102 CYTOLOGICAL TECHNIQUE 



cavities in the dyed cytoplasm, and they are not nearly so con- 

 spicuous as in sections coloured with a cationic dye. Red blood- 

 corpuscles will take any anionic dye that is able to enter them 

 (p. 108). 



It must not be supposed that basiphil objects are necessarily 

 resistant to coloration by anionic dyes, or acidophil ones to 

 coloration by cationic dyes. Proteins that are predominantly 

 acidic contain some basic amino-acids, and conversely. Thus all 

 the proteins in a cell can eventually be dyed to some extent by any 

 dye, whether basic or acid. However, by controlling the times 

 during which the dyes act, it is easy to ensure that the basiphil 

 objects will be dyed by cationic (basic) dyes and the acidophil 

 ones by anionic (acid) dyes, each almost to the exclusion of the 

 other. 



The amphoteric objects in cells can be dyed with either cationic 

 or anionic dyes, by controlling the pH. In practice one usually 

 wants to leave the cytoplasm uncoloured, or else to dye it differ- 

 ently from the chromatin. If a cationic dye is used in slightly 

 acid solution (methyl green, for instance, in a weak solution of 

 acetic acid), the chromatin will be coloured: but the amphoteric 

 cytoplasm, having been rendered basic by the acidity of the 

 solution, will have very little affinity for the cationic dye, and the 

 cytoplasm will therefore appear colourless or nearly so. Alterna- 

 tively one may use a cationic dye in acid solution first, and then 

 an anionic one of contrasting colour. The amphoteric cytoplasm 

 will have some affinity for the anionic dye, and it can thus be 

 coloured differentially. 



Since most tissue-constituents will eventually be coloured to 

 some extent by any dye, it may be desirable to limit the period of 

 dyeing, so that some of them will appear colourless when others 

 have already been quite strongly dyed. In other words, dyeing 

 may be arrested at a particular stage instead of being allowed to 

 go to completion. This i?, progressive dyeing, so called because the 

 depth of colour becomes progressively greater until the desired 

 end is reached. In regressive dyeing, on the contrary, the tissues 

 are allowed to take up an excess of the dye, and the latter is then 

 partly removed, with careful control under the microscope, until 



