20 Animal Micrology 



hydrochloric acid (0.1 to 1 per cent.) is used to decolorize the 

 tissues. For formulae see Appendix B. 



Hematoxylin is a compound containing the coloring matter of 

 logwood. The hematoxylins follow well almost any of the fixing 

 agents; they are especially recommended after fluids containing 

 chromic acid or its salts. According to Mayer, the active agent 

 in these stains is a compound of hematein with alumina. The 

 hematein is produced by the oxidation of hematoxylin. The so- 

 called "ripening" is simply this change, which is brought about 

 by exposing the hematoxylin solution to air. If the pure hematein 

 is used in making the stain, therefore, the latter will be ready 

 for use immediately, because it need not undergo the ripening 

 process (see reagent 47, Appendix B). For formulae see 

 Appendix B. 



Anilin is a colorless coal-tar derivative, and is the base from 

 which many of the numerous coal-tar dyes are made. The anilins 

 are brilliant stains of all colors. They are used almost exclusively 

 for staining sections or thin membranes, and are of great service 

 to the microscopist, although, as a rule, they fade in time. 



The basic anilin stains, such as methyl green, methyl violet, 

 gentian violet, methylen blue, safranin, Bismarck brown, toluidin 

 blue, and thionin are usually nuclear stains. On the other hand, 

 the acid anilin stains, such as acid fuchsin, eosin, erythrosin, light 

 green, orange G, bleu de Lyon, nigrosin, benzopurpurin, and 

 aurantia are ranked as cytoplasmic stains. These stains must be 

 made up fresh every two or three weeks, as they frequently spoil 

 if kept much longer. 



The metallic substances used for color differentiation operate 

 principally as impregnations rather than as stains. The coloring 

 matter is held physically as a precipitate or reduction product in 

 certain of the tissue elements. The commonest reagents of this 

 class in use are silver nitrate and gold chloride. 



The different tissue elements frequently show affinity for 

 different stains, consequently it is a common practice to use more 

 than one stain. Very decided contrasts may thus be produced, 

 such as red and blue, red and green, green and orange, etc. It 



