282 



PROTOPLASM 



entrance into the living membrane is a function of their ether 

 solubiUty. This means that the copper ferrocyanide membrane 

 operates as a sieve and that the permeabihty of the plasma 

 membrane is a question of relative solubility. 



While the foregoing evidence is against the sieve hypothesis, 

 there is other evidence in its favor. 



M. H. Jacobs concludes that the pore theory is the only one 

 that can be seriously considered as offering an adequate explana- 

 tion of the behavior of the erythrocyte in the presence of ions. 

 S. C. Brooks regards the cell membrane as a sieve with pores the 

 diameters of which change with surrounding conditions. The 

 sieve, therefore, is not a fixed one but is capable of constant 

 change and adjustment. A change in the size of the pores of a 

 living or a nonliving membrane could be brought about by 

 aggregation (coagulation), peptization (dispersal), change in sign 

 and magnitude of electric charge, and the orientation of surface 

 molecules. When small particles aggregate into larger ones, the 

 spaces between the particles will become larger; the sieve, 

 therefore, coarser. The reverse change takes place on peptiza- 

 tion. A positively charged membrane would repel positive 

 particles and thus keep them out ; a negatively charged membrane 

 would permit them to enter (no other factors interfering). If 

 polar molecules are oriented at the surface, they will form a 

 molecular sieve. We may compare the surface molecules to logs 



