STAINING 133 



case in isolated instances, there are many facts which show that 

 it cannot be accepted as a general law. It is diflicult to see what 

 purely chemical relationship can exist between complex, sub- 

 stituted, diazo-sulphonates, as a large number of these specific 

 dyes are, and the chemical components of cells. Moreover, 

 although methylen blue and other thiazines are specific vital 

 stains for nervous tissue, certain safranin azo-dyes — ^diazin- 

 green, for example — ^which have no chemical relationship to the 

 former, are also vital nerve stains ; while similar compounds of 

 the safranin series itself have no such property. See Michaelis, 

 Chemie der Farbstoffe, 1902, p. 104. We have already seen how 

 complex are the physical factors that intervene in such a simple 

 case as the staining of paper, and to these may be added questions 

 of solid solution, distribution between phases, diffusibility, and 

 so forth. Indeed, it would seem that each individual case of 

 specific staining requires investigation by itself. 



242. Objects of Staining. Most constituents of cells are, in 

 their natural state, either colourless or only faintly coloured. 

 Thus they are only visible if their refractive indices differ from 

 those of the media in which they are immersed. Such, for 

 example, are fatty globules and the granules of many secreting 

 cells. But, as seen thus, it is not an easy matter to judge of their 

 true forms. This is greatly facilitated by staining them either 

 more deeply than, or of a different colour from, their surroundings. 

 If colourless glass beads, although they are easily seen by refrac- 

 tion, could only be observed from the direction of a line through 

 the hole in the centre, the recognition of their true form would be 

 difficult. Immersing them in a medium of the same refractive 

 index as themselves would render them invisible. But if they 

 were made of coloured glass and immersed in such a medium, 

 they would be readily detected and their shape recognised. 



The chief object of histological staining is then to cause certain 

 constituents of the cells to take on a different intensity of tint 

 from others. This may be done in various ways, as will be seen 

 later. It is usual to distinguish two kinds of selective staining, 

 histological and cytological selection. In the former an entire 

 tissue or group of tissue eleinents is prominently stained, the 

 elements of other kinds present remaining colourless or being 

 differently stained, as in the impregnation of nerve endings by 

 the silver and gold reduction methods. In the latter the stain is 

 taken up or retained by some constituent element of the cell, 

 such as the chromatin of the nucleus or an element of the cyto- 

 plasm. 



The nuclear stains are of importance in marking out the eon- 

 tours and relations of the tissues making up regions or organs as 

 a whole, and are thus of special value to the embryologist and 

 morphologist. 



