PROPERTIES OF PROTOPLASMIC MEMBRANES 141 



bility to water as well as to dissolved substances. The 

 character of the bioelectric variation accompanying 

 secretion also indicates this. 



Antropoff^ has studied the influence of the water- 

 permeability of osmometer membranes on the rate of 

 osmotic transfer of water. He reaches the general 

 formulation : 



i.e., the rate of passage of water into an osmometer 

 -5^ j at any time is proportional to the permeability of 



the membrane to water (a) and to the osmotic pressure 

 of the solution (P) less the hydrostatic or other pressure 

 (Pi) resisting the transfer. The rate of osmotic transfer 

 of water is thus proportional to the product of the 

 effective osmotic pressure into the permeability to water. 

 This formulation defines the conditions for perfect 

 membranes; i.e., those which are permeable to water 

 and impermeable to solute. 



In the Arbacia egg the permeability to water is 

 altered in a remarkable manner as a result of fertilization. 

 A quantitative measure of this permeability may be 

 obtained by measuring the rate at which water enters 

 or leaves the egg in dilute or concentrated sea water. 

 This rate is found to be increased about fourfold after 

 fertilization, indicating that this process is associated 

 with a marked increase in permeability to water. If 

 mixed fertilized and unfertilized eggs are placed in dilute 

 or concentrated sea water, the former undergo change 

 of volume much more rapidly than the latter, and 



^ Antropoff, Z. physik. Chem., LXXVI (1911), 721. 



