The fine structure of the intracytoplasmic organelles of mycobacteria was studied by means of electron microscopy of ultrathin sections. A well-preserved nuclear apparatus was obtained by fixation with OsO4 in acetate-veronal buffer, containing calcium and tryptone, or in collidine-HCl buffer, followed by uranyl-acetate treatment and embedding in araldite. A low density nuclear region was filled with fine fibrils, 30 A in diameter, in parallel or concentric arrangement. A membranous organelle, tentatively designated as "lamellar structure," consists of unit membranes in lamellar arrangement. The thickness of each lamella in this membranous organelle coincides with that of the three-layered cytoplasmic membrane Moreover, the continuity of this unit membrane with the cytoplasmic membrane was demonstrated.

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