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Taste buds of rabbit circumvallate papillae were studied with the electron microscope at intervals from six hours to 11 weeks after section of the glossopharyngeal nerve distal to the petrosal ganglion. Regenerated nerves appeared beneath the epithelium at 21 days but new buds first appeared at 25 days, after nerves had penetrated the basement membrane. Intimate contact of nerves with epithelial cells appears to be a precondition for taste bud renewal. Early appearance of cells resembling basal cells (type I V ) followed by relatively simultaneous appearance of type I, I1 and I11 suggest independent origins for these three types. The data support a humoral hypothesis of trophic action but do not rule out a role for impulse transmission.

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The fine structure of rat thymic lymphocytes from early prophase to late telophase of mitosis is described, using material fixed at pH 7.3 either in 1 per cent OsO4 or in glutaraldehyde followed by 2 per cent OsO4. The structure of the centriolar complex of interphase thymocytes is analyzed and compared with that of centrioles during division. The appearance of daughter centrioles is the earliest clearly recognizable sign of prophase. Daughter centrioles probably retain a secondary relation to the primary centriole, while the latter appears to be related, both genetically and spatially, to the spindle apparatus. The nuclear envelope persists in recognizable form to help reconstitute the envelopes of the daughter nuclei. Ribosome bodies (dense aggregates of ribosomes) accumulate, beginning at late prophase, and are retained by the daughter cells. Cytokinesis proceeds by formation of a ribosomefree plate at the equator with a central plate of vesicles which may coalesce to form the new plasma membrane of the daughter cells. Stages in the formation of the midbody are illustrated.

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