342 J. DAVIDSON 



which are known to find their way into the cell. The entrance 

 into the cell of these substances, in contradistinction to lipoid- 

 soluble substances is physiological in character, depending upon 

 the regulatory power of the cell. No equalization of concen- 

 tration is reached within and outside of the cell but a certain 

 equilibrium is established between the two concentrations. 



The regulatory power of the cell is, according to Nathanson, 

 of such a nature that it allows of exosmosis of a substance, dis- 

 solved in the cell sap, toward a higher, concentration. Thus 

 he found that when a cell is transferred from a solution of a 

 higher concentration into a solution of a lower concentration, 

 exosmosis from the cell to the medium takes place till a new 

 equilibrium is established, even when the concentration of the 

 new solution is greater than that of the cell sap. The explana- 

 tion suggested by Nathanson is that different concentrations 

 of the outside medium affect the permeability of the plasma- 

 membrane in a different manner. This, however, could only 

 explain the establishment of an equilibrium between the con- 

 centration of the cell sap and that of the outside solution but 

 would not explain the diffusion of a dissolved substance in the 

 direction of a higher concentration, as it is hard to conceive 

 that permeability should be different with reference to exosmosis 

 than endosmosis. Nathanson's results could, perhaps be ex- 

 plained on the assumption that there is a certain solubility ratio 

 for the substances in question between the cell sap and water, 

 or that the dissolved substances enter into chemical combina- 

 tions with the substances of the cell sap which are subject to 

 the law of mass action. On the whole, however, one must 

 agree with Ruhland^^ that the experimental methods used by 

 Nathanson are too crude for his results, with reference to the 

 regulatory power of the cell to be considered reliable. 



The mechanism of exchange with reference to lipoid-insoluble 

 substances is, according to Nathanson, -^ somewhat analagous 

 to that of an artificial semipermeable membrane. He assumes 

 that there is a certain affinity between the membrane and water 



" L.c. 



28L.C. 



