STUDIES IN. ARTIFICIAL PARTHENOGENESIS. 335 



The table shows clearly that more water leaves the fertilized 

 than the unfertilized eggs. This can not be due to an increased 

 permeability to water for an increase of this sort could produce 

 no such effect. It must be due to a loss in the rigidity of the 

 plasma membrane. In order to make this relation clear, I shall 

 quote from p. 1 54 of the second paper of this series : ' ' The plasma- 

 membrane of the Arbacia egg is a protein gel. As such it pos- 

 sesses a certain degree of rigidity. Suppose a hypothetical 

 system completely surrounded by an extremely rigid semi- 

 permeable membrane. If such a system were placed in a con- 

 centrated solution no exosmosis could take place, for if the 

 membrane were perfectly rigid, there could be no removal of 

 solvent from the system without the production of a vacuum. 

 But the membrane would be subjected to a considerable pressure 

 which would tend to make it rearrange its particles in such a 

 fashion that the volume enclosed within it might be lessened. 

 Whereas an extremely rigid membrane would resist such forces 

 one with only a certain degree of rigidity would yield (in the 

 case of sufficient pressure) and exosmosis would be possible. 

 Thus osmosis in an enclosed system depends to some extent at 

 least on the rigidity of the confining membrane. These con- 

 clusions apply in some measure to the sea-urchin egg, for the 

 vitelline membrane possesses a slight degree of rigidity." They 

 apply even more directly to the Cumingia egg, for its vitelline 

 membrane, which is also its plasma .membrane, is stifter than 

 that of Arbacia. Any loss in the rigidity of this membrane favors 

 either endosmosis or exosmosis. That is why the fertilized eggs 

 of Cumingia take up more water from hypotonic solutions and 

 lose more water to hypertonic sloutions than do the unfertilized 

 eggs. 



Osmotic change in unfertilized eggs is fairly rapid and no one 

 can deny that the plasma membrane is permeable to water. If 

 then R. S. Lillie's measurements are correct and water enters 

 fertilized eggs more rapidly than unfertilized eggs in hypotonic 

 solutions and leaves them more rapidly in hypertonic solutions 

 than it seems certain that the egg plasma membrane has not 

 markedly increased its permeability to dissolved substances as 

 a result of fertilization. For such an increase in permeability, 



