GAUGING THE DIFFUSION 67 



while we experiment with them; and it must be borne 

 in mind that since sodium citrate is detrimental, leuco- 

 cytes or other cells placed in it gradually lose vitality 

 all the time, and that they are under experimental 

 conditions. 



The laws of diffusion or rather what we know of 

 them are simple in their experimental application; 

 but they are difficult to describe. 



There are two methods by which it may be known 

 when substances have diffused into a cell. If the 

 diffusing substance consists of a colouring matter which 

 will combine with or otherwise colour the molecules of 

 protoplasm within the cell, one can see the extent of the 

 diffusion by watching the progress of the coloration. 

 The other method consists in the use of a substance 

 which has a specific action on the cell and causes it to 

 give a definite response which will tell us w r hen the 

 substance has diffused in. Of the tw r o methods, the 

 former is obviously the better, for by seeing the gradual 

 staining of the morphological elements of the cell one 

 can more accurately gauge the extent of the diffusion 

 than one can by measuring roughly the degree of a 

 response such as excitation of amoeboid movements or 

 even cell-division. It is, of course, possible to employ 

 a combination of both methods, by which much can be 

 learnt; in fact, in this book I shall describe what has 

 been observed, in the first place, by using colouring 

 substances only, afterw r ards a combination of stain and 

 other substances, and lastly by experimenting with 

 other substances by themselves. 



Suppose the jelly on which a given cell is resting 



