Organic Substances 111 



2) For acetalphosphatides.^^—\Jse unfixed or formalin- 

 fixed tissues. Block pre-formed aldehydes by treating the 

 sections with a 2 per cent solution of either phenylhydrazine 

 or hydroxylamine-HCl in an acetate buffer of pH 4.5-5.5 for 

 6-12 hours at room temperature. Rinse thoroughly in distilled 

 water. Treat sections with a 2 per cent solution of HgCU for 

 15 minutes. Rinse in distilled water. Stain, etc., as under 1, 

 above. 



Danielli^^ finds that hydrolysis in 0.1 N HCl for 15 minutes 

 at room temperature is preferable to treatment with HgCk 

 because the latter may have oxidative side effects. 



3) For aldehydes secondary to the oxidation of unsatu- 

 rated fatty acids. ^^—Tresit sections with HgCl2 or 0.1 N HCl; 

 block aldehydes with one of the reagents mentioned under 

 2, above; wash and expose them to air for several days (for 

 instance, in a flat dish containing a shallow layer of water ) . 

 Stain, etc., as under 1, above. 



It is always advisable to extract a control section with sev- 

 eral changes of alcohol-ether or acetone and to carry it 

 through the entire procedure. The fat-free blank will help to 

 rule out reactions given by nonlipid substances. 



C. PROTEINS, AMINO ACIDS, AND PRODUCTS OF 

 PROTEIN METABOLISM 



The demonstration of the presence of protein substances 

 as such is of relatively minor importance in histochemistry 

 because proteins are ubiquitous components of all tissues. 

 Occasionally, however, it may be necessary to ascertain the 

 protein or nonprotein nature of certain structures, such as 

 granulations. The identification of the individual amino acids 

 is of much more interest but possible only ta a few special 

 instances. 



Proteins 



The methods for the demonstration of proteins can be 

 divided into two groups: precipitation and digestion tests. 



1. The precipitation tests are based on the fact that many 

 proteins retain their aflBnity for protein precipitants even 



