428 J. GRAY 



with the zona pellucida, i. e. with a colloid of opposite sign. 

 Mutual precipitation must occur, giving rise to the fertilization 

 membrane. This membrane is impermeable to the remainder 

 of the protein {d), which draws in water through the fertilization 

 membrane by osmosis. In this way the fertilization membrane 

 is extruded from the protoplasmic surface of the egg. 



The evidence for such an analysis of membrane formation 

 is strong, (i) There is no doubt that the protein within the 

 fertilization membrane has an opposite charge to that of the 

 zona pellucida. (ii) If the zona pellucida be removed prior to 

 fertilization no fertilization membrane is formed, but the egg 

 is activated in a perfectly normal way, (iii) By micro-dissection 

 it can be shown that the fertilization membrane is much 

 tougher than any membrane possessed by the unfertilized egg. 



It must again be emphasized that the essential act of activa- 

 tion is the emulsitication of the vitelline membrane, and not 

 the formation of the fertilization membrane or the extrusion of 

 the latter by absorption of water. 



If the above analysis of ' membrane formation ' be accepted, 

 the question arises how can the spermatozoon act as an emulsify- 

 ing agent ? Loeb holds that the sperm introduces a specific 

 ' cytolysin ' into the egg-surface. The evidence is, however, 

 against this view : (i) in order that artificial membrane forma- 

 tion may occur as quickly as a normal fertilization membrane, 

 fairly high concentrations of emulsifying agents are necessar}' : 

 it is quite impossible for one spermatozoon to introduce suffi- 

 cient quantities into the egg. (ii) Membrane-forming substances 

 are not in any way specific, whereas spermatozoa are markedly 

 so. The only alternative seems to be that the spermatozoon 

 emulsifies the egg-surface by a different means to that effected 

 by artificial agents. The action of the spermatozoon is at 

 first local, and evidence has already been put forward in support 

 of the view that its activating action on the egg is essentially 

 a physical process. The suggestion made is that the destruction 

 of the surface lipoid film is brought about by the spermatozoon 

 electrolytically. Thin lipoid films are, according to Hardy, 

 electrically charged, and are sensitive to the electric current. 



