THE STAINING OF BACTERIA. 103 



These can be readily removed by placing the sections, 

 before staining, for a few minutes in equal parts of Gram's 

 iodine solution (p. in) and water, and then washing out 

 the iodine with methylated spirit. 



To save repetition we shall in treating of stains suppose 

 that, with paraffin sections, the above preliminary steps 

 have already been taken, and further that sections cut by a 

 freezing microtome are also in spirit and water. 



THE STAINING OF BACTERIA. 



Staining Principles. To speak generally, the protoplasm 

 of bacteria reacts to stains in a manner similar to the 

 nuclear chromatin, though sometimes more and sometimes 

 less actively. The bacterial stains par excellence are the 

 basic aniline dyes. These dyes are more or less compli- 

 cated compounds derived from the coal-tar product aniline 

 (C 6 H 5 . NH 2 ). Many of them have the constitution of salts. 

 Such compounds are divided into two groups according 

 as the staining action depends on the basic or the acid 

 portion of the molecule. Thus the acetate of rosaniline 

 derives its staining action from the rosaniline. It is there- 

 fore called a basic aniline dye. On the other hand, 

 ammonium picrate owes its action to the picric acid part of 

 the molecule. It is therefore termed an acid aniline dye. 

 These two groups have affinities for different parts of the 

 animal cell. The basic stains have a special affinity for the 

 nuclear chromatin, the acid for the protoplasm and various 

 formed elements. Thus it is that the former the basic 

 aniline dyes are especially the bacterial stains. 



The number of basic aniline stains is very large. The following 

 are the most commonly used : * 



Violet Stains. Methyl-violet, R-5R (synonyms : Hoffmann's violet, 

 dahlia). 



Gentian-violet (synonyms : benzyl-violet, Pyoktanin). 



Crystal violet. 



1 For further information on this subject the student is referred to 

 Rawitz, " Leitfaden ftir histologische Untersuchungen," Jena, 1895, from 

 which the synonyms used in the text are taken. 



