206 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 



the fact that if we use extract of dead sperm, the membrane- 

 forming substances will reach the egg in higher concentrations 

 than if a single spermatozoon of a foreign species reaches it. 

 It seems that a living spermatozoon must come in close contact 

 with the fertilization cone of the egg before membrane forma- 

 tion is possible. This seems possible only for the foreign 

 spermatozoon if we raise the alkalinity of the sea-water through 

 the addition of some NaOH. 



When we use the eggs of another sea-urchin, S. franciscanus, 

 the result is entirely different. Even the living spermatozoa of 

 starfish or the shark, or even of warm-blooded animals like the 

 fowl, bring about the membrane formation in this egg in 

 normal sea-water. But no egg develops; they all behave in 

 this case as if only the membrane formation with butyric acid 

 had been called forth . We see from this that we cannot expect 

 that all species of sea-urchins behave alike in every respect. 



T. B. Robertson has worked out a method which permits 

 the extraction of a substance from the testicles of the sea- 

 urchin which produces membrane formation in the sea-urchin 

 egg. 1 



1 T. B. Robertson, Archiv /. Entwicklungsmechanik, XXXV, 64, 1912. 



