TAKES PLACE AT VARIOUS RATES 73 



jelly is suitably prepared to stain the nuclei of leuco- 

 cytes in a given time, it will stain the nuclei of all 

 the leucocytes in that time, and it will always do so. 

 There will, of course, be a few exceptions among 

 individual cells which have died or which have become 

 achromatic, but generally speaking all the cells obey 

 the rules of their class. In some classes of cells, how- 

 ever, such as those of the epidermis, we have not yet 

 succeeded in causing anything to diffuse into them at 

 all; and in some of the larger cells, such as some 

 epithelial cells, only a few types will absorb sub- 

 stances in vitro; yet if some of the cells of a class in a 

 specimen will absorb a substance at a certain rate, the 

 others of the same class, which are not achromatic, 

 will also absorb the substance at the same rate. It 

 must therefore be grasped that the individual cells of 

 a class will absorb substances in the same way as each 

 other, and the diffusion into them will be influenced 

 by the usual factors in the same way in each cell of the 

 class; but substances diffuse into the cells of different 

 classes at different rates. 



Now we come to an extremely important factor 

 which has not been mentioned before, and which is the 

 last one to be taken into consideration. It is the 

 "coefficient of diffusion." 



We may prepare a jelly containing a certain con- 

 centration of stain, alkali, and salts which will allow 

 a certain amount of diffusion of the stain into a certain 

 class of cells at a certain temperature in a certain 

 number of minutes. Another class of cells may then 

 be tried on a film made from the same jelly under 



