Observations were made using scanning and transmission electron microscopy and were correlated with measurements of the mean surface area and the percentage area occupied by secretory cells to provide precise interpretation of the morphological data. In cyclic and anoestrous possums the oviducts were lined with elaborate folds of mucosa except in the portion of the isthmus adjacent to the uterus where the pattern was much simplified. The folds decreased in height and number after ovariectomy.

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... cretion was maximal between oestrus and Day 2 of the cycle when most ovulations take place. Extrusion of vesicles was observed in the ampulla and isthmus at this stage although the process was localized to some extent because groups of cells with actively extruding vesicles were interspersed with others which contained vesicles but otherwise appeared inactive. This is interpreted as a device whereby a constant supply of secretion can be maintained over several days. Secretory activity declined between Days 3 and 8 and the epithelium in mid-cycle (Days 8\p=n-\18)appeared relatively quiescent. By Day 24 there was morphological and morphometric evidence of a return to oestrous conditions, although vesicles had not yet reappeared in secretory cells. Ciliogenesis was prevalent at this time and is probably related to the ovum transport that will occur several days later.