DETERMINATION OF SEX 315 



one. If a sperm with four chromosomes unites with an 

 egg, which invariably carries four, a female results; the 

 fertilization of an egg by a sperm with three chromosomes 

 gives rise to a male. There are thus male- and female- 

 producing sperms, but only one type of egg. 



In a number of other species there are differences be- 

 tween the shapes of the chromosomes and even in many 

 animals where these are not apparent there is evidence 

 derived from breeding experiments that here likewise a 

 chromosome mechanism regulates sex. 



