336 



HOMER W. SMITH AND G. II. A. CLOWES. 



occurs. The cessation of fertilization with increasing acidity is 

 very abrupt, but the critical H-ion concentration may be most ac- 

 curately indicated by the pH at which only 50 per cent, of the eggs 

 fertilize. This critical H-ion concentration is pH 6.8 for Arbacia 

 eggs, and 7.0 for Aster las eggs. In solutions slightly more alkaline 

 than these, all the eggs fertilize ; and in solutions slightly more 



TABLE I. 



A. ACID BLOCK TO FERTILIZATION IN Asterias. 

 Per Cent, of Eggs Fertilized.' 



B. ACID BLOCK TO FERTILIZATION IN Arbacia. 

 Per Cent, of Eggs Fertilized. 



pH 



C. COMPARISON OF ACID BLOCK IN Asterias AND Arbacia IN 

 SAME pH SOLUTIONS. 9-21-23. 



Per Cent, of Eggs Fertilized. 



