194 RALPH S. LILLIE. 



the cell this isolation is due to the presence of an impermeable 

 surface film. A marked degree of impermeability to the great 

 majority of its diffusible constituents is thus indispensable to the 

 continued existence of a highly complex heterogeneous system 

 like the cell. To certain substances like oxygen and carbon di- 

 oxide the plasma-membrane appears quite freely permeable, 

 though even in the case of these substances the permeability 

 seems to differ greatly at different times. 1 In general the con- 

 clusion seems justified that the presence of this impermeable 

 boundary is an indispensable condition of the most fundamental 

 chemical characteristics of living matter. We may say indeed 

 that just as the high development of chemistry would have been 

 impossible without the use of vessels in which the interacting 

 substances could be confined and isolated from the surroundings, 

 so the development in phylogenesis of the complex chemical spe- 

 cificity of living organisms has depended on the isolation of the 

 protoplasmic chemical system from its surroundings by a physi- 

 cally impermeable boundary. The impermeability of the plasma 

 membrane thus appears not as a merely incidental character of 

 living cells, but as a primary condition both of their development 

 and of their continued existence. 



The non-permeability of cells to many electrolytes is an espe- 

 cially significant characteristic. Neutral salts of the alkali and 

 alkali earth metals are almost invariable constituents of proto- 

 plasm ; yet the plasma-membrane, as the investigations of Over- 

 ton and others have shown, is impermeable or difficultly permea- 

 ble to many if not all of these. This implies impermeability to 

 the undissociated molecules and to one or other or both of the 

 two classes of ions resulting from dissociation. If a membrane is 

 partially permeable to both ions of an electrolyte the chances are 

 that it will be unequally permeable to these ; and the case is con- 

 ceivable that it should be freely permeable to one ion but not to 

 the other. Ostwald 2 in 1890 first directed attention to this possi- 

 bility ; such a membrane, if interposed between two unequally 

 concentrated solutions of the electrolyte, would then be the seat 

 of an electrical potential difference, since there would be a sepa- 



x The permeability to fat-solvents is an incidental, not a vital, peculiarity, since 

 these substances are not normally present in protoplasm or its surroundings. 

 2 Zeitschrift fur physikalische Chemie, 6, 1890, p. 71. 



