THE DETERMINATION OF SEX 267 



^-chromosome becomes a male. Since both types of 

 sperm are ordinarily formed in equal numbers, the 



FEMALE 



Secondary 

 Oocyte 



Matured 

 Ovum 



FIG. 81. Sex determination in the case of heterogametic males. 



chances that a male- or a female-determining sperm 

 will reach the egg in the process of fertilization, are 



