Observations on epoxy r esin embdded sections of tissue mast cells of the iris of Triturus pyrrhogaster (Boie) revealed the following additional findings to our previous study on methacrylate embedded sections.
1. The vacuoles frequently seen within the middle disk of light cell type mast cells in methacrylate embedded sections do not appear so often in epoxy resin embedded sections, therefore rather some of them are thought to have been the result of sublimation by electron beam. Further, there is a localized swelling of the low density layer of the middle disk with decreased density of the high density layer passing through this area and this may be the primary step of the process of vacuole formation resulting in the separation of the middle disk.
2. The dark colored granules, which predominate in the dark cell type mast cell, initially consist almost entirely of matrix of high density and a definite middle disk is gradually formed in the matrix. With the maturation of the granule, there occurs a marked reduction of the density of the matrix, probably due to liquefaction, and an increase of volume. Occasional fusion of granules may occur due to the partial disappearance of the limiting membrane. On the other hand within the middle disk, swelling develops with separation and the middle disk gradually dissolves and disappears in the liquefied matrix. In other words, with maturation of the granules, there is a transition from dark cell to light cell.
3. Occasionally, cylindrical or scroll-like lamellar structures appear within the matrix of light cell type mast cells but these were seen previously in immature mast cells also and are presumed