. Cytological technique; the principles underlying routine methods. Histology -- Technique; Cytology -- Technique. THE CAUSES OF DIFFERENTIAL DYEING 103 certain tissue-constituents have lost the colour while others retain it. During this regressive process the parts become more distinct from one another than they were when all parts held too much of the dye. The regressive stage of the process is therefore called differentiation. Cationic dyes are frequently used regressively. It is customary to use acids as differentiating agents. They act, in principle, in exactly the same way as when they a
. Cytological technique; the principles underlying routine methods. Histology -- Technique; Cytology -- Technique. THE CAUSES OF DIFFERENTIAL DYEING 103 certain tissue-constituents have lost the colour while others retain it. During this regressive process the parts become more distinct from one another than they were when all parts held too much of the dye. The regressive stage of the process is therefore called differentiation. Cationic dyes are frequently used regressively. It is customary to use acids as differentiating agents. They act, in principle, in exactly the same way as when they are mixed with the dye: reliance is placed on the ability of the acid to render the am- photeric tissue-constituents basic, and thus make them let go of the cationic (basic) dye. Alkalis can be used in the same way to differentiate acid dyes. An alternative way of differentiating is to soak the tissue in a fluid in which the tendency of the dye to remain ionized is reduced or eliminated, but in which the dye is soluble. Ethanol, either absolute or in strong aqueous solution, is often used for this purpose. The dye is rapidly lost from those tissue-constituents that hold little of it. If differentiation be stopped as soon as these constituents appear colourless, the objects that still retain a considerable amount of the dye will contrast strongly with them. The tissue must now be brought quickly into a fluid, such as xylene, in which the dye is insoluble. Acidic tissue-constituents, lacking any basic groups, can in certain circumstances be dyed by anionic (acid) dyes. Thus methyl blue, an anionic triarylmethane dye, will colour cellulose cell-walls. Indeed, it is used in the textile industry for dyeing cotton, and is often called cotton blue. The structural formula for cellulose shows the absence of any group that would be H OH O-i. r-O H^COH H Part of a cellulose molecule OH. Please note that these images are extracted from scanned page images that may have been digitally
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