86 Specificity in Fertilization 



2- or 4-cell stage normal plutei will result. When the 

 sperm is added while the eggs are in active partheno- 

 genetic cell division the individual blastomeres into 

 which a spermatozoon enters will also form a fertiliza- 

 tion membrane, but such blastomeres perish very 

 rapidly. It is not yet possible to state why it should 

 make such a difference for the possibility of develop- 

 ment whether the spermatozoon enters into a blasto 

 mere when at rest or when it is in active nuclear division, 

 although the idea presents itself that in the latter case 

 an abnormal mix-up and separation of chromosomes 

 and other constituents may be responsible for the fatal 

 result. Whatever may be the explanation of this 

 phenomenon it proves to us that it is not the process 

 of development in itself which acts as a block to the 

 entrance of a spermatozoon into an egg which is already 

 fertilized. x 



When the spermatozoon enters the egg of the sea 

 urchin it calls forth the formation of a membrane 

 the fertilization membrane. It might be considered 

 possible that this membrane formation or the alteration 

 underlying or accompanying it is responsible for the 

 fact that an egg once fertilized becomes immune against 

 a spermatozoon. We shall see in the next chapter 

 that it is possible to call forth the membrane in an 

 unfertilized sea-urchin egg by treating it with butyric 



1 Loeb, J., Arch. f. Entwcklngsmech., 1907, xxii., 479; Artificial Par- 

 thenogenesis and Fertilization, Chicago, 1913, p. 240. 



