122 Microscopic Histochemistry 



tate ("chromaffin reaction"); ^2' ^^ in the case of dichromates 

 the precipitate will contain brownish chromium dioxide.^* In 

 the case of a- and p-diphenols, which are powerful reducers, 

 these reactions are prompt; the reducing power of m-di- 

 phenols is much weaker, and the reactions take place over a 

 period of hours rather than minutes. The corresponding 

 aminophenols and diamines behave in an analogous way, 

 but, since they are not known to occur in tissues, unless ad- 

 ministered parenterally ( arsphenamine ) ,^^ their histochem- 

 ical significance is very limited. 



On account of the effects of fixation, the situation in tissues 

 is somewhat more compHcated than in the test tube. Non- 

 protein phenols appear to be soluble in alcohol; at least, they 

 are not preserved by alcohohc fixatives. Formalin, on the 

 other hand, undergoes, condensation reactions (of the bake- 

 lite type) with phenols, resulting in the formation of highly 

 insoluble resin-like substances^^ of varying molecular sizes. 

 The identifying reactions of the condensation products may 

 differ sharply from those of the parent phenols. This is easy 

 to understand, since the hydroxymethyl groups entering the 

 benzene ring have a tendency to occupy the coupling ( ortho 

 and para) positions; folding of the large polymer molecules 

 formed later in the course of the reaction may cause instances 

 of unpredictable steric hindrance or the approximation of 

 reactive groups. 



The changes in the chemical properties of phenols by form- 

 aldehyde can be studied conveniently by the use of Cou- 

 jard sildes.^^ The phenols in question are dissolved in serum 

 or dilute gelatin, and marks are made on slides with the solu- 



12. Veme, J.: Bull. Soc. chim. biol., 5:227, 1923. 



13. Gerard, P., Cordier, R., and Lison, L.: Bull, d'histol. appliq. a la 

 phvsiol., 7:133, 1930. 



14. Ogata, T., and Ogata, A.: Beitr. z. path. Anat. u. z. allg. Path., 

 71:377, 1922-23. 



15. Jancso, N. von: Ztschr. f. d. ges. exper. Med., 61:63, 1928; Jancso, 

 N. von: Arch. f. exper. Zellforsch., 6:444, 1928. 



16. Coujard, R.: Bull, d'histol. appliq. a la physioL, 20:161, 1943. 



17. Gomori, G.: Arch. Path., 45:48, 1948. 



