1912] On the Distribution of Potassium in Renal Cells 391 



object is attained the diffusion of the salts in the sections is reduced to a 

 minimum. The slightest thawing permits diffusion of the salts, in which 

 case the distribution of the potassium compounds, as observed, would 

 be other than that obtaining in the living tissue. Even with the greatest 

 care the knife will warm a little and cause surface diffusion in cutting. 

 It is thus necessary to prepare a large quantity of material and examine 

 each section carefully under the microscope after it has been appro- 

 priately treated. Nearly all the drawings of the plates of this contri- 

 bution have been made from sections in the preparation of which these 

 precautions were observed, and in which surface diffusion, consequently, 

 had been almost wholly prevented. The sections were cut at 10 to 20 /x, 

 the thickness depending on the firmness of the tissue. 



The sections were allowed to remain in the reagent from three- 

 quarters of an hour to an hour and a half, according to convenience. 

 The longer time was preferable to attain better fixation of the tissue. 

 They were then washed gently from half an hour to an hour in ice-cold 

 water which was frequently changed. This dissolved the uncombined 

 reagent and the precipitates which the reagent forms with creatin or 

 ammonium salts. Immediately after washing, the sections were mounted 

 on a glass slide in a mixture of equal parts of 50 per cent, glycerine and 

 concentrated ammonium sulphide solution. 



After this treatment the distribution of potassium in the sections 

 was revealed by the localization of the black precipitate due to the forma- 

 tion of cobaltous sulphide. The staining of the preparation by the 

 reagent itself varies in colour from a deep brownish yellow to a light 

 gray. In animal tissues I have found the nuclei, if apparent, to be so 

 from their lighter colour as compared with the rest of the section. In 

 nerve cells the nuclei are stained much deeper, and Professor Macallum 

 states that in vegetable cells they are stained still deeper and of a reddish 

 shade. This is probably due to the nitrous acid reaction affecting the 

 cytoplasm and the nuclear material differently. 



That the uncombined reagent is readily removed from the sections 

 by washing with ice-cold water has been many times shown in prepara- 

 tions of which Fig. i is representative. This was drawn from a section 

 which, immediately after it was cut, was put in distilled water to remove 

 all the potassium salts present, then placed in the reagent and treated 

 further as described. The absence of a black reaction or of a dark 

 shading in the preparations is evidence that all the cobalt reagent has 

 been extracted. 



Frogs were largely used, other animals as occasion offered. The 

 frogs had been kept in a cool tank until Januaiy, and so were con- 

 sidered to have ceased to secrete actively. Hence they are referred to 



