THE DETERMINATION OF SEX 



633 



food the element that determines whether or not the female is 

 to be fertile. This is a sex distinction that cannot hold in the 

 higher forms, where fertilization is necessary to development, 

 whatever the sex, showing that one species may differ from 

 another, even in seeming fundamentals, and teaching us caution 

 in making sweeping generalizations. 



In wasps. Von Siebold l conducted investigations with the 

 fertilized and with the unfertilized ova of the wasp, Nematus 

 ventricosus, each kind of which produces both sexes under 

 certain conditions. 



DEVELOPMENT OF FERTILIZED OVA 



DEVELOPMENT OF UNFERTILIZED OVA 



From this we conclude that in general fertilized ova produce 

 females, but not exclusively, the proportion of females being in 

 some accord with temperature or food, or both. 



Here the unferti- 

 lized ova produced 

 males except when 

 the conditions of de- 

 velopment were so 

 favorable as to 

 shorten the larval 

 period to the utmost, 

 leading Geddes and 

 Thomson to remark 

 that even "where 

 the production of 

 males is the normal condition, favorable environmental influ- 

 ences appear to introduce females." 



1 Geddes and Thomson, The Evolution of Sex, pp. 48, 49. 



