THE PROBLEM OF ACTUAllON 259 



tion might perhaps suffice in a formal sense for the 

 special case under consideration by Heilhrunn, that of 

 the sea urchin. But it can hardly apply to other cases 

 where the cortical changes present a different morpho- 

 logical form {Nereis, for instance, F'ig. 2). Loeb has 

 suggested that the sperm bears a lysin which acts on 

 the surface of the egg; the objections to this concep- 

 tion have been already considered (p. 245), and it 

 seems to the writer untenable. R. S. Lillie (1914) has 

 suggested the idea that contact of the sperm produces 

 an electrical depolarization of the membrane which 

 releases an ''impediment to the chemical interaction 

 forming the primary event in the response'' (p. 614). 



There are good reasons for believing that the propa- 

 gation of the cortical change from the point of impact 

 of the spermatozoon requires a measurable time ele- 

 ment. It has been asserted by several observers, begin- 

 ning with Fol (1877), that the fertilization membrane 

 can be seen to arise first at the point of penetration and 

 to spread thence as fast as the eye can follow it; this 

 is a very delicate observation, owing to the difficulty 

 of seeing the precise point of fertilization at the moment 

 of impact. It is therefore difficult to say how much 

 maybe due to subjective impressions in this observation. 



Just (19 1 9) has, however, recently observed a case 

 in the sand dollar {Echinarachniiis), where the rate of 

 the cortical change can be readily followed by the eye; 

 membrane elevation can be observed to j^roceed as a 

 wave around the egg, beginning at the point of i)ene- 

 tration of the sperm. The sperm enters the egg in 

 about twenty to fifty seconds after insemination; the 

 membrane begins to arise at the point of entrance 



