The structure of the ciliary epithelium of the adult albino rabbit has been studied by electron microscopy. Material was fixed in osmium tetroxide and embedded in epoxy resins. Two hitherto unappreciated features of the non-pigmented epithelial layer are described. First, the "infolded plasma membranes" described by previous workers are shown by serial sections to be projections or interdigitations from adjacent cells. Second, the "rows of vesicles" described by previous workers are shown by serial sections to be part of an unusual form of smooth-surfaced tubular endoplasmic reticulum. The tubules are highly convoluted and extensively interconnected. They are arranged in sheets, so that a cross-section through a sheet gives the appearance of a row of vesicles. The other structural features of the ciliary epithelium are also described. Previous workers have reported that Diamox, which inhibits the secretory activity of the epithelium, causes profound structural changes. An effort has been made to confirm these reports under carefully controlled experimental conditions. It was found that secretion could be inhibited by a maximally effective dose of Diamox without the occurrence of any detectable structural changes. The physiological significance of these findings is discussed.

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