Specificity in Fertilization 85 



tinate, e. g., the egg of franciscanus can be fertilized 

 by sperm of purpuratus, although the egg sea water 

 of franciscanus causes no agglutination of the sperm of 

 purpuratus. When the jelly surrounding the egg of 

 the Californian sea urchin S. purpuratus is dissolved 

 with acid and the eggs are washed, the eggs will not 

 cause any more sperm agglutination; and yet one hun- 

 dred per cent, of such eggs can be fertilized by sperm. 1 

 5. It is well known that if an egg is once fertilized 

 it becomes impermeable for other spermatozoa. This 

 cannot well be due to the fact that the egg develops; 

 for the writer found some time ago that eggs of Stron- 

 gylocentrotus purpuratus which are induced to develop 

 by means of artificial parthenogenesis can be fertilized 

 by sperm. The following observation leaves no doubts 

 in this respect. When the unfertilized eggs of pur- 

 puratus are put for two hours into hypertonic sea water 

 (50 c.c. of sea water +8 c.c. 2 J^ m NaCl) and then trans- 

 ferred into sea water it occasionally happens that a 

 certain percentage of the eggs will begin to divide into 

 2, 4, 8 or more cells, without developing any further. 

 When to such eggs after they have remained in the 

 resting stage for a number of hours or a day, sperm is 

 added, some or all of the blastomeres form a fertiliza- 

 tion membrane and now begin to develop into larvae; 

 and if the spermatozoon gets into a blastomere of the 



1 Loeb, ]., Jour. Exper. ZooL, 1914, xvii., 123; Am. Naturalist, 1915, 

 xlix., 257. 



