

268 



GENETICS 



B 



I 



Figure 52, A, Sea urchin egg fertilized by mollusk sperm. Spindle 

 forming for the first division of the large egg nucleus. The sperm 

 nucleus a small black mass at the right of the lower end of the 

 spindle; it takes no part in the division. Redrawn from Kupelwieser 

 (1909). 



B, Section of a i6-cell stage of a sea urchin egg that was fertilized 

 by annelid sperm. In the large upper cell the sperm chromosomes are 

 separated as a dark mass from the dividing egg chromosomes. Re- 

 drawn from Kupelwieser (1912). 



In such cases there Is, seemingly, incompatibility between 

 the two types of chromosomes; those of one type are cast 

 out and destroyed. 



