206 ARTIFICIAL PARTHENOGENESIS AND FERTLIIZATION 
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.! 
1T. B. Robertson, Archiv f. Entwicklungsmechanik, XXXV, 64, 1912. 
