Specificity in Fertilization 77 



the proper time, indicating that fertilization- has been 

 accomplished. The same result can be obtained if the 

 eggs are transferred into a neutral mixture of NaCl+ 

 MgCl 2 +CaCl 2 (in the proportion in which these salts 

 exist in the sea water) or into a neutral mixture of 

 NaCl+MgCl 2 +KCl+CaCl 2 . In such neutral mix- 

 tures the eggs form fertilization membranes and begin 

 to segment. The eggs are not fertilized in a neutral 

 solution of NaCl or of NaCl+KCl. 1 



It is, therefore, obvious that if we diminish the al- 

 kalinity of the solution surrounding the egg and deprive 

 this solution of CaCl 2 we establish the same block to the 

 entrance of the spermatozoon of Arbacia into the egg 

 of the same species as exists in normal sea water for 

 the entrance of the sperm of the starfish into the egg of 

 purpuratus. 



The "block" created in this way, to the entrance of 

 the sperm of Arbacia into the egg of the same species 

 is also rapidly reversible. 



We reach the conclusion, therefore, that the speci- 

 ficity which allows the sperm to enter an egg is a sur- 

 face effect which can be increased or diminished by an 

 increase or diminution in the concentration of OH as 

 well as of Ca. The writer has shown that an increase 

 in the concentration of both substances may cause an 

 agglutination of the spermatozoa of starfish to the 



1 Loeb, J., Science, 1914, xl., 316; Am. Naturalist, 1915, xlix., 

 257- 



