128 STAINING. 



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 refraction, 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 im- 

 mersed 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 elements 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 cytoplasm. 



The nuclear stains are of importance in marking out the contours 

 and relations of the tissues making up regions or organs as 

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

 morphologist. 



At one time, it was thought to be possible to distinguish between 

 ' basophilous ' ' and ' acidophilous ' tissue elements, according to 

 their affinity for basic or acidic dyes. EHRLICH (Du Bois Reymond's 

 Archiv., 1879, p. 571) thought that the basic dyes have a special 

 affinity for the chromatin of nuclei and the acidic dyes for the 

 cytoplasm and intercellular substances. But we have already seen 

 that the same substance may take up either kind of dye, according 

 to the conditions present. Most staining processes are undertaken 

 on cells which have been acted on by fixing reagents or by the so- 

 called ' mordants," and these may reverse the natural behaviour 

 to dyes. EHRLICH 's statement only applies in fact to cover-glass 

 preparations fixed and dried by heat, without the action of reagents. 

 The acidic colours, orange and acid fuchsin, although they stain 

 cytoplasm, may give good chromatin differentiation when used as 

 regressive stains. Methylen blue is basic, but stains nerves. The 

 widely used carmin and hsematoxylin are both acidic dyes, but in 

 combination with alum they give nuclear stains. Other instances 

 might be given, but these will suffice. 



