomes, the two sorts being ecjually 



numerous. 



Egg ii+sperm 11 produces a zygote 22 (2AO, a female 

 Egg ii+sperm 10 produces a zygote 21 (2iV-i), a male 



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



2.Y but the female 2^^+. 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 



