The mast cells too seem suited for this purpose since their specific substance is strongly characterised by its peculiar metachromatic staining, and is further especially readily recognisable, since by its great affinity for basic dyes it remains plainly stained, even in preparations that are almost quite decolorised. In fact appearances of the mast cells are not infrequently found, which must be referred to excretory processes of this kind.
In the first place it is occasionally seen that the mast cell granulation is dissolved within the cell, and diffuses in solution into the nucleus. In place of the well-known picture of the mast cell (see page 76) of a colourless nucleus, surrounded by a deeply stained metachromatic granulation, a nucleus is present intensely and homogeneously stained in the tint of the mast cell granulation, surrounded by a protoplasm shewing but traces of granules.
Still more convincing is the presence of a peculiar halo of the mast cells, described by various authors. Ehrlich first shortly mentioned this halo in his book on the oxygen requirements of the organism. A few years ago, Unna, whose notice Ehrlich's remark had no doubt escaped, described an analogous condition as follows: "in some nodules the mast cells appeared in part twice as large as usual, especially with the new mast cell stain (polychrome methylene blue, glycerine ether mixture). This was caused by the staining of a large round halo, in the centre of which lay the peculiar long-known mast cell, consisting of blue nucleus, and an areola of deep red granules. Higher magnification shewed that the halo was not granular, but very finely reticular; although it exhibited exactly the same red colour as the granules. It was consequently a spongioplasm peculiar to these mast cells."
The appearance of the mast cells described by Unna may also be artificially produced, by allowing a preparation that is stained with the oxygen containing analogue of thionin, oxamine, to remain for some time in lævulose syrup or watery glycerine. Evidently part of the dyed mast cell substance is dissolved and retained in the immediate neighbourhood. But as Unna possesses great experience of the mast cells and is a complete master of the methods of their demonstration, one must suppose that the halos described by him were preformed, and did not arise during the preparation of the specimen.