104 CYTOLOGICAL TECHNIQUE 



thought Hkely to attract an acid dye. It is supposed that the 

 linkage is through a hydrogen bond," for it will be remembered 

 that hydrogen can in certain circumstances act as though it were 

 bivalent. Such a bond may tie the hydrogen of a hydroxy 1 group 

 of cellulose to the nitrogen of an amino-group (or substituted 

 amino-group) in a dye. Now methyl blue, and certain other 

 anionic triarylmethane dyes that will colour cellulose, do contain 

 nitrogen in a substituted amino-group. 



The hydrogens of the peptide groups of nylon are thought to 

 form hydrogen bonds with the amino-groups of certain dyes in 

 much the same way. If so, it is likely that the peptide groups of 



c=o 



i 

 NH 



I 



Peptide group in nylon or protein 



protein could behave similarly. This may account for the ten- 

 dency of certain acid dyes to colour tissue-constituents without 

 appearing to discriminate between those that are positively and 

 negatively charged. 



It is thought by some that dyes are first brought close to 

 particular tissue-constituents by the interaction of electrical 

 charges, and are then tied closely to them by 'short-range' forces; 

 that is to say, by covalent or hydrogen bonds. ^^^ Whether a close 

 tying-up of this sort does or does not occur finally as a general 

 rule in microtechnical dyeing, the distribution of dyes is in the 

 main determined by the electrical charges, density, and per- 

 meability of the tissue-constituents. 



It has already been remarked (p. 86) that dye-ions are usually 

 taken up without change of colour. An important exception to 

 this rule must now be briefly mentioned. 



Certain pure (unmixed) dyes have the property of dyeing par- 

 ticular tissue-constituents in a colour that differs from that of the 

 solution. This is called metachromasy and the dye is called 



