No. 581] ENTRANCE OF THE SPERMATOZOON 265 



duee a reversible block to the spermatozoa of the same 

 species. That means, it should be possible to find solu- 

 tions in which the egg does not suffer for a long time, in 

 which the sperm lives for a long time, and in which the 

 sperm of the same species is intensely active and attacks 

 the egg with the greatest eagerness and yet is not able to 

 enter; while if the medium is but slightly changed the 

 sperm enters the egg at once. The writer carried out 

 such experiments a year ago in Pacific Grove and last 

 summer in Woods Hole and found this to be true. 9 



For the purpose of these experiments the ovaries and 

 testes of the sea urchins were not put into sea water but 

 into a pure m/2 NaCl solution (after several washings in 

 such a solution) and kept in such a solution. Several 

 drops of sperm and one drop of eggs were in one experi- 

 ment put into 2.5 c.c. of a neutral mixture of m/2 NaCl 

 and 3/8 m MgCl 2 in the proportion in which these two 

 salts exist in the sea water. In such a neutral solution 

 no egg of A rbacia or of purpuratus is fertilized no matter 

 how long they remain in the solution, although the sperm 

 is very active. If the eggs and sperm are transferred 

 into the same solution which contains in addition 1 drop 

 of a 2V/100 solution of NaOH (or NH,, or benzylamine, 

 or butylamine) or 8 drops of w/100 NaHC0 3 , most and 

 often practically all the eggs at once form fertilization 

 membranes and begin to segment at the proper time. 



The same result can be obtained if the eggs are trans- 

 ferred 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 + 

 CaCl 2 + KC1. In such a neutral mixture the eggs form 

 fertilization membranes and begin to segment. 



The eggs will not be fertilized if transferred into a 

 neutral solution of NaCl or of NaCl + KG. 



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

 ity of the solution surrounding the egg and if we deprive 

 this solution of CaCl 2 we establish the same reversible 



»Loeb, Science, X. S., XL, 316, 1914. 



