76 Specificity in Fertilization 



might block the entrance of the sperm of sea urchin into 

 eggs of their own species. This was found to be cor- 

 rect; when we put eggs and sperm of the same species 

 of sea urchin into solutions whose concentration of Ca 

 or of OH is too small, the sperm, although it may be 

 intensely active, cannot enter the egg. 



For the purpose of these experiments the ovaries 

 and testes of the sea urchins were not put into sea 

 water, but instead into pure m/2NaCl and after several 

 washings in this solution were kept in it (they remain 

 alive for several days in pure m/2 NaCl). Several 

 drops of such sperm and one drop of eggs were in one 

 series of experiments put into 2.5 c.c. of a neutral mix- 

 ture 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 eggs of Arbacia or purpuratus are 

 not fertilized no matter how long they remain in it, 

 although the spermatozoa swim around the eggs very 

 actively. That no spermatozoon enters the eggs can 

 be shown by the fact that the eggs do not divide (al- 

 though they can segment in such a solution if previously 

 fertilized in sea water or some other efficient solution). 

 When, however, eggs and sperm are put into 2.5 c.c. 

 of the same solution of NaCl+MgCl 2 , containing in 

 addition one drop of a N/ioo solution of NaOH (or NH 3 

 or benzylamine or butylamine) or eight drops of m/ioo 

 NaHC0 3 , most, and often practically all of the eggs 

 at once form fertilization membranes and segment at 



