166 PHYSICAL BASIS OF HEREDITY 



At the time when the polar bodies are extruded from 

 the egg, the two X^s separate, one passing out, the other 

 remaining in the egg. Every egg is left with one X 

 (Fig. 68). 



In the male, the X and Y conjugate and separate at one 

 of the maturation divisions, so that each sperm contains 

 either an X- or a Z-chromosome (Fig. 68). Fertilization 

 of any egg {X) by an X-bearing sperm produces a female 



9 cr 



XX XV 



Fig. 68. — Scheme showing the relation of the sex-chromosome to sex-determination. 



XX-XY type. 



(XX). Fertilization of any egg (X) by a F-bearing sperm 

 produces a male (XY). 



Since the two kinds of spermatozoa are produced in 

 equal numbers, females and males will be equal in num- 

 ber. The mechanism is self-perpetuating. 



The Inheritance of Factors Carried by the Sex- 

 Chromosomes in the Drosophila Type 



Since the son gets his one X-chromosome from his 

 mother, and the Y from his father, he inherits factors 

 carried by the sex-chromosomes in a different way from 



