No. 581] ENTRANCE OF THE SPERMATOZOON 



263 



on the nature of the block to the entrance of hetero- 

 geneous sperm it was necessary first to find the means by 

 which this block could be overcome. He succeeded in 

 showing that the egg of the sea urchin 8. purpuratus can 

 be fertilized by the sperm of starfish, brittle stars, and 

 holothurians in sea water (or other balanced solutions) 

 if their alkalinity was a trifle higher than that of ordinary 

 sea water (e. g., in a solution of 50 c.c. sea water -f- 0.6 c.c. 

 2V/10 NaOH). 7 Godlewski* succeeded by the same 

 method in the fertilization of the egg of the sea urchin 

 with the sperm of crinoids 



The most important fact found out in this connection 

 was the following, namely, that the fertilization of the 

 egg of purpuratus by the sperm of Asterias only takes 

 place while both eggs and sperm are in this hyperalkaline 

 solution. If eggs and sperm are put into these solutions 

 separately and if then from time to time sperm and eggs 

 so treated are transferred into normal sea water, as a 

 rule not a single egg is fertilized; while with the same 

 material when eggs and sperm are together in the hyper- 

 alkaline solution as many as 100 per cent, of the eggs may 

 be fertilized. The effect of the alkali is, therefore, rap- 

 idly reversible; the eggs when put from the hyperal- 

 kaline sea water free from sperm into the normal sea 

 water containing very motile sperm of Asterias can not 

 be fertilized; when put back into hyperalkaline sea water 

 containing Asterias sperm they will be fertilized rapidly. 



This rapid reversibility of the effect of the XaOH in- 

 dicates that it must be confined to the surface of the egg 

 and the spermatozoon or both ; and this is corroborated 

 by the fact that the NaOH does not enter the cells. One 

 of the forces which determine the entrance of the sper- 

 matozoon into the egg may be surface tension and the 

 phenomenon of the entrance may be comparable or pos- 

 sibly identical with the phenomenon of phagocytosis. 



Godlewski mentioned that he occasionally observed a 



