INFLUENCE OF HYDROGEN ION CONCENTRATION. 343 



REACTION OF SPERM WITH IMMATURE Astcrias EGGS. 



When immature Astcrias eggs are inseminated in sea water, 

 several sperm usually enter each egg before the fertilization mem- 

 brane is formed. Subsequently the germinal vesicle breaks down 

 and the cytoplasm acquires a mottled appearance, each sperm be- 

 ing the focus of a localized cytolytic process. Such prematurely 

 fertilized eggs never attempt to divide. If sperm are added to 

 immature eggs at various H-ion concentrations, a block appears at 

 the same point as in the fertilization of the mature egg, viz., pH 

 7.0. On the alkaline side of this point the sperm enter the eggs, 

 causing membrane elevation and the changes described above. On 

 the acid side the sperm do not react with the eggs in any way ; in 

 the course of time, a varying proportion of these unfertilized eggs 

 will maturate, depending on the H-ion concentration, and these, 

 if they are returned to sea water and inseminated, will fertilize and 

 develop normally. 



SUMMARY. 



When Arbacia, Asterias and Chtztopterus eggs are inseminated 

 in COo-free sea water of varying H-ion concentration, a block to 

 fertilization appears at a H-ion concentration which is constant, 

 and apparently characteristic for each species. If the block is de- 

 fined by the H-ion concentration at which 50 per cent, of the eggs 

 fertilize, these H-ion concentrations are : Arbacia, pH 6.8 ; As- 

 tcrias, pH 7.0; and Chcetopteriis, pH 7.1. 



This block to fertilization is complete, in that eggs either ferti- 

 lize and develop normally, or do not fertilize at all ; and it is per- 

 fectly reversible, in that eggs which do not fertilize on the acid 

 side of the block will fertilize immediately if they are returned to 

 solutions on the alkaline side of the block and inseminated with 

 fresh sperm. 



In sea water more alkaline than pH 9.8 to 10.0 the fertilization 

 process in both Arbacia and Asterias eggs is either incomplete or 

 impaired. Apparently there is no alkaline block to fertilization 

 corresponding in its complete reversibility to the block which ap- 

 pears around neutrality. 



In Arbacia there is an increased incidence of polyspermy within 

 a very narrow range centering at pH 7.2, indicating some critical 



