STUDIES OF FERTILIZATION 577 



As the result met the anticipated objection and did not appear 

 to He in the main hne of the research, the experiment was per- 

 formed only three times in all, with essentially similar results. 



To explain such a result one of two hypotheses is possible: 

 (1) that shell secretions destroy the inhibitor; (2) that they unite 

 with its inhibiting side-chain, and thus possess a combining group 

 identical with the ovophile group of the fertilizin; though they 

 do not possess the spermophile group. The reaction takes place 

 rapidly; hence the second assumption is more likely. 



3. SUMMARY OF PART III 



In this section we have shown: 



1. That the blood of male and female sea-urchins contains a 

 varying amount of a substance which inhibits fertilization with- 

 out exerting any injurious effect upon either the sperm or the 

 ova. This substance is probably more abundant in the blood of 

 mature individuals than in others. 



2. The inhibitor does not act by blocking the combination of 

 the sperm receptors with the fertilizin, for it has no inhibiting 

 effect upon the agglutinating action of the fertilizin. 



3. It acts by occupying the ovophile group of the fertilizin, 

 thus preventing action of the latter upon the egg by union with 

 egg receptors. 



4. This conclusion is demonstrated by the fact that the inhibitor 

 may be entirely neutralized by a sufficient quantity of the aggluti- 

 nating substance. 



IV. GENERAL DISCUSSION 



The essential conception that results from the experiments is 

 that the formation of the fertilization membrane is due to acti- 

 vation of an ovogenous substance, which I have named fertilizin 

 to express this idea. Following insemination any excess of the 

 fertilizin is bound, or neutralized, by another ovogenous sub- 

 stance, which I have named anti-fertilizin, and polyspermy is 

 thereby prevented. The experiments demonstrate that reactions 

 in fertihzation occur as described in sections II and III, but the 



