THE CELLULAR BASIS 157 



is due to the fact that all the male-producing 

 spermatozoa degenerate and that only female- 

 producing spermatozoa become functional. 

 Possibly experimental alterations of the sex 

 ratio, such as Hertwig, King and others have 

 brought, about may be explained in a similar 

 way. At least the chromosomal theory of sex 

 determination is so well supported in so many 

 cases and has been found to apply in so many 

 instances where at first it seemed impossible of 

 application, that it ought not to be abandoned 

 until unmistakable evidence can be adduced 

 against it.* 



C. THE MECHANISM OF HEREDITY 

 The mechanism of heredity, as contrasted 

 with the mechanism of development, consists 

 in the formation of particular kinds of germ 

 cells and in the union of certain of these cells 

 in fertilization. We have briefly traced the 

 origin, maturation and union of male and fe- 

 male sex cells in a number of animals, and in 

 these phenomena we have the mechanism of 



* More recent work by Riddle, Goldschmidt, Lillie, et ah in- 

 dicates that chemical substances such as "sex hormones" or 

 enzymes (see pp. 337-339) may modify the chromosomal de- 

 termination of sex, and Goldschmidt suggests that even chromo- 

 somal determination may be due to such enzymes. 



