118 CYTOLOGICAL TECHNIQUE 



with a mordant. Haematein is an example. This has a dirty red- 

 dish colour when dissolved in water, but becomes clear blue in 

 the presence of the aluminium ion. The addition of acid restores 

 the original colour. Purpurine does not change colour markedly 

 when it associates with aluminium (though alizarine changes from 

 yellow to scarlet). 



It always surprises students to learn that the mordant — the very 

 substance that attaches the dye to the tissue — can also be used to 

 remove the dye from it. The fact is that everything depends on the 

 relative abundance of mordant and dye. When a section has been 

 dyed, the total amount of dye in it is very small. If the section is 

 now placed in a solution of the mordant, the latter is present in 

 enormous excess. The dissolved aluminium competes with the 

 attached aluminium for the minute amount of purpurine that is 

 colouring the tissue. The dye redistributes itself. If sufficient time 

 be allowed, no visible trace of dye will remain in the tissue. In 

 practice one looks at the section under the microscope from time 

 to time. When the objects that it is desired to show clearly are still 

 strongly coloured, but the surrounding parts of the tissue have 

 become colourless or scarcely tinged, one washes away the 

 differentiating fluid with water. 



While the dye is being extracted by the mordant, coloured 

 clouds may be seen in the solution surrounding the section. Thus 

 the solution extracts the dye, though both mordant and dye are 

 present in it. It follows that whether a mordant/dye solution dyes 

 or extracts depends on the relative abundance of dye and mor- 

 dant; that is to say, on the mordanting quotient. For any particu- 

 lar mordant and dye one can find a concentration of each that 

 neither increases nor decreases the intensity of colouring of 

 tissue-constituents that have already been dyed by the same mor- 

 dant and dye.^^ In experiments of this sort it is best to work with 

 sections that have been exposed to the standard aluminium 

 purpurine solution (p. Ill) until chromatin has been strongly 

 coloured but cytoplasm only feebly. The mordanting quotient of 

 an aluminium purpurine solution that neither increases nor de- 

 creases the intensity of colouring of such a section is called the 

 critical mordant quotient .^^ If the quotient of a solution is lower 



