272 



GENETICS 



when the ova of Sphaerechinus are fertilized by sperm of 

 Paracentrotus, the sperm chromosomes are not eHminated, 

 but take part in development, and the larvae produced are 

 intermediate. It appears that the Paracentrotus chromo- 

 somes can operate in the cytoplasm of Sphaerechinus, but 



o*- 



B 



Figure 54, Two figures showing the lagging behind of the male 

 nucleus and chromosomes in eggs of Sphaerechinus fertilized by sperm 

 of Paracentrotus, after the egg nucleus has begun to enlarge for 

 development. In A, the egg nucleus is enlarged and in it the chromo- 

 somes have appeared, while the sperm nucleus (to the left) is small 

 and without nuclei. In B, the egg chromosomes, at the right, are 

 approaching division, while the sperm chromosomes, to the left, form 

 a small separate group less advanced in development. See text. After 

 Herbst (1909). 



most of the Sphaerechinus chromosomes cannot operate in 

 the cytoplasm of Paracentrotus. 



Thus it is clear that in different cases there are various 

 degrees of antagonism between the chromosomes of one or- 

 ganism and the cytoplasm (or the chromosomes) of an- 

 other. The results of crossing depend on the degree of this 

 antagonism. 



