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The opossum prostate is highly complex cytologically, containing six different secretory cell types, each possessing a recognizably different secretory product. Secretory granules in cells of the first segment contain numerous particles 60k in diameter of high electron density. They are transferred to the lumen by apocrine secretion. Both granules and lumina contain iron (Turnbull blue reaction), Segment two possesses t h e e cell types. Type A cells have spherical secretory granules containing acid mucopolysaccharides. In type B cells, secretory granules are elongate and possess densely packed protein filaments, 80 A in diameter. Secretory granules in type C cells contain crystalloids of hexagonally packed cylinders with a spacing of 600 A. Third segment cells contain few organelles and have a broad irregular microvillar border. Most of the cytoplasm is filled with glycogen particles. The secretion is apocrine. The lumen contains lipoprotein lamellae, probably derived from shed microvilli, and also membrane bound glycogen globules originating from the cyto-

Albino rats increased their intake of water soon after they were given a load of 0.12 molar lithium chloride in the stomach. Alterations in blood volume and tonicity could not account for the magnitude of the thirst observed, which served to facilitate the renal excretion of the toxic lithium ions.

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