The effect of a series of photographic developers on the final silver-staining picture has been investigated. Ten common developers were used, but of these only hydroquinone, chloroquinol, pyrogallol, and p-aminophenol, were found to be of general use. The other developers were either so weak in their action that the final staining was light and incomplete, or so powerful that a differentiated nerve staining was not produced.

For silver staining to be effected nuclei of reduced silver should be present in the section. These nuclei act as centres for the deposition of additional silver reduced by the developer; the additional silver may either be derived from that combined with the sections during impregnation or from the developing solution itself. Whether or ot the additional silver is deposited in such a way as to produce differentiated nerve staining depends on the properties of the developer and on the composition of the developing solution. The redox- and ‘bromide’-potentials, the sulphite and hydrogen n concentrations in the developing solution, and the protective action of the tissue components of the section all play a part in determining the final staining picture.

A new glycine-containing physical developer and a gold thiocyanate physical developer are described.

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