172 PROBLEMS OF FERTILIZATION 



eggs alone, of sperm alone, or of both, before insemina- 

 tion in normal sea-water does not increase the percent- 

 age of fertilization. We are therefore probably dealing 

 with a rapidly reversible modification of the surface of 

 one or both kinds of gametes. 



With reference to the question of balance of salts, 

 the only systematic experiments are those of Loeb 

 (1914, 191 56), who found that, for the fertilization of 

 eggs of the sea urchin, the presence of Ca and OH ions 

 is very important. Eggs and sperm washed in neutral 

 N/2 NaCl will not fertilize in this salt alone, nor in 

 combinations of two or more of NaCl, MgCl 2 , and KC1 

 in the proportions and concentrations in which these 

 salts exist in sea-water, though the spermatozoa may 

 be very active and fill the jelly of the eggs. But the 

 addition of CaCl 2 to NaCl, or to NaCl and MgCl 2 , or 

 to NaCl and MgCl 2 and KC1 in the sea-water propor- 

 tions will induce normal fertilization; this will happen 

 even more promptly and certainly if a little NaOH is 

 added at the same time. Loeb states (1914) that cal- 

 cium possesses an almost specific action for fertilization 

 of the sea urchin egg, and it is important to note that it 

 increases sperm agglutination also, according to the 

 same author. 



We shall not inquire here just how the CaCl 2 or 

 NaOH acts in such cases, but will hold the recorded 

 facts for discussion in connection with other data. 



VII. OTHER BLOCKS TO FERTILIZATION 



Any environmental defect that prevents fertilization 

 may be considered as a block, and in that sense the 

 present section is a continuation of the preceding. It 



