644 THE PHYSIOLOGY OF REPRODUCTION 



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, 1 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 

 parthenogenetic or a winter " egg), and that the two kinds 

 of eggs are stated to arise in different parts of the ovary. More- 

 over, Heape suggests that the marked difference between the 

 death-rate of men and women during famines, 2 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 quantity 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 proportion of the sexes of the ova pro- 

 duced, and therefore of the young born, is effected in all animals 

 in which the ripening of the ovarian ova is subject to selective 

 action ; 



" (2) That when no selective action occurs in the ovary the 

 proportion of the sexes of ovarian ova produced is governed by 

 laws of heredity." 



Having arrived at these conclusions, Heape next adduces 

 evidence that certain external forces may affect the proportion 

 of the sexes in dogs. It is shown that amongst greyhounds 

 conception during the period from August to November is most 

 favourable to the production of males under the conditions of 

 breeding at present practised, and this result is attributed to 

 a selective action on the ova produced at this time. There 

 is evidence also that among dachshunds and Basset hounds 

 the seasons affect the proportions of the males and females 

 born. The bloodhound returns seem to show that an excessive 

 production of males is associated with inbreeding. Further, 

 there is statistical evidence that a higher proportion of males is 

 produced in the larger litters, that the larger dogs produce the 



1 Issakowitsch, loc. cit. 



2 Mclvor, Madras Census Reports, 1883. 



