96 



CYTOLOGICAL TECHNIQUE 



^ Br Br 



C^\^ 



Na+ -O 

 Br 



Br 



COO- Na+ 



Skeleton-formula for 

 xanthene dyes 



Eosin Y 



The name refers to the pinkish-orange colour of the sky at dawn 

 (Greek eds, dawn). It will be noticed that eosin has two different 

 auxochromes, and that bromine is here used as a modifier. In 

 related dyes chlorine and iodine partially or wholly replace the 

 bromine, and a bluish tinge is thus imparted. 



Thiazine dyes. Here sulphur and nitrogen link two rings (Greek 

 theion, sulphur; French azote, nitrogen; the a in thiazine is short, 

 as in azote). Thionine, a blue dye with a reddish tinge, is the 



^^N 



HoN 



S. ^^ .NH. 



N^\^ 



Skeleton-formula for 

 thiazine dyes 



ci- 



Thionine 



simplest member of the group. The thiazines used in micro- 

 technique are all cationic. They are valuable dyes for nucleo- 

 proteins. Several of them, including thionine, are 'metachro- 

 matic'; that is to say, they impart quite different colours to 

 different substances (p. 104). This is a most valuable property, 

 since it gives important indications of chemical composition. 

 Azure B is another metachromatic thiazine dye. It is trimethyl 

 thionine ; that is to say, thionine modified by the substitution of 

 three methyls for three of the four hydrogens of the amino- 

 groups. 



AZO DYES 



These contain no quinonoid ring, apart from a few that have 

 both azo and quinonoid chromophores. The name refers to the 



