ONTOGENESIS 243 



C. That sex-determination depends upon the age 

 of the ovum at the time of fertilization. 



Thury (1863) and Busing (1883) taught that 

 when the ovum was fertilized soon after ovula- 

 tion, it developed into a female; if long after 

 ovulation, into a male. 



Hensen was of the opinion that females re- 

 sulted when both ova and spermatozoa were in 

 the most active condition at the time of fertiliza- 

 tion. 



Vernon, in cross fertilizing certain echinoderms, 

 thought that the fresher gamete exerted the 

 greater influence in sex determination. 



D. That sex determination depends upon certain 

 conditions of fertilization. 



Canestrini long ago suggested that the sex of 

 the individual was determined by the number of 

 spermatozoa that enter the ovum at the time of 

 fertilization. But as it is now well known that 

 only one spermatozoon enters to fertilize the 

 ovum, of course this theory fails. 



That conditions of fertilization do have some- 

 thing to do with sex determination is, however, 

 quite certain. Thus, in bees the spermatozoa 

 of the male are contained sometimes during the 

 several years of the life of the queen, in living 

 condition, in a special receptacle made to receive 

 them, from which they can be liberated to fertil- 

 ize the eggs or not, at the will of the queen. 

 Ordinarily the eggs are fertilized and all give rise 

 to females, but unfertilized eggs all develop into 

 males. 



II. Theories increasing in validity with increasing 

 knowledge of cytological science: 



E. That the sex of the individual is hi every case 

 predetermined in the ovum. 



This must not be construed to mean .that one 

 ovary produces male eggs and the other female 



