THE KINDS OF STAINS. 53 



CHAPTER VII. 



STAINING. 



91. The Kinds of Stains. Stains are either General or 

 Special (otherwise called Specific, or Selective, or Elective). 

 A general stain is one that takes effect on all the elements 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 different tone. To obtain this differentiation 

 is the chief object for which colouring reagents are employed 

 in microscopic 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 promi- 

 nently stained, the elements of other sorts present in the 

 preparation 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, namely, either the nucleus or the extra- 

 nuclear parts. 



Stains that thus exhibit a selective affinity for the sub- 

 stance of nuclei, or nuclear stains, form at present by far the 

 most important class of stains in zootomy at any rate. 

 What the zootomist wants, and the histologist too, in the 

 great majority of cases, is either to differentiate the intimate 

 structures of cells by means of a colour reaction, in order to 

 study them for their own sakes, or to have the nuclei of 

 tissues marked out by staining in the midst of the unstained 

 material in such a way that they may form landmarks to 



