2 E. E. JUST. 



not been wanting that the membrane forms as a wave that 

 sweeps over the egg. 



Fol first intimated that the membrane lifts from the egg as a 

 wave dwelling on the rapidity with which the process is com- 

 pleted. Unfortunately he used eggs under pressure. Mem- 

 brane formation as a progressive wave beginning at the site of 

 sperm-entrance has been observed by Wilson who, however, is 

 not at all sure. He says (in a footnote) : " I have often observed 

 that the formation of the membrane, in Toxopneustes, proceeds 

 like a wave from the entrance-point around the periphery, but 

 this is often irregular." Ries followed the fertilization with the 

 cinematograph; he observed changes which indicated that the 

 membrane forms first at one point. 



In 1915 Dr. Tennent informed the writer that he had observed 

 in cases a wavelike membrane formation. According to other 

 workers, however, it would appear that no part of the cortex 

 takes precedence in lifting off the membrane. Thus, Harvey 

 ( r io) who worked with Toxopneustes, Hipponoe and Arbacia 

 says: "As observed in the living egg, almost immediately 

 (iJ/2 to 3 minutes) after addition of sperm the membrane sub- 

 stance becomes separated from the egg surface by spaces. These 

 spaces fill with a fluid, unite and enlarge, thus pushing out the 

 membrane some little distance. 1 This statement must refer to 

 the egg of Toxopneusles (with which Wilson worked) or to that 

 of Hipponoe for Heilbrunn after long experience with the egg of 

 Arbacia was never able actually to follow the membrane ele- 

 vation; he believed that in this egg the membrane lifted simul- 

 taneously from all parts of the cortex. 



Loeb has followed membrane formation in the egg of Strongy- 

 locentrotus purpiiratus by lowering the temperature of the sea- 

 water which retards the process. He gives eight figures of the 

 process described. 'The beginning of the process shows itself 

 in a roughening of the hitherto smooth surface of the egg. This 

 is due to the formation of countless tiny vesicles which stand out 

 on the surface of the egg. These small droplets quickly increase 



1 The reader must not conclude from this statement that Harvey believes that 

 the unfertilized egg possesses a membrane. In this paper he makes a categorical 

 statement that the membrane arises at fertilization. In his 1914 paper he reaffirms 

 this statement. 



