No. 581] ENTRANCE OF THE SPERMATOZOON 



267 



lack of CaCl 2 or NaOH or both to the entrance of the 

 spermatozoon is removed immediately after these sub- 

 stances are added. The block must be due merely to a 

 change in the physical condition of the surface (which 

 may be based on a rapidly reversible chemical reaction). 



In these experiments the XaCl can not be replaced by 

 isotonic sugar solutions. The same fact was found by 

 the writer to be true for heterogeneous hybridization. 



It is of importance to call attention to the fact that 

 the abolition of the block in the case of heterogeneous 

 hybridization depends upon the same substances, CaCl 2 

 and NaOH (or some other alkali), which make normal 

 fertilization possible. The influence of electrolytes on 

 the fertilization of the egg of purpii ratiis by the sperm of 

 Asterias is parallel to the influence of the same electro- 

 lytes on the fertilization of the same egg by the sperm of 

 purpuratus; only the concentrations differ, and always 

 in the same sense. The forces at work are, therefore, 

 apparently the same in both cases; but we can only ex- 

 press surmises as to their nature. The role of salts as 

 well as the rapid reversibility indicate that they are 

 forces situated at the surface of the egg and the sperma- 

 tozoon or both. In the first place we may think of sur- 

 face tension conditions and in this respect it is possible 

 that the entrance of the spermatozoon into the egg may 

 be determined by such forces in a way similar to the 

 process of phagocytosis. In the second place it may be 

 that previous to the action of surface tension forces an 

 alteration in the degree of fluidity of the egg surface may 

 be required (e. g., that physical change which finds its 

 expression in the formation of the fertilization cone). 

 Thirdly, it may be possible that before the surface ten- 

 sion forces can act the spermatozoon must agglutinate 

 with the egg surface and that this agglutination is de- 

 termined by certain specific substances or by certain salts 

 (CaCl 2 and NaOH) or by both. 



Brief mention should be made of the block discovered 

 by Godlewski 11 to the entrance of a spermatozoon into 



