THE CELL WALL 51 



lipoid conipoiinds, and others proteins without lipoids, (Robertson *^ 

 suggests that there is a supei-fieial film of concentrated protein about 

 the cells, underlaid by a discontinuous lipoid layer.) There is, fur- 

 thermore, evidence that the entire cell substance has a profound effect 

 upon diffusion within the cell, so that it is at present impossible to 

 say whether the osmotic phenomena of cells depend upon a cell mem- 

 brane or upon the entire cell substance. ^^-^ It may be that there are 

 membranes or surfaces within the cell, as postulated in the foam 

 structure hypothesis of protoplasm, or that a homogeneous protoplasm 

 develops surfaces where in contact with substances entering from the 

 outside. 



Many facts indicate that either the delicate external membrane 

 of animal cells or the entire cytoplasm has the features of a semi- 

 permeable membrane, to the extent of permitting certain substances 

 to diffuse through and not others. Had they the property of some 

 of the artificial semipermeable membranes, of letting water pass 

 through but holding back almost absolutely all crystalloids, the re- 

 sult would be the development of an enormous disproportion in the 

 pressure between the inside and the outside of the cell. Furthermore, 

 the exchange of nutritive material and excretion products between the 

 blood and the cells would be impossible. But permitting some sub- 

 stances to pass into the cell results in their accumulation within the 

 cell, until they are in sufficient concentration to neutralize the osmotic 

 pressure exerted on the outside of the cell. As evidence of this elec- 

 tive permeability we have the fact that the proportion of certain salts 

 within the cell is quite different from what it is in the fluids bathing 

 them; e. g., animal cells generally contain more potassium and less 

 sodium than the fluids surrounding them. The inorganic constituents 

 of red cells are different from those of the plasma, the corpuscles 

 not containing any calcium at all, while the magnesium seems to 

 enter them freely; in other words, the red coi'puscle seems to be 

 impermeable to calcium and permeable to magnesium. If the salts 

 in a corpuscle are in smaller proportion than in the surrounding fluid, 

 it indicates that the cell membrane is not freely penneable for them ; 

 if in greater proportion, that some constituent of the cell is holding 

 them in combination, possibly as ion-protein compounds. Probably 

 inorganic salts are present in the cell by virtue of both physical and 

 chemical influences, some simply diffusing in and out, others com- 

 bining with the proteins and being held chemically. 



The intercellular substance varies greatly in different tissues. In 

 the case of the supportive tissues it is the important element, and 

 the cells seem to exist chiefly for the purpose of forming and keeping 

 it in repair as it is worn out. In the epithelial and secreting tissues, 

 however, the intercellular substance is reduced to a minimum, except 



"Jour. Biol. Chem., 1908 (4), 1. 



4ia See Kite, Amer. Jour. Pliysiol., 1915 (37), 282. 



