56 



HEREDITY AND SEX 



the sea-urchin two spermatozoa may enter a single 

 egg. They both unite with the egg nucleus (Fig. 31). 

 Each brings in 18 chromosomes. The egg contributes 

 18 chromosomes. There are in all 54, instead of 36 

 chromosomes, as in normal fertilization. 



FIG. 31. Dispermy and its effects in egg of sea urchin. (After 

 Boveri.) 



Around these chromosomes a double system of 

 threads develops with four poles. The chromosomes 

 become unequally distributed on the four spindles that 

 develop. Each chromosome then divides, and half of 

 each goes to the nearest pole. To some of the poles 

 many chromosomes may pass, to other poles fewer. 



