414 EDITH PINNEY 



same interpretation. Lillie (9, p. 191) has tabulated the results 

 of Baltzer's crosses. From his table it is seen that Echinus and 

 Strongylocentrotus cross reciprocally with no elimination. The 

 eggs of both species eUminate Sphaerechinus chromosomes at 

 the first cleavage. The egg of Sphaerechinus, on the other hand, 

 tolerates the male chromatin of either species. In addition 

 the two sorts of eggs eliminate Arbacia chromosomes, but not 

 until the blastula stage is reached. 



In connection with these echinoderm crosses one should re- 

 member that cross-fertilization was only possible after treating 

 the eggs with alkaline solutions. The primary effect of this 

 treatment is to alter the normal cortical reaction of the egg to 

 the foreign sperm. If it affects the physical character of the 

 cytoplasm, it is more than probable that it does this in a uni- 

 form manner so that the same relative conditions obtain in the 

 treated eggs as would exist in eggs that were not treated. 



Norman (13) showed a difference between the eggs of Cteno- 

 labrus and Fundulus, forms which behave differently in crossing, 

 by subjecting them to the action of heat: 30°C. was suff.cient 

 to stop segmentation in Ctenolabrus, while it required a tempera- 

 ture of 38^C. to produce the same effect in Fundulus. This 

 indicates some specific difference in the cytoplasm of these two 

 eggs. It is not unreasonable to suppose that other fish eggs 

 would show different points of susceptibility to heat in this 

 regard. Whether such susceptibility points would follow the 

 taxonomic afF.nities of species or show independent variation 

 would be an interesting point to determine in this connection. 



The evidence so far accumulated seems to me to point to the 

 variation in the physical factors controlling mitosis as one basis 

 upon which the lack of correlation between developmental suc- 

 cess in fish hybrids and taxonomic relationships can be explained. 

 If this view is correct, it should be possible to reproduce these 

 phenomena experimentally in both straight-fertilized eggs and 

 in crosses. In that direction lies the hope of further analysis. 



