MEMBRANE POTENTIALS 229 



researches of Pfeffer and of de Vries on plant cells and of Hamburger 

 on animal cells it has been assumed that most of the water soluble 

 substances, such as particularly sugar and NaCl, cannot penetrate 

 the cell membrane. Those substances which do penetrate (urea, 

 narcotics) are, according to Overton and Hans Meyer, lipoid-soluble. 

 In later years this hypothesis has not been sustained. Many inves- 

 tigations have since demonstrated that anions diffuse freely through 

 the membrane of blood corpuscles, and that only the cations are non- 

 diifusible. This question cannot be very well discussed in greater 

 detail at this juncture. But, if this theory'- is at all correct, then all 

 conditions and prerequisites are present for the formation of mem- 

 brane potentials. Only the non-diffusible ion is in this case not the 

 protein-ion, but the common alkali metal cation. Since such cations 

 are present in the tissue fluids in much greater molar concentrations 

 than the proteins, it becomes quite possible that they give rise to 

 considerable membrane potentials. The only difficulty lies in the 

 fact that thus far it has not been found possible to construct a satis- 

 factory model for the study of such processes, or to reproduce a 

 membrane which would be impermeable to cations alone, in the same 

 sense in which phase boundary potentials have been so successfully 

 reproduced artificially. This leads us to the following conjecture: 



If we recall Haber's glass chains or Beutner's ''oil chains," it wiU 

 be remembered that they are considered as chains "reversible only 

 for cations or only for anions." We could restate this by saying that 

 the oil-membrane is "permeable" only for cations (or only for anions). 

 This impermeability is not by any means of the same Idnd as that of 

 the collodion membrane for protein. In the latter case the imperme- 

 abiUty is explained on the basis of a spatial disparity between the 

 pores of the membrane and the diffusing molecules, while in the 

 foraier case it is based upon the limited solubility of the ions of the 

 aqueous solution in the medium of the "oil". In the former case the 

 membrane is represented as a system of very fine canals through 

 which all the (non-colloidal) molecules, of the solvent as well as of 

 the solutes, circulate freely, the solvent in the pores being the same 

 (water) as in the two solutions. In the latter case the membrane is 

 conceived of as a foreign phase interposed between the two solutions 

 and representing a solvent totally different from those of the solu- 

 tions on either side of it. 



This brings us back to the first statement of this section in which 



