no PROTOPLASMIC ACTION AND NERVOUS ACTION 



which has an entirely different composition. The 

 maintenance of the normal vital properties requires that 

 the essential diffusible constituents of protoplasm should 

 not be lost to the surroundings; it is also evident that 

 too ready an entrance of substances from the outside 

 would interfere with the stability of protoplasmic 

 composition. The presence of a semi-permeable bound- 

 ary layer appears thus to be a necessary condition for the 

 preservation of the normal chemical organization of the 

 cell. It is readily seen, for example, that the existence of 

 a simple diffusion equilibrium between surrounding 

 medium and protoplasm would prevent the latter 

 from acquiring the special crystalloidal content which is 

 characteristic of it. Hence, living cells are enabled to 

 survive and develop largely by virtue of being inclosed 

 by surface-films which are impermeable to crystalloidal 

 compounds of the foregoing classes. 



But since these compounds do in fact gain entrance 

 to the cell, at least at certain times, it is clear that the 

 problem of cell-permeability is not a simple one. Appar- 

 ently we must conclude that the entrance or exit of 

 substances by simple diffusion is in most cases a different 

 phenomenon from their entrance or exit under physio- 

 logical conditions. The processes of absorption and 

 secretion are in fact special activities, requiring the 

 performance of work by the cell. The distinction 

 between a passive or purely physical permeability and 

 an active or physiological permeability thus seems a 

 necessary one. 



The conditions of passive permeability are of interest 

 chiefly because of the light which they throw upon the 

 physical and chemical nature of the substances composing 



