124 PROBLEMS OF FERTILIZATION 



occurs, as we have seen, in the blood of the sea urchin 

 and is present there in greater concentration than in 

 the egg water; so it is certainly not tissue specific as 

 is the iso-agglutinin. 



It is therefore certain that the iso-agglutinins of 

 Arbacia and of Nereis are without action on the 

 heterologous sperm. But these forms belong to differ- 

 ent phyla. How is it as between more closely related 

 forms? Just (1919) has found that egg water of Arba- 

 cia agglutinates the sperm of Arbacia and Echinarach- 

 nius; the egg water of the latter will agglutinate its 

 own sperm but not that of Arbacia. The relations are 

 thus similar to Arbacia and Nereis; and correspond- 

 ingly Just was able to show that two distinct substances 

 are involved in the Arbacia egg water. Loeb (1914, 

 p. 125) notes that the egg water of the sea urchin 

 Strongylocentrotus purpuratus will agglutinate both its 

 own sperm and that of S. franciscanus but the latter 

 not so strongly; the egg water of S. franciscanus will 

 agglutinate its own sperm but not that of S. purpuratus. 

 Here again the specificity is not reciprocal, and this 

 suggests the possibility that the hetero-agglutinating 

 substance of S. franciscanus may be a distinct substance 

 from the iso-agglutinating substance. Loeb, however, 

 did not investigate this possibility. 



The results thus far obtained are too meager to 

 permit a generalization on the specificity of the agglu- 

 tination reaction. There is a considerable degree of 

 specificity in such reactions, as we have seen, but we do 

 not yet know how far this extends. 



Certain observations indicate an absence of agglu- 

 tinating effect of egg water on specific sperm of certain 



