332 L. V. HEILBRUNN. 



effect from increased permeability to water. Apparently there 

 is a dilemma. 



As a matter of fact it appears to be rather far-fetched to 

 assume a change in permeability to water, since we know the 

 plasma membrane to be at all times permeable to it. R. Lillie's 

 results can be much more simply explained on the basis of my 

 conception of the plasma membrane (see Heilbrunn, '15). Nor- 

 mally before fertilization it is a more or less rigid structure and 

 as such resists the entrance or exit of water from the cell. I 

 showed by measurement ('15, pp. 155-158) that when the 

 membrane was made less rigid as a result of membrane swelling 

 then water left the cell more readily. After fertilization the 

 plasma membrane either itself becomes less rigid, as when 

 membrane swelling occurs, or it is replaced by a less rigid mem- 

 brane as a result of membrane elevation. Hence water enters 

 and leaves the cell more rapidly. 



It is easy enough to decide between R. Lillie's interpretation 

 and mine. If the difference is simply one of relative permeability 

 to water, then in hypotonic or hypertonic solutions the water 

 should enter or leave the fertilized eggs more rapidly, but the 

 final equilibrium point should be the same for both fertilized and 

 unfertilized eggs. However on the basis of my view, not only 

 should the water enter and leave the eggs more rapidly, but the 

 actual equilibrium state should be altered. In hypertonic solu- 

 tions, more water should leave the fertilized eggs and in hypotonic 

 solutions more water should enter them. In the case of hypo- 

 tonic solutions Lillie's own figures seem to show that at equilib- 

 rium more water has entered the fertilized eggs than the 

 unfertilized. 1 



1 Lillie's measurements were made on the egg of the sea-urchin Arbacia. In 

 this egg the presence of the elevated vitelline membrane or fertilization membrane 

 introduces a complication. As is well known this membrane is a stiff structure. 

 When fertilized sea-urchin eggs are subjected to hypotonic solutions they increase 

 in size rapidly until they reach the elevated membrane. Then further increase in 

 diameter is dependent on the power of the eggs to stretch or rupture the membrane. 

 R. S. Lillie does not state which occurs. As a matter of fact these experiments 

 on endosmosis were done in September when the Arbacia season is practically 

 over. At this time the normal membrane elevation is difficult to obtain, and 

 usually, unless the sperm concentration falls within certain very narrow limits, 

 the membrane swells at fertilization. For eggs with swollen membranes the 

 experiment is uncomplicated, as in Cumingia. 



