SPECIFICITY IN FERTILIZATION 187 



others the foreign sperm may produce cortical changes 

 but fail to penetrate; or the sperm may penetrate, and 

 perish without uniting with the egg nucleus; or it may 

 unite and be secondarily eliminated in the first or later 

 cleavages; or again, without elimination, the combina- 

 tion may prove its incompatibility by abnormalities 

 in development appearing rapidly or slowly; in yet 

 other cases the hybrid may develop fully but remain 

 sterile. Finally we may have completely fertile hybrids 

 from certain crosses of closely related species. The 

 blocks to hybrid fertilization are thus not the same in 

 all cases; even in the case of cortical block it is quite 

 conceivable that we have different causes operative in 

 different cases. This block seems to be readily remov- 

 able in some instances by mere increase in alkalinity 

 of the medium as first shown by Loeb, but this is 

 by no means always so. The internal blocks on the 

 other hand are not controllable by means hitherto em- 

 ployed. 



From the standpoint of preservation of the species 

 it makes but little difference in what stage hybrid fer- 

 tilization exhibits its incompatability so long as the 

 hybrid does not breed. But from the standpoint of 

 fertilization problems we need consider only the incom- 

 patibilities of the stages of fertilization itself. 



A brief systematic survey of the field will prepare 

 the way for consideration of the problems. The 

 groups in which the possibilities of hybrid fertilization 

 have been most fully investigated are the echinoderms, 

 teleosts, and amphibia. In some groups, as in insects, 

 birds, and mammals, mating behavior constitutes a 

 serious obstacle to close investigation, 



