258 THE AMEBIC AN NATURALIST [Vol. XLIX 



stances) the specific block can be overcome if we slightly 

 increase the alkalinity of the sea water. The spermato- 

 zoon can only enter the foreign egg while both sperm and 

 egg are in the hyperalkaline sea water, whereas if the egg 

 and sperm are treated separately with hyperalkaline so- 

 lution (no matter how long) and put together in a suffi- 

 ciently large quantity of normal sea water no egg can be 

 fertilized 1 while fertilization will take place as soon as 

 the hyperalkalinity is restored. This shows that the 

 change (brought about by the hyperalkaline sea water) 

 which makes the fertilization possible is rapidly reversible, 

 as we should expect it to be if it consisted merely in a 

 physical change at the surface of the egg. To this series 

 of facts, others might be added which point in the same 

 direction. In this paper we intend to discuss a little 

 more fully the various conditions which block or favor 

 the entrance of a spermatozoon into an egg, in order to 

 form an idea of the nature of the forces which control 

 these phenomena. 



II 



1. When the unfertilized eggs of 8. ptirpuratus are 

 treated for two hours with hypertonic sea water (50 c.c. 

 sea water + 8 c.c. 2\ m NaCl or Ringer solution) the 

 eggs of certain females will develop into blastulo?, gas- 

 trulae and plutei, while the eggs of other females can not 

 be caused to develop in this way. These individual dif- 

 ferences coincide possibly with those observed by the 

 writer in regard to spontaneous membrane formation in 

 the eggs of different females 2 and it is possible that only 

 the eggs of such females of purpuratus can be induced to 

 form larvae through a mere treatment with hypertonic sea 

 water in which the latter can induce the cortical changes 

 underlying the membrane formation. Whatever the na- 

 ture of the individual difference may be, purpuratus eggs 



