138 ESSAYS OF A BIOLOGIST 



factors in male and female, and that this difference 

 of balance dates from the moment of fertilization, 

 and normally determines sex. 



Various agencies may alter the balance. The 

 chromosomes themselves may vary in what w^e must 

 vaguely call their potency ; or external agencies may 

 affect it. As a result, we sometimes obtain strange 

 abnormal individuals, in which the balance has 

 been upset ; in them development results sometimes 

 in organisms permanently intermediate between male 

 and female, sometimes in a change of sex at some 

 period of development. 



In insects the chromosomes appear to be pre- 

 dominant throughout life. In vertebrates, however, 

 they seem to play their chief role in early development, 

 ending by building up either a male or a female gonad 

 in the early embryo. This, once produced, takes 

 over what remains of the task of sex-determination. 

 It secretes a specific internal secretion which in a 

 male acts so as to encourage the growth of male 

 organs and instincts, to suppress those of females ; 

 and vice versa in a female. 



As a result of this difference we find that castration 

 in insects, even followed by engrafting of a gonad 

 of opposite sex, produces no effect upon other sexual 

 characters ; whereas it exerts a profound effect upon 

 mammals or birds. 



As a second result, we find that in vertebrates the 

 gonads form part of what has been called the chemical 



