CHAPTER XI. 



STAINING. 



201. The Molecular Processes involved in Staining. — The 



question whether the phenomena of staining and of industrial dyeing- 

 are chiefly of a chemical order, as held by some, or chieily of a physical 

 order, as held by others, is outside the province of this book. See 

 (besides works on chemistry, amongst which may be mentioned Bbne- 

 DIKT and Knecht, The Chemistry of the Coal-tar Colours, London, 

 1889) Fischer's Fixinmg, Fdrbimg unci Ban des Protoplasmus, Jena, 

 G. Fischer, 1899 ; Pappenheim's Grundriss der Farbcheiiiie, Berlin, A. 

 Hirschwald, 1901 ; and the articles in Encycl. mile. Technik. 



202. Histological Staining : Specific, Nuclear, and Plasmatic. 



■ — Stains are either general or special (otherwise called SjDe- 

 cifiCj or Selective^, or Elective). A general stain is one 

 that takes effect on all the elenients of a preparation. A 

 special, specific, selective, or elective stain is one that takes 

 effect only on some of them, certain elements being made 

 prominent by being coloured, the rest either remaining 

 colourless or being coloured with a different intensity or in 

 a diffei'ent tone. To obtain this dijferentiation is the chief 

 object for which colouring reagents are employed in micro- 

 scopic anatomy. 



Two chief kinds of this selection may be distinguished — ■ 

 histological selection and cytological selection. In the former 

 an entire tissue or group of tissue elements is prominently 

 stained, the elements of other sorts present in the prepara- 

 tion remaining colourless or being at all events differently 

 stained, as in a successful impregnation of nerve-endings by 

 means of gold chloride. This is the kind of stain that 

 is generally meant by a specific stain. In the latter the 

 stain seizes on one of the constituent elements of cells in 

 general, for instance, either on the chromatin of the nucleus, 



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