THE FACTORS WHICH DETERMINE SEX 68i 



may be explained on this view: "While statistically the father 

 might be shown to be responsible, physiologically 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,pr6f)0rtion 

 of the sexes in the offspring is, in all likelihood, determined by 

 Mendelian laws. But it is pointed out that in many animals only £t 

 small proportion of the ova formed in the ovary ever reach maturity, 

 the remainder undergoing degeneration and ultimately absorption 

 (see p. 151). 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^ 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 the ovum, for one reason or 

 another, is unable to assimilate the nutriment provided for it. It is 

 possible, therefore, that nutrition may in this way exercise a selective 

 action as regards sex. In this connection it is interesting to note 

 that, according to Issakowitsch,^ the nutritive conditions prevailing 

 in the ovary of the daphnid Simocephalus are determinative as to 

 the kind of egg which will develop {i.e. whether it will be a partheno- 

 genetic or a " winter " egg), and that the two kinds of eggs are stated 

 to arise in different parts of the ovary. Moreover, Heape suggests 

 that the marked difference between the death-rate of men and women 

 during famines,^ for example, may be reproduced among male and 

 female ova in the ovary when that organ is subjected to conditions 

 of a homologous kind. 



Heape's general conclusions are summarised as follows : — 



" (1) That through the medium of nutrition supplied to the ovary, 

 either by the quantitj or by the quality of that nutrition, either by 

 its direct effect upon the ovarian ova or by its indirect effect, a 

 variation in the prqportion of the sexes of the ova produced, and 



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

 B., vol. Ixxvi., 1905. 



2 Issakowitsch, loc. cit. 



^ M'lvor, Madras Census Reports, 1883. 



22 A 



