THE MODE OF ACTION OF VITAL DYES 293 



more striking way by the use of colouring agents of a very special 

 kind, which develop colour or become more intensely coloured on 

 reduction. These are the tetrazolium salts. Their reduction-pro- 



/-\_n^^-Y 



;NOo 



lodo-nitro-tetrazolium 



I 



ducts, which are insoluble, are called formazans. lodo-nitro- 

 tetrazolium is a pale yellow substance, soluble in water. It is 

 particularly suitable for the purpose because it reacts readily and 



H X ^NOo 



Formazan of iodo-nitro-tetrazolium 



its reddish-purple formazan stays immobile at the place of its 

 formation in the tissues. ^^^ The azo chromophore, to which it 

 owes its intense colour, will be noticed; so will the absence of any 

 auxochrome. 



Succinic dehydrogenase is able to remove hydrogen from 

 succinate and transfer it to tetrazolium compounds, which mark 

 the site of this reaction by the deposition of formazan. It has been 

 shown that if mitochondria, isolated by differential centrifuging, 

 are treated with a tetrazolium salt in the presence of succinate, so 

 much formazan is deposited on them that the matted clumps of 

 mitochondria and coloured precipitate appear almost black. In 

 the absence of succinate the reaction does not occur. *^^ 



lodo-nitro-tetrazolium is luckily very innocuous, and indeed 

 cells from chick-embryos can be cultured in solutions at 0-25%. 

 In living nerve-fibres of the ray, Hughes ^^^ has been able to see 

 particles of formazan, apparently deposited in rows along the mito- 

 chondria. This very interesting observation requires confirmation. 

 It seems to open the way towards a dynamic histochemistry of the 

 living cell. 



When a vital dye has been taken up by the cytoplasm of a cell 

 or by a cell-inclusion, it is easily removed by alcohol and other 



