SELECTIVE PERMEABILITY 349 



son and Lepeshkin relegate the role of the lipoids as factors con- 

 ditioning the selective permeability of cells to a secondary place. 



(4) There is no adequate method of testing the permeability 

 of living cells. The observ'ational methods (appearance of 

 color, change of color, appearance of precipitates) are limited 

 only to certain classes of substances, mostly unessential, and 

 are conclusive only with reference to positive results (when 

 permeability is demonstrated), but leave the matter unsettled 

 when negative results are obtained. The plasmolytic method 

 leads to results so contradictory that it can hardly be used as a 

 reliable test of permeability^ in all cases. 



(5) It is possible that the universally accepted explanation 

 of the plasmolytic phenomena is not correct in spite of its plausi- 

 bility and conformitj^ to so many known facts. It is possible 

 that there are factors involved in the failure to recover from 

 plasmolysis other than the inability of the substance in solution 

 to enter the cell. It is possible, for instance, that the substances 

 which produce plasmolysis and are at the same time known to 

 find their way into the cell produce certain reversible changes 

 in the components of the plasma-membrane which prevent 

 the cell from regaining its original volume notwithstanding the 

 equalization of the concentration within and without of the cell. 



The writer acknowledges his indebtedness to Dr. Lewis Knud- 

 son, upon whose recommendation this article was written, for 

 valuable information with reference to the bibliography on the 

 question. 



