CHAPTER XI. 



STAINING. 



198. 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 011 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, 

 histoloyical 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, for instance either on the chromatin of the 

 nucleus, or on one or other of the elements that go to make 

 up the cytoplasm. 



Stains that thus exhibit a selective affinity for the sub- 

 stance of nuclei nuclear or chromatin stains form at present 

 the most important class of stains for the embryologist or 

 zootomist. What the zootornist or embryologist wants, in the 

 great majority of cases, is not so much to differentiate the 

 intimate structures of cells by means of a colour reaction, in 

 order to study them for their own sakes, as the cytologist 

 does, as merely to have the nuclei of tissues marked out by 



