STUDY OF FERTILIZATION 427 



There seems, therefore, reasonable grounds for beUeving that 

 the mifertiHzed egg is surrounded by a continuous lipoid film, 

 and that membrane formation occurs when this film is emul- 

 sified. When this occurs, liowever, we have seen that an electro- 

 negative colloid is liberated ; we must suppose, therefore, that 

 either (a) this colloid exists in the lipoid layer as a dispersed 

 phase or (h) lies immediately between the lipoid layer and the 

 protoplasmic surface. 



We have still to determine the origin of the fertilization 

 membrane itself. Each unfertilized egg is surrounded by 

 a wide gelatinous zona pellucida : this substance appears to 

 be of a proteid nature — it is readily soluble in dilute acids, and 

 so we may infer that it is electro-positive. If the zona pellucida 

 is not removed before fertilization then the electro-negative 

 colloid set free when the lipoid layer of the vitelline membrane 

 is emulsified will come into contact with a colloid of opposite 

 electrical charge. Mutual precipitation must occur — and this, 

 it is here suggested, is the origin of the fertilization membrane. 

 If, however, the zona pellucida be removed prior to fertiliza- 

 tion, then no fertilization membrane is formed (McClendon, 25 ; 

 Gray, 8). Nevertheless the egg develops normally.^ 



The complete mechanism of ' membrane formation ' may 

 therefore be as follows : 



The unfertilized egg is surrounded by two membranes — the 

 hyaline vitelline membrane, and the gelatinous zona pellucida. 

 The vitelline membrane consists of a continuous lipoid structure, 

 in which an electro-negative protein {d) [see Text-fig. 1] 

 exists in solution as a dispersed phase (or below the lipoid 

 structure is a layer of electro-negative protein). The zona 

 pellucida consists of an electro-positive protein. When the 

 continuous lipoid phase of the vitelline membrane is destroyed 

 by emulsification, the enclosed protein (d) comes into contact 



' Since writing the above I have found that a similar conchxsion had been 

 reached by McClendon (see ' Internat. Zeit. fiir Phys. Chem. Biologie ', 

 vol. i, p. 163, 1914); although the eggs of Toxopneustes eTxamined 

 by this author appear to have the reverse charges on the respective colloids 

 when compared with those of Echinus, which were used by the present 

 writer. 



