SPECIFICITY IN FERTILIZATION 



191 



succeeds when Strongylocentrotus is the male, but always 

 succeeds when Arbacia is the male. He states that, 

 curiously enough, the Hertwigs obtained the exact oppo- 

 site result with these genera in another locality. Some 

 authors state only their successful cross-fertilizations, 

 and the facts with respect to the reciprocals are not 

 stated. 



Balzer (1910) has, however, paid particular atten- 

 tion to this question, and his results may be tabulated 

 as follows: 



NOTE. The reciprocal crosses bear the same number. 



Only three of these cross-fertilizations were success- 

 ful in sea-water, then only occasionally, and quanti- 

 tative data were not given, viz.: Sphaerechinus $ X 

 Strongylocentrotus 6; Strongylocentrotus $ X Arbacia <$; 

 Echinus $ X Sphaerechinus $. The fertilizations re- 

 corded were therefore made in hyperalkaline sea-water 

 according to Loeb's method. Very great variability in 

 the capacity for hybridization even by this method was 



