58 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 



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, 



