=Neutral red= is highly suitable for the study of vital granule-staining, a dye recommended by Ehrlich, and employed successfully since that time by Przesmycki, Prowazek, S. Mayer, Solger, Friedmann, Pappenheim and others. This dye was prepared by O. N. Witt from nitrosodimethylamin and metatoluylendiamin, and is the hydrochloric acid salt of a base which is soluble in pure water, yielding a fuchsin red colour, but which in weak alkaline solution--the alkalinity of mineral water suffices--is a yellow-orange hue.
Now neutral red is characterised by a really maximal affinity for the majority of the granules. Ehrlich was able by the aid of this dye to demonstrate granules, even in some vegetable cells. Moreover the method of using it is the simplest conceivable, as subcutaneous or intravenous injection, or even feeding, in the higher animals stains the granules; with frog's larvæ and invertebrates, to allow them to swim in a dilute solution of the dye is often sufficient. The staining also succeeds in "surviving" organs, and is best effected by allowing small pieces to float in physiological salt solution, to which a trace of neutral red is added, under plentiful access of air. When the object is macroscopically red it is ready for examination.
The finest results are naturally given by organs that are easily teased out, _e.g._ flies' eggs, or the Malpighian canals of insects. The staining solution is to be chosen so that the act of staining does not last too long, but on the other hand too high a concentration must not be used. About 1/50000 to 1/100000 is recommended, so that the protoplasm and nucleus remain quite uncoloured. Artificial products with this method cannot entirely be excluded, and, _e.g._ in plant-cells containing tannin, are to be explained by the production and precipitation of the salt of tannic acid. However it is not difficult for the experienced to recognise artificial products as such in individual cases. The kind of granulation, the typical distribution, a comparison with neighbouring cells, the combination of various methods, the comparison of the same object under vital and "=survival=" staining, facilitate judgment and obviate mistakes of this kind.