118 Microscopic Histochemistry 



demonstrate urea at relatively low concentrations led FeyeP** 

 to believe that urea is not filtered through the glomeruli. 

 Probably the best method is that of Oliver.^^ 



Method 



Fix small pieces of tissue in a mixture of 6 g. of xanthydrol, 

 35 ml. of alcohol, and 65 ml. of acetic acid for 6-12 hours. 

 Heat the mixture gently and filter it before use. Dehydrate in 

 alcohols, embed in paraffin. Counterstaining of the sections 

 is permissible. The crystals are best recognized by their bire- 

 fringence. 



Uric Acm 



Uric acid occurs in the tissues in the form of acid sodium 

 urate, a substance of a very typical crystalline structure, spar- 

 ingly soluble in water, insoluble in alcohol. Although some- 

 what soluble in dilute alkalis, it is practically insoluble in 

 ammonia. Treatment with ammonia converts it into the cor- 

 responding ammonium salt, the crystalline structure of 

 which is different from that of sodium urate. 



Heavy-metal salts of uric acid are quite insoluble and can 

 be converted into colored compounds ;^^ however, such reac- 

 tions lack specificity ( phosphates and carbonates behave the 

 same way). This applies also to most of the silver methods 

 recommended for its demonstration.^^ However, uric acid is 

 one of the few argentaffin substances (see p. 58) occurring 

 in the animal body, and this property can be utilized for its 

 identification. This has been done, although under nonopti- 

 mal conditions, by Gersh.^* The following method gives very 

 sharp and selective pictures. 



30. Feyel, P.: Compt. rend. Soc. de biol., 114:1155, 1933. 



31. OUver, J.: J. Exper. Med., 33:177, 1921. 



32. Saint-Hilaire, C: Ztschr. f. physiol. Chem., 26:102, 1898. 



33. Courmont, J., and Andre, C: J. de phys. et de path, gen., 7:255, 

 1905; Tomita, W.: Tr. Jap. Path. Soc., 17:190, 1927. 



34. Gersh, I.: Anat. Rec, 58:349, 1934. 



