THE FACTORS WHICH DETERMINE SEX 643 



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 which 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 off- 

 spring produced may be explained on this view ; " while 

 statistically the father might be shown to be responsible, physio- 

 logically the mother controls the governing influence." 



It is assumed that in normal cases both sexes of ova and 

 spermatozoa are probably produced in the gonads in equal 

 quantities, and that in those females which shed all their ova 

 the proportion of the sexes in the offspring is, in all likelihood, 

 determined by Mendelian laws. But it is pointed out that in 

 many animals only a small proportion of the ova formed in the 

 ovary ever reach maturity, the remainder undergoing de- 

 generation and ultimately absorption (see p. 156). It is inferred, 

 therefore, that the proportion of the sexes among the ova which 

 survive and are discharged must depend directly upon the 

 causes which lead to the degeneration of some ovarian ova 

 and the continued development of others. On this view it is 

 held that the ova are subject to the same law of natural selection 

 as other organisms, and that in some cases the male ova are best 

 fitted to survive, and in other cases the female ones. 



Heape l has shown further that in the ovary of the rabbit 

 two kinds of degeneration prevail, and that in one kind it is the 

 follicle which first begins to undergo atretic changes, and that 

 in the other kind it is the ovum that is earliest affected. The 

 former condition is regarded as evidence that the available 

 supply of nutriment is insufficient for the maintenance of all 

 the ova in the ovary, while the latter is interpreted to mean that 



1 Heape, " Ovulation and Degeneration of Ova in the Rabbit," Proc. Roy. 

 Soc., B., vol. Ixxvi., 1905. 



