300 DYEING 



differs only in possessing methyl groups attached to two of the 

 aryl rings. It has the advantage of being rather darker and thus 

 showing small lipid globules more clearly, and is one of the most 



H 



/ N— N=N— ^ \— N/" ^CHs 

 \ / \ / I 



H 



Sudan black B^""^ 



frequently used lysochromes. Sudan black B is a more complex, 

 almost black lysochrome, introduced by Lison."^''^^^ It colours 

 lipids with particular intensity. This appears to be due not only to 

 the darkness of its colour, but also in part to particularly high 

 solubility in lipids. 



Although colouring by lysochromes is so simple in principle, 

 and sometimes in fact as well, yet some of the most commonly- 

 used synthetic lysochromes do not act in exact accordance with 

 expectation. This subject deserves rather careful study, because it 

 throws an interesting sidelight on the nature of colouring processes 

 in general. 



Whereas the carotenoid lysochromes leave the cytoplasm un- 

 touched, Sudan III and IV show some tendency to colour it, as 

 though they were capable of acting to a very small extent as dyes. 

 This has not been explained. It is conceivable that in the presence 

 of particular substances the -N-N^ group might be changed to a 



H 



second azo group (-N=N-) and the =0 to the auxochrome -OH. 

 On the possibility of a transformation of this sort, see an old but 

 very interesting paper by Alichaelis.^^* 



Sudan black also sometimes behaves unexpectedly, for it tends 

 to be retained in certain tissue-constituents when the section is 

 placed in a solvent such as 70% alcohol after colouring. The two 

 -N- groups suggest themselves as potential auxochromes. It 



H 



has been shown ^^^' ^^^ that if these are blocked by acetylation 

 with acetic anhydride, the colorant now dissolves out from the 

 tissues like any other lysochrome. 



It is a matter of particular interest that Sudan black, when not 

 acetylated, has a special tendency to be taken up by phospholipids 



