68o THE PHYSIOLOGY OF REPRODUCTION 



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 offspring which result 

 therefrom, will thus be influenced." 



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 themselves 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 gradations 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 ^ that an ovum of one sex must always be fertilised by a 

 spermatozoon of the opposite sex is adopted, 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 conferred 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 will be barren. Diising ^ claimed that the 

 statistical results he obtained from a study of the mating of 

 Thoroughbred horses indicated the dominant influence of the male 

 parent on the sex of the offspring. Any sire that usually produces 

 spermatozoa of one sex only can be fertile, as a rule, only with mares 

 wbich produce ova of the other sex, and to such an extent he 

 determines the proportion of the sexes of the offspring for which he 

 is responsible. But where the sperm of both sexes is uniformly 

 produced, the sire must be fertile with all mares producing ova, and 

 as only one ovum is produced by each mare, the responsibility for 

 the sex of the offspring then lies solely with the female parent." 



The opinion is expressed that much of the evidence cited to show 

 the dominating influence of the male parent on the offspring produced 



' Evidence on this point, including some of that adduced by Heape, is cited 

 below in dealing with hermaphroditism and the latency of sexual characters. 



'^ Castle, "The Heredity of Sex," Bull. Museum Gompt. Zool., B[arvard, 

 vol. xL, 1903. 



^ Diising, loc. cit. 



