106 CYTOLOGICAL TECHNIQUE 



Strongly the dimeric and polymeric forms of the dye, and hold 

 the dye to themselves in its dimeric or particularly its polymeric 

 form.i^°' ^2^' ^^^' ^'^ A small molecule would not be able to hold 

 a high polymer of a dye, but a polymeric substance, with numerous 

 negative charges arranged on its surface at regular and convenient 

 distances apart, might be able to do so.^^^ It thus appears that the 

 mode of attachment of the dye causes the shift of colour. 



Metachromasy is important in cytological technique because 

 it acts as a pointer towards chemical composition. 



Density 



Make two gelatine gels, one containing 5% and the other 20% 

 of gelatine. Fix these in formaldehyde solution (or in a mixture 

 containing formaldehyde). Cut a section of each on the freezing 

 microtome, at the same thickness (say 15 //). Allow an aqueous 

 solution of any dye to act on both sections for the same period, 

 which must be sufficient to allow complete permeation. Rinse 

 with distilled water. It is not surprising that one section is more 

 strongly coloured than the other. The result is due solely to the 

 fact that one section is denser than the other: that is to say, it 

 contains more colourable matter than the other, and therefore 

 takes up more dye. 



One often reads that the cytoplasm of a particular cell is 'dense'. 

 How did the writer know ? The depth of colouring may be due to 

 quite different causes. If one had not made the gelatine gels one- 

 self, one would not have known the cause of the difference in 

 colouring. The density of the two sections might have been 

 exactly the same, but one of them might have been made of a 

 basic and the other of an acidic protein, and they would neces- 

 sarily have taken up different amounts of any particular dye. 



Anyone who performs the experiment just described is likely 

 to be surprised by the darkness of the section that contains more 

 gelatine. There is only four times as much gelatine, but it appears 

 to have taken up much more than four times as much dye. This 

 is, in fact, an illusion. Let us suppose that we have used a black 

 dye, and that the section made from the v/eaker gelatine gel stops 



