S per)U(itogenesis 505 



number must also be even, and all the maturated eggs necessarily 

 possess the same number of chromosomes, 14. Applying Wilson's 

 ('06b) formula for sex determination to the Poeciloptera, we have 

 the foUovs^ing: 



I Egg (14 chromosomes) + Spermatozoon (14 chromosomes) 

 = Female (28 chromosomes). 



II Egg (14 chromosomes) + Spermatozoon (13 chromosomes) 

 = Male (27 chromosomes). 



Here again it is possible to apply Castle's ('00) theory of sex as a 

 Mendelian character, w^hich has been so fully elaborated and 

 applied to the case of the odd chromosome by Wilson. It involves 

 the assumption of two kinds of eggs, male and female, as well as 

 the two kinds of spermatozoa which are actually to be observed. 

 It also involves the assumption of selective fertilization: an egg 

 bearing the female determinant must be fertilized by a spermato- 

 zoon with the male determinant, while an egg bearing the male 

 determinant must be fertilized by a spermatozoon with the female 

 determinant. In case II of the above formula when the egg is 

 fertilized by the spermatozoon without the odd chromosome, the 

 sex determinant must be introduced by the egg; and as in this case, 

 a male is produced, the eggs fertilized by a spermatozoon without 

 an odd chromosome must bear the male determinant, and the 

 chromosome which has disappeared in the males must be the one 

 with the female character. So in case I, where the egg is fertil- 

 ized by the spermatozoon with the odd chromosome, the sperma- 

 tozoon must bear the male character and the egg the female; as 

 this combination always results in a female, it is necessary to 

 assume that the male character is recessive and the female domi- 

 nant. The above formulae can be extended to show these assump- 

 tions and will read thus : 



I 9 Egg (14 chromosomes) + (d") Spermatozoon (14 chro- 

 mosomes) = 9 (d^) Female (28 chromosomes). 



II (d") Egg (14 chromosomes) + (o) Spermatozoon (13 chro- 

 mosomes) = (d^) (o) Male (27 chromosomes). 



This is the part of Wilson's theory that deals with the case 

 presented by Poeciloptera and presumably the other Hemiptera 

 Homoptera. The facts as far as they go are not at variance with 

 the theory. 



