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The chorioid of the stingaree Dasyatis sabina contains large numbers of melanocytes and guanophores, the latter forming an oriented reflecting layer on the inner surface . The crystals contained in these cells have been chemically analyzed and consist of guanine (Nicol and Van Baalen, 1969) . At deeper levels both kinds of cells are commingled, generally preserving their separate identities . Examining electron micrographs of these cells, however, we found several instances in which guanine crystals and melanocytes are present in the same cell (Figs . I and 2) ; such cells occur well within the oriented reflecting layer . Guanophores contain stacks of superposed crystal sacs, each enclosing a crystal chamber possessing a crystalline inclusion (Fig . 1) . It is common experience that crystals such as these drop out when thin sections are being made for electron microscopy, leaving characteristic clefts (Arnott, 1966 ; Bevelander and Nakahara, 1969 a, b; Arnott and Nicol, 1970) . Crystal sacs are bounded by a membrane, as are melanosomes . In cells containing both kinds of inclusions, the crystal sac may be independent of the melanosome (Fig . 1), or the melanin granule and the crystals may occur within the same membrane (Fig . 2) . A " melanosome" in Fig. 2 contains one pigment granule and two crystalline inclusions . It seems as if occasional pigment cells of the chorioid are indeterminate and have the ability to form both crystal sacs and melanosomes . Perhaps, this indicates a common type of stem cell for both melanocyte and guanophore . A mixed cell containing both dark pigment granules and

OCCURRENCE OF MELANOSOMES AND OF CRYSTAL SACS WITHIN THE SAME CELL IN THE TAPETUM LUCIDUM OF THE STINGAREE