256 RECENT CYTOLOGY 



The facts clearly prove, in the first place, that the 

 unpaired heterotropic chromosome alternates between 

 the two sexes in alternate generations, passing from 

 the male to the female in the production of females, 

 and from the female to the male in the production of 

 males (see the diagram). 



Assuming that these particular chromosomes are 

 really concerned in the determination of sex, Wilson 

 suggests the following interpretation on Mendelian 

 lines. Since the heterotropic chromosome is the only 

 one present in the male, it must represent the male 

 determinant. But, since spermatozoa which contain 

 this chromosome produce only females, the maternal 

 mate of the male heterotropic chromosome, already 

 present in the egg, must be a dominant female de- 

 terminant. And in the process of fertilization which 

 gives rise to males the heterotropic chromosome 

 derived from the egg must represent the male deter- 

 minant. Two different sorts of eggs are therefore 

 produced presumably in equal numbers which con- 

 tain the male and female determinant respectively ; 

 the former are fertilized only by spermatozoa lacking 

 the heterotropic chromosome and vice versa. The 

 combinations which arise in this way may be repre- 

 sented as (m)f and m. 



A selective process of fertilization is therefore a sine 

 qua non for this explanation it must be impossible 

 for a spermatozoon bearing the male determinant to 

 fertilize an egg in which a male determinant is already 

 present in other words, only eggs containing the 

 female determinant can be fertilized by sperms which 



