By means of a new staining procedure (using a mixture of Astra blue and safranin 0 ) , studies on the polysaccharides of the mast cell granules were made in adult and embryonic connective tissues of the rat. According to their staining reactivity toward these dyes, it was found that, in the adult tissues, mast cells can be classified into three groups: (1) Astra blue positive mast cells, or "blue" cells; ( 2 ) safranin positive mast cells, or "red" cells, which are the most numerous cells;
( 3 ) intermediate forms, which exhibit affinity for both dyes, or "mixed" cells. In the embryo, mast cells appear first on the fifteenth-sixteenth day of development as small round cells, the cytoplasm of which contains flakes of Astra blue positive material; rapidly, this material becomes granular, and on the twenty-first day (just before birth), a large mast cell population exists in the subcutaneous connective tissue of the embryo, containing only cells of the "blue" type. Eight to 15 days after birth, "red" and "mixed" cells are visible. Progressively, the mast cell population becomes of the adult type. It is thought that these variations in the staining reactivity correspond to the evolution of the cell: the "blue" forms are the younger cells and the "red" forms are the adult mast cells. This is probably related to a progressive increase of the number of strong acidic groups which are present in the polysaccharidic molecule.