138 ESSAYS OF A BIOLOGIST 



are present in the fertilized egg and a female results; 

 when the latter, only one, and the offspring is male.^ 



Putting the matter in the broadest terms, we can 

 say that there is a different balance of hereditary 

 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 we 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 develop- 

 ment, ending by building up either a male or a fe- 

 male gonad in the early embryo. This, once pro- 

 duced, takes over what remains of the task of sex- 

 determination. It secretes a specific internal secre- 

 tion 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 



5 See Goldschmidt, '23; Morgan, '19; Doncaster, '14. 



