SPECIFICITY IN FERTILIZATION 219 



The various methods used to induce hybrid fertiliza- 

 tion staling of eggs, high concentration of sperm, use 

 of alkalies or other chemicals have therefore this one 

 feature in common, that they destroy the chemical or 

 physical integrity of the cortex of the egg. Thus is 

 rendered possible a form of reaction common to all 

 gametes, the inclusion of the spermatozoon within the 

 egg. Specific factors then do not apparently intervene 

 until after the meeting of the germ nuclei. 



If there is indeed a specific factor in the cortical 



reaction it can hardly be of a purely physical character, 



though physical factors must be of significance in any 



case; for the specific factor must include not only 



the incompatibilities in hybrid fertilization but also 



those in self-fertilization where the physical resemblance 



of the gametes excludes a purely physical explanation 



. of the phenomenon. We are therefore forced to the 



[ conclusion that there is a chemical specificity, more or 



~ less narrow, in the union of the gametes. The only 



phenomenon in which we can so far detect a closer 



approach to this problem is that of sperm agglutination 



by egg secretions. 



A warning is in place here against too much reliance 

 on the phenomenon of specific agglutination in the 

 field of immunology by way of comparison. It may 

 turn out that there is a much closer relationship be- 

 tween these phenomena than we can now detect, or 

 they may represent only superficially similar phenomena 

 with the factor of specificity in common, and that only 

 to a certain extent, at present unknown. 



We have the very striking fact reciprocal to speci- 

 ficity in fertilization, that spermatozoa, of certain forms 



