72 DIFFUSION OF SUBSTANCES INTO LIVING CELLS 



retard diffusion themselves according to their strength; 

 that is to say, that if a certain amount of diffusion of a 

 substance will occur from a neutral jelly, the addition of 

 an acid will delay that diffusion in direct proportion to 

 the amount of acid present. As a matter of fact, acids 

 play only a very small part in these researches, for it 

 has been our endeavor to copy the conditions found 

 in the body as much as possible, and cells do not 

 normally come into contact with acids to a great extent. 

 For this reason, as will be shown later, we actually take 

 steps to eliminate the consideration of acids from our 

 experiments, in order to simplify matters. 



The foregoing, then, are the factors which increase 

 or decrease the diffusion of substances into living cells. 

 We have no right, of course, to assert that all alkalies 

 increase and all salts retard the diffusion of substances 

 into cells, for we have not tried them all; but as far as 

 we have experimented they seem to obey a general 

 rule. As has already been stated, one can only touch 

 on the main principles of this subject of the passage of 

 substances into individual cells, about which little was 

 known before this jelly method of in-vitro staining 

 was invented. 



Up to the present I have used the expression "cell" 



in its widest sense. Cells exist as individuals, and as 



individuals in classes. One may say that polynuclear 



'neutrophile leucocytes are a class of cell, and that 



erythrocytes are another class of cell. 



The diffusion of substances into cells is generally 

 the same in individuals of a class, but it presents great 

 differences in the various classes. For instance, if a 



