48o HEREDITY AND SEX 



and the spermatozoon as a male gamete. Each is a complete 

 gamete — a vehicle of the potentialities of both sexes ; neither 

 has any bias towards masculinity or femininity, though as cells 

 they are expressions of the fundamental contrast of the sexes. 



In connection with the theory that the fertilised egg has in 

 it the complete potentiality of both sexes, there are several 

 facts to be noted. Many of the lower animals, such as earth- 

 worm, leech, snail, and sea-squirt, are normally and intimately 

 hermaphrodite ; in many cases and in very varied degrees 

 casual hermaphroditism is common at almost every level up 

 to and including birds and mammals ; there are quaint cases, 

 especially in ants, bees, and butterflies, of gynandromorphism, 

 where one part of the body has distinctively masculine characters 

 while another part is as distinctively feminine ; in a number of 

 cases, such as tadpoles, it seems certain that the developing 

 organism passes through an embryonic or even a larval phase 

 of hermaphroditism. 



Now, so much being clear, that every germ-cell is the vehicle 

 of the potentialities of both sexes, the problem is what factors 

 determine which set of potentialities will find expression. It 

 is plain that a decision as to these possible determining factors 

 will depend a good deal on the time when sex is defined. Thus, 

 if the embryo lingers for a while with evident expressions of 

 both male and female gonads and associated structures, it is 

 evidently of use to look for factors operative during this period. 

 This is a question for embryologists. 



In the higher animals (mammals and birds) it is possible at a 

 very early date in embryonic life to tell whether the developing 

 organism will turn into a male or into a female, though in the 

 very earliest stages it seems quite impossible to determine whether 

 the primordium, or rudiment of the reproductive organs which 

 has been laid down, is going to become a testis or an ovary. 



But in lower vertebrates, such as frogs, the period of embryonic 

 indifference seems to be greatly prolonged, and it seems as if 



