42 LECTURE III. 



vessel of water: can we then directly compare a turgid cell to 

 a turgid bladder, the cell-wall corresponding to the membrane 

 of the bladder, the cell-sap to the syrup ? The answer to the 

 question is that such a comparison is inaccurate, for it has 

 been found that the cell-wall is not able, like the membrane 

 of the bladder, to hinder the escape by exosmosis of the con- 

 tained liquid to such an extent as to become turgid. It is the 

 lining layer of protoplasm, the primordial utricle as we have 

 termed it, which offers the necessary resistance and prevents 

 the escape of liquid by exosmosis : it is to this that the turgidity 

 of the cell is to be ascribed. We can now compare the turgid 

 cell to the turgid bladder: the cell- wall and the primordial 

 utricle together correspond to the membrane of the bladder, 

 the cell-wall supplying the necessary elasticity, the primordial 

 utricle the necessary resistance to exosmosis. 



We will consider, in conclusion, the absorption into the 

 cell 'of substances in solution. Speaking generally, we may 

 say that the passage of any dissolved substance through a 

 membrane is determined by the size of its molecules ; if the 

 molecules of the substance are smaller than the intermolecular 

 interstices of the membrane then they will be able to traverse 

 the membrane. It is upon this that the formation of Traube's 

 artificial cells depends, the formation of a membrane of 

 precipitation being due to the fact that the intermolecular 

 interstices of the precipitate are smaller than the molecules of 

 the substances producing it. Further it has been observed 

 that membranes of precipitation are permeable to some salts 

 and not to others; for instance, water and potassic chloride 

 will traverse a membrane of cupric ferrocyanide, but the 

 chlorides of barium and of calcium, the sulphates of potassium 

 and ammonia, and barium nitrate, will not traverse it. The 

 most conclusive proof is afforded by the possibility of dimin- 

 ishing the size of the intermolecular interstices of a membrane, 

 and of thereby modifying its permeability. Thus when a 

 solution of gelatin comes into contact with a solution of tannic 

 acid, a membrane is formed which is permeable to ammonium 

 sulphate. If ammonium sulphate be added to the solution 

 of gelatin, and a soluble salt of barium to the solution of 



