46 CELLULAR 



stances as are insoluble in it or unable to unite 

 with it chemically. 



In this connection we may recall the fact 

 that a monomolecular film on the surface 

 of water does not prevent the evaporation of 

 water. It is evident that water molecules pass 

 through the oil. In this case the escape of water 

 may not be governed by the solubility of the 

 water in the oil but by the size and speed of the 

 water molecules and the direction in which they 

 strike the surface. If we extend this idea to other 

 kinds of molecules we must also consider that 

 the shape of the molecules may be of importance 

 in some cases. It is of course quite possible that 

 the surface of the protoplasm is a non-aqueous 

 monomolecular layer. 



Let us now endeavor to picture a mechanism 

 by which certain substances may accumulate in 

 the cell. Suppose that undissociated molecules 

 of a weak base penetrate until their concentration 

 is the same inside and outside (the acidity and 

 other conditions being the same inside and out- 

 side) . If now the cell produces acid, some of the 

 molecules of the weak base will dissociate and 



