METACHROMASY 257 



authors doubt whether the dye ever exists in a polymeric form, 

 though they do not dispute the reahty of the y-band. 



The necessity for the presence of water if metachromasy is to 

 appear was recognized long ago, and Hansen ^^^ even gave in- 

 structions for making balsam preparations in such a way that some 

 water would be retained in the final mount. It has been claimed *^^ 

 that a molecule of water is actually incorporated in the dimeric ion 

 of the dye, and indeed lies between and links the two ions. If this 

 could be substantiated, the effect of dehydrating agents would be 

 readily understandable. 



When a metachromatic dye is presented to the tissues, there is a 

 selective uptake of the forms of the dye responsible for the a, p, 

 and y troughs. These forms will here be provisionally called re- 

 spectively the monomeric, dimeric, and polymeric. The dye may 

 be at such a low concentration that it is monomeric, yet certain 

 tissue-constituents will take it up almost entirely in the polymeric 

 form; or again, it may be so concentrated that it contains a high 

 proportion of dimers, yet certain tissue-constituents wdll take it 

 up as monomers.^^^ It is evident that the various tissue-constituents 

 are extremely diverse in their affinity for monomers, dimers, and 

 polymers. 



In general, small molecules have no affinity for the polymeric 

 forms of dyes. It is when a substance becomes itself polymeric, 

 and particularly when it exhibits itself as a gel or film, that it 

 aflPords an opportunity for the attachment of polymeric dyes. 

 Gels or films, however, have no tendency to take up metachro- 

 matic dyes unless they possess many negatively charged points on 

 their surfaces, ready to attract the positive charges of the basic 

 dye-ions. 



Sylven *^* lays special stress on the need for numerous negative 

 charges on the surface to be dyed, suitably spaced out. If the 

 conditions of the surface are appropriate, the arrangement of the 

 dye in polymeric form may be facilitated. If the negative charges 

 on the surface are too distant from one another, or too irregular, or 

 too weak, the tendency to attract the dye-ions and hold them 

 firmly in polymeric form will be small. We see here a hypo- 

 thetical explanation for the variation between diflferent chromo- 

 tropes in the intensity of their metachromatic reactions. It is quite 

 possible that when the dye is taken up in polymeric form, a w'ater- 

 molecule is intercalated between each dye-ion and the next; and if 

 so, a substance would be particularly chromotropic if its negative 



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