50 STAINING. 



CHAPTER VII. 



STAINING. 



91. The Kinds of Stains. The chief end for which colouring 

 reagents are employed in microscopic anatomy, is to obtain 

 a selective staining of organs. In a selective stain, certain 

 elements are made prominent by being coloured, the rest either 

 remaining colourless or being coloured of a different intensity 

 or of a different tone. 



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 preparation 

 remaining colourless or being at all events differently stained, 

 as in a successful impregnation of nerve-endings by means of 

 gold chloride. 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 substance 

 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 sake, 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 catch the eye, which is then 

 able to follow out with ease the contours and relations of the 

 elements to which the nuclei belong ; the extra-nuclear parts 

 of these elements being expressly left unstained in order that 

 as little light as possible may be absorbed in passing through 

 the preparation. Possibly this may be an irrational procedure, 

 but it has hitherto been found in practice to be the most 

 efficient for general work. 



