CHAPTER 13 



Metachromasy 



The words metachromatic and metachromatism were intro- 

 duced in 1876 ^' ^ in reference to the changes in colour undergone 

 by certain substances when heated. The adjective, however, is used 

 in biology in a different and very special way. The corresponding 

 noun is metachromasy or metachromasia. 



If a section is dved with toluidine blue, manv tissue-constituents 

 will be coloured blue, but if there is any cartilage in the preparation 

 its matrix will be dyed purple or red. It might be thought that the 

 dye was impure, owing to faults in manufacture or subsequent 

 changes in chemical composition, but the effect is observed 

 equally well with pure specimens of the dye. This is a typical 

 example of metachromasy. Toluidine blue is said to be a meta- 

 chromatic dye. The matrix of cartilage is called a chromotrope: 

 that is to say, a substance capable of altering the colour of a 

 metachromatic dye. The corresponding adjective is chromotropic. 

 The word orthochromatic is used to mean non-metachromatic. 

 Thus whatever is coloured blue by toluidine blue is said to be dyed 

 orthochromatically, and dyes that do not give metachromatic 

 effects are called orthochromatic. 



Certain dyes are not stable in solution, but gradually give rise to 

 other colouring agents, which are then present as impurities and 

 can be separated by suitable means. The presence of such im- 

 purities of spontaneous origin is called allochromasy. This has no 



necessarv connexion with metachromasv, but an orthochromatic 



■J • ' 



dye may give rise to metachromatic impurities by allochromasy. 

 Solutions of Nile blue are allochromatic, for they contain not only 

 the ions that one would expect, but also another substance, an 

 oxazone (p. 301); in addition to this, however, the cation of the 

 dye is feebly metachromatic. 



Metachromasy was discovered independently three or perhaps 

 four times during the year 1875. The evidence suggests that 

 Jiirgens ^^^ was the first to report his findings publicly. He used 



243 



