166 PHYSICAL BASIS OF HEREDITY 



At the time when the polar bodies are extruded from 

 the eggy 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 



Female XX XYM-'^ 



->^^ 



^ 



i^ggs yY yv ^P®"^^ Y 



Female XX XY^ale 



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



XX-XY type. 



(XX) . Fertilization of any egg (X) by a Z-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 



