THE CAUSES OF DIFFERENTIAL DYEING 105 



metachromatic; the tissue-constituents that are dyed in a differing 

 colour are called chromotropes (colour-turners). 



All the metachromatic dyes that are commonly used in micro- 

 technique are basic, and they all act on the same chromotropes. 

 The metachromasy of acid dyes will not be considered in this 

 book. 



The metachromatic dyes do not form a chemical group by 

 themsehes. On the contrary, they are distributed among all the 

 major groups except the azo. Methyl violet is a metachromatic 

 triarylmethane dye, for instance; thionine and azure B are meta- 

 chromatic thiazines. (Commercial thionine is often adulterated 

 with a red dye, and may then give false metachromatic colours. ^°^) 

 It is a curious fact that when all the hydrogens of the amino- 

 groups of a dye are replaced by methyl, the resulting substance is 

 not metachromatic.^^" Thus crystal violet (p. 93) is not meta- 

 chromatic (though in commerce it is often adulterated with the 

 strongly metachromatic methyl violet). 



The colour-shift caused by chromotropes is always in the 

 same direction. Green dyes become bluish; blue dyes are reddened 

 to purple or red; red ones become orange or yellow. This means 

 that the absorption-maximum of the dye moves in all cases 

 towards the shorter wave-lengths. 



Chromotropes are necessarily acidic, since they have affinity 

 for cationic dyes. Their negative charges are due to the possession 

 of sulphuric, phosphoric, or carboxyl groups. Many of the most 

 familiar chromotropes are sulphuric esters of polysaccharides of 

 high molecular weight. ^^"^ These are often mucosubstances. Thus 

 the matrix of cartilage, the secretions of certain mucous glands, 

 and the granules of the basiphil cells (Mastzellen) of connective 

 tissue give metachromatic colours because they contain chon- 

 droitic acid, mucoitic acid, and heparin respectively. Not all 

 chromotropes, however, are mucosubstances. Agar, strongly 

 chromotropic, lacks an amino-group. 



The cause of metachromasy has not been established with 

 certainty. The metachromatic dyes have a tendency to form 

 dimers and polym.ers, of the metachromatic colour, in aqueous 

 solution. Chromotropes appear to be substances that favour 



