PROBLEMS OF FERTILIZATION 



interpreted as due to a discharge of numerous minute 

 refringent droplets previously present. In some other 

 cases a membrane appears to be formed as a result 

 of fertilization, separated from the surface of the egg 

 by a narrow perivitelline space. Such a membrane is 

 hence often called the fertilization membrane. In the 

 sea urchin (Fig. 5) the appearance of this membrane 



a 



FIG. 5. The formation of the fertilization membrane in the egg of 

 the sea urchin Strongylocentrotus purpuratus: a, unfertilized egg sur- 

 rounded by spermatozoa; b, the same egg about two minutes later after 

 the entrance of the spermatozoon (from Loeb, Artificial Parthenogenesis 

 and Fertilization, p. 17; by permission of the author). 



is a very obvious and reliable indicator of fertilization. 

 Fol, who first observed it, regarded it as a device for 

 the prevention of polyspermy, as it begins to form at 

 the point of penetration of the spermatozoon and spreads 

 over the entire periphery "with a rapidity that would 

 be inconceivable if one did not witness it." Whether it 

 is a preformed delicate membrane that is merely elevated 

 from the surface of the egg or a kind of secretion from 

 the egg can hardly be answered from observation alone, 



