1. The change in form of the mammalian red cell from that of a biconcave disc to that of a sphere in saline, serum, or plasma to which lecithin has been added is studied in detail. The transformation is reversible, and is unaccompanied by a change in cell volume. Different quantities of lecithin are necessary to bring about the shape change in the cells of different animals.

2. A similar change from disc to sphere is produced by the addition of the dyes of the fluorescein series to red cells in saline. The order of increasing activity is: fluorescein, eosin, erythrosine, and rose bengal. Haematoporphyrin also produces a disc-sphere transformation, and in all these cases the spheres can be turned back into discs by the addition of serum or plasma. The shape change is not accompanied by a volume change, and, in the case of rose bengal, it is apparently brought about when there are scarcely sufficient dye molecules to cover the cell surface.

3. The disc-sphere transformation which occurs between a slide and a closely applied cover-glass is discussed in the light of recent experimental results. The cause of this transformation is still obscure.

4. It is pointed out that in all cases in which the disc-sphere transformation occurs (with the exception of the case in which it occurs between slide and cover-glass), the change in form is ultimately followed by lysis. The component of the cell membrane responsible for the special shape seems to break down earlier, and some times very much earlier, than does the component responsible for the semi permeability, but the breakdown of the former is invariably followed by the break down of the latter. The breakdown in the shape component, however, appears to modify the permeability and electrical properties of the cell surface remarkably little.

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