642 THE PHYSIOLOGY OF REPRODUCTION 



from the generative glands may be regulated by selective 

 action, exerted in accordance with the resultant of a variety of 

 extraneous forces. If this be true, the proportion of living 

 male and female ova and spermatozoa which are freed from the 

 generative glands, and the proportion of the sexes of the off- 

 spring which result therefrom, will thus be influenced." A 

 similar suggestion had been made by Schultze ^ and also by 

 Morgan.^ 



Heape is of opinion, however, that just as there is evidence 

 that adult animals are never purely male or female,^ so it is 

 probable that the sexual products {i.e. the gametes) are them- 

 selves similarly constituted. According to this view, an ovum 

 or a spermatozoon may possess dominant male or female 

 characters as the case may be, and recessive characters of the 

 opposite sex. " In such cases the possibility of infinite grada- 

 tions of sexual differentiation in an individual would be vastly 

 increased, and from the point of view of heredity, such complex 

 conditions carry with them factors of the highest importance." 



Ova and spermatozoa in which the characters of one sex 

 are dominant are referred to as being male and female, and 

 Castle's conclusion is accepted, that an ovum of one sex must 

 always be fertihsed by a spermatozoon of the opposite sex, but 

 whether the sex of the adult is determined by the ovum or by 

 the spermatozoon is a question which is left open, as it may 

 admit of a different answer for different species of animals, or 

 even for different individuals. Heape says, however, that even 

 if that be so, the sex of the ovum must be regarded as bearing a 

 regular relation to the sex of the embryo as surely as if it con- 

 ferred its own sex. 



" On this assumption a female parent producing ova of one 

 sex only will give birth to embryos of one sex, unless the male 

 parent possesses no spermatozoa of the opposite sex wherewith 

 to fertilise it, in which case the union wiU be barren. Diising * 

 claimed that the statistical results he obtained from a study of 



' See below (p. 652 et seq.). 



^ Morgan, loc. cit. 



3 Of. Castle (see above, p. 638). Evidence on this point, including some 

 of that adduced by Heape, is cited below in dealing with hermaphroditism 

 and the latency of sexual characters. 



* Diising, loc. cit. 



