l8o DYEING 



is a second link between the two rings, through an atom that may 

 be oxygen, sulphur, or nitrogen. The three groups of dyes are 

 respectively the oxazines, thiazines, and azines. 



Oxazines 



The link through oxygen will be noted in the structural formula 

 for the basic vital dye, brilliant cresyl blue. Among the small 

 group of oxazine dyes used in microtechnique, a high proportion 



+ 



pj pi I I I 



Brilliant cresyl blue 



can be used vitally. This applies to Nile blue A, which is also used 

 in a roundabout way in histochemistry for the distinction between 

 certain groups of lipids (p. 301). 



Several of the oxazine dyes, coelestine blue B among them, can 

 be used w^ith a mordant (p. 215). 



Thiazines 



In these dyes, several of w^hich are among the most important 

 used in microtechnique, the second part of the link between the 



H n/\/S\^\^NH, CI- 



I I I I 



Thio7iine 



two rings is formed by an atom of sulphur. The simplest is thi- 

 onine, a violet dye. The thiazine dyes used in microtechnique are 

 all basic. 



The four hydrogens attached to nitrogen can be replaced by 

 methyl groups. One is so replaced in azure C, two in azure A, 

 three in azure B, all four in methylene blue (see p. 268). As the 

 replacement occurs, so the absorption maximum shifts towards the 

 longer wave-lengths, and methylene blue is a pure blue dye. 

 Toluidine blue is closely related to azure A. 



The thiazine dyes are useful for staining chromatin. Their chief 

 virtue, however, is that they have a particularly strong tendency 

 towards metachromasy: that is to say, towards the dyeing of 

 different tissue-constituents in different colours. This subject is 



