The most striking feature of vital stains is the peculiar selective affinity which they possess for certain cells, and the wide difference and often extreme contrast in the affinity of very closely related dyes. Two views are held concerning the nature of vital staining: The first, advocated by Ehrlich, regards the chemical properties and is based on the assumption of receptors for certain parts of the molecule of the dye. The second, chiefly supported by Overton, takes the physical properties of dyes into account. The tendency at present is to consider the latter as the more probable explanation.
Evans and Schulemann1 in a recent article report experiments with dyes of the benzidin group, made for the purpose of determining the factors which govern the elective action of these dyes. They found that it is not the position of the hydroxyl group, not the entire periamido complex or the presence