SPECIFICITY IN FERTILIZATION 211 



of an immunity reaction in a very general sense. 'I'his 

 implies that a certain chemical differentiation of the 

 gametes is necessary for the fertilization reaction, and 

 that such differentiation may be lacking. It seems to 

 the writer that this step in interpretation is along the 

 right line, but it is clear that it needs to be carried 

 farther by more investigation. Nothing would probably 

 contribute more to a comprehension of the biochemical 

 factors on which the fertilization reaction depends than 

 the solution of the problem of self-sterility. 



The solution might be carried a step farther if we 

 were to assume that the egg produces an agglutinating 

 substance necessary to bind the sperm for the cortical 

 reaction, but that the agglutinating substance does not 

 operate on own sperm in the case of Ciona. We have 

 seen in chapter iv that ova actually do produce a 

 sperm-agglutinating substance, and the relation of this 

 to fertilization is discussed in the next chapter; the 

 foregoing assumption is therefore not a mere fancy. 



Before discussing the matter further let us very 

 briefly review the analogous phenomena in flowering 

 plants, in which, as we have already stated, a relatively 

 small number exhibit the phenomenon of physiological 

 self-incompatibiHty in pollination. Stout (1916) and 

 East (191 7) have given excellent historical reviews of 

 the literature of this subject. It should be borne in 

 mind in comparing plants with animals that the 

 incompatibility in the former consists in inhibition of 

 growth of the pollen tube and not incomixitibility of 

 the actual gametes. There is no evidence that self- 

 steriHty in plants is ever due to incomi)atibility of the 

 actual gametes. 



