Artificial Parthenogenesis 103 



bility for the sperm of foreign species. Thus the eggs 

 of Strongylocentrotus purpuratus require a higher con- 

 centration of sperm extract than the eggs of S. fran- 

 ciscanus. For the latter the amount of foreign cell 

 constituents which suffices to call forth membrane 

 formation is so small that contact with almost any- 

 foreign living spermatozoon produces this effect; and 

 as a rule no previous sensitizing action of SrClz is 

 required. When we bring the unfertilized eggs of S. 

 franciscanus into contact with the living sperm of star- 

 fish or shark or even of fowl, the eggs form a fertiliza- 

 tion membrane without previous sensitization. A 

 specific substance from the foreign spermatozoon 

 causes membrane formation before the spermatozoon 

 has time to enter the egg. The effect is the same as if 

 artificial membrane formation had been called forth 

 with butyric acid, i. e., they begin to develop and 

 then disintegrate unless they receive a second short 

 treatment. 



When, however, we treat the eggs with the watery 

 extracts from the cells of their own or closely related 

 species we find that these extracts are utterly inactive, 

 even if used in comparatively strong concentrations. 

 This agrees with the results given in Chapter III. 



These phenomena lead to a very paradoxical result; 

 namely that while in the case of foreign sperm we can 

 cause membrane formation by both the living and the 

 dead spermatozoon, only the living spermatozoon of 



