Organic Substances 77 



acid until sections are suflBciently differentiated. Refloat on 

 water once more; mount on slides. Dry slides, remove paraffin 

 with xylene, and mount section in balsam. This technique 

 avoids all dehydrating agents and preserves metachromasia 

 very well. 



II. NUCLEIC ACIDS 



The three components of the nucleic acids are ( 1 ) purine 

 and pyrimidine bases; (2) pentose or desoxypentose sugar; 

 and (3) phosphoric acid. The histochemical methods for 

 their demonstration will accordingly be divided into three 

 groups, depending on which of the components is identified. 

 The specificity of the identifying reactions can be checked 

 by extraction techniques, enzymatic or other, which remove 

 nucleic acids in a selective way. 



1. The purine and pyrimidine bases can be demonstrated 

 by the ultraviolet absorption method only. Recently, Danielli^^ 

 has proposed another method which may be called descrip- 

 tively "double azo-coupling." It is based on the theory that 

 purine and pyrimidine bases will couple with diazonium salts. 

 The resulting azo dye is, however, only pale yellowish in 

 shade and unsuitable for direct observation. For this reason, 

 Danielli uses a tetrazonium compound ( of benzidine or dia- 

 nisidine ) which attaches itself to the base by only one of its 

 diazo groups; the other free, diazonium group can then be 

 coupled with highly chromogenic naphthol, and an intensely 

 colored (purplish) azo dye results. Tyrosine, histidine, and 

 tryptophane give similar color reactions; benzoylation, how- 

 ever, will abolish their coupling ability while it will leave that 

 of the purine and pyrimidine bases intact. 



The chemical basis of this method does not appear to be 

 sufficiently firm. 



First of all, chemical studies on the azo-couphng of purines 



81. Danielli, J. F.: Symp. Soc. Exper. Biol., 1:101, 1947. 



