282 DYEING 



solution of methylene blue. The hyaline substance of the region 

 became violet, while the external parts (so-called bdtonnets) were 

 blue. A few years later Ancel ^ reported the red vital colouring 

 by methylene blue of globules in this region. This has recently 

 been confirmed by Roque/'-^^ who used toluidine blue. 



These early records are mentioned here because some writers 

 suppose vital metachromasy to be a modern discovery. ^^^ The 

 dyes used in more modern work have been new methylene blue,^^ 

 azure B,^^^ and toluidine blue.^^^' ^^^' ^^^ 



The inclusions that are coloured metachromatically during life 

 are by no means necessarily 'volutin' granules. In the higher 

 animals they commonly contain or consist of mucopolysac- 

 charides.^^^' ^^^ They sometimes occur in the part of the cell that is 

 blackened by the 'Golgi' techniques. ^^^ 



There is one striking difference between orthochromatic and 

 metachromatic vital colouring. An object that is chromotrope in 

 life is commonly chromotrope also in fixed preparations. In other 

 words, the chromotrope substances are not easily altered by the 

 fixation of the cell, but remain in position and corltinue to show 

 their characteristic effect upon dyes. Lipid globules, on the con- 

 trary, are either not retained in preparations of dead tissues, or 

 else do not colour specifically with the same dyes that coloured 

 them in life. 



Since the cytoplasmic inclusions could not be coloured unless 

 the dye passed through the ground cytoplasm, it is natural enough 

 that the latter is often slightly tinged. This is usually not helpful, 

 but in one particular case the colouring of the ground cytoplasm is 

 the object desired. It was discovered by Ehrlich ^^^ in 1887 that 

 certain nerve-fibres and nerve-endings, especially in the taste- 

 papillae of the frog, are coloured during life by methylene blue. 

 He says that thionine and its dimethyl derivative give metachro- 

 matic colouring of nerve-endings, and this is probably the earliest 

 mention of vital metachromasy. This colouring of nerve-fibres is 

 due to the uptake of the dye by the axon. The cytoplasm of the 

 body of the nerve-cell also becomes coloured. Other kinds of cells 

 may be dyed in the same way, but they begin to lose their colour 

 while the axons are becoming darker; in the mammals, hair- 

 follicles and sebaceous glands retain the colour longer than other 

 non-nervous tissues. ^^^ Ehrlich's discovery was of great importance 

 to neurology, for it simplified the tracking of nerve-fibres, while 

 avoiding artifacts due to metallic impregnation. 



