ig8 PROBLEMS OF FERTILIZATION 



the cortical resistance to the hybrid fertilization must 

 be broken down by staling of the eggs or dilution 

 of the sea-water, or by modification of the chemical 

 environment, or by high concentration of sperm. Inter- 

 class crosses require in all cases apparently the action 

 of some foreign agent on the egg; and the same is 

 true of the interphylum crosses considered, though in 

 this case the concentrated sperm may itself exert a 

 cytolytic action on the egg, which favors penetration. 



The specific factor that appears at the end of the 

 fertilization process evidences itself usually in elimina- 

 tion of chromatin; but it is readily conceivable that 

 such a result may not be evident; the fertilization 

 might to all appearance be perfectly normal, and yet sub- 

 sequent events might demonstrate the incompatibility of 

 the union. 



The nature of these specificities need not concern 

 us here, as they can be considered more profitably 

 after other data have been considered. 



b) Teleosts: A great many experiments have been 

 carried out in the cross-fertilization of various species 

 of teleosts, between species, genera, families, and orders. 

 Thus Newman (1915) records seventy-eight heterogenic 

 crosses between members of different families or orders 

 of teleosts belonging to fourteen species, and Moenk- 

 haus (1910) records eighteen. 



Every cross-fertilization attempted was more or less 

 successful in the sense that some or even a large per- 

 centage of the eggs segmented; no artificial treatment 

 of the eggs or the spermatozoa appeared to be neces- 

 sary in order to secure these results, unlike echinoderm 

 crosses. Even in the most distant heterogenic crosses 



