362 PROTOPLASMIC ACTION AND NERVOUS ACTION 



the first visible effect of insemination is an alteration of 

 the egg surface, beginning at the point of entrance of 

 the spermatozoon. A liquefactive or secretory change 

 occurs in the protoplasmic surface layer, in consequence 

 of which a thin surface-film is separated to form the 

 fertilization membrane; this process of separation is not 

 simultaneous at all points on the surface, but progresses 

 in a wavelike manner from the point of sperm-entry 

 to the opposite pole, which it reaches about 20 seconds 

 later (at 20°). During the propagation of this dis- 

 turbance over the egg surface, the latter is altered in 

 such a manner that the plasma membrane loses tempora- 

 rily its normal tenacity and coherence; this effect is 

 readily demonstrated by placing the eggs at this time 

 in dilute sea water (60 vols, fresh plus 40 sea water) 

 in which they undergo immediate and rapid disintegra- 

 tion. The breakdown of the surface layer can be seen 

 to begin at the region where the fertilization membrane 

 is beginning its separation. This period of instability 

 lasts only for the brief period (about one minute) during 

 which the ''cortical reaction" is travehng over the egg 

 surface; within about a minute after insemination the 

 original resistance to dilute sea water has returned, 

 showing a restitution of the normal coherent surface layer. 

 Thus a characteristic surface-change, apparently accom- 

 panied by a local disintegration or disorganization of the 

 plasma membrane, constitutes the first reaction of this 

 egg to fertilization; this change is propagated as a wave 

 over the cell surface and is followed by a reconstructive 

 process restoring the original condition. As we have 

 already seen, there are indications that a propagated 

 surface-change of a similar kind, only with different time- 



