The cytoproct or cell anus of Paramecium caudatum was studied, using light optics and electron microscopy, at known times before, during and following food vacuole egestion. This was accomplished by microscopically observing single cells, fixing these cells at specific times and finally serial sectioning these individually processed cells. The cytoproct, at rest, is a long narrow ridge along the posterior suture. It contains 2 uniquely positioned components which identify this structure as the cytoproct: piles of fibres along the inside surfaces of the ridge, and microtubules passing from the epiplasm at the summit of the ridge down into the cytoplasm. The plasma membrane is continuous over the top of the ridge. The cortical basal bodies adjacent to the ridge have bundles of microtubules passing into the cytoplasm from an opaque plaque at their proximal ends. These 2 sets of microtubules may function in guiding the food vacuoles to the cytoproct. A model is presented in which motive forces generated between the microtubules and the food vacuole membrane bring the food vacuole to the cytoproct and, in addition, pull the cytoproct lips apart so that the food vacuole membrane and plasma membrane come into contact and fuse together, thus opening the food vacuole to the outside. The plasma membrane increases in area between the parting lips, possibly, as the result of intercalation of membrane vesicles into the plasma membrane at the top of the ridge. Immediately after this opening is formed the food vacuole membrane changes from a smooth topography to a highly convoluted one. The membrane is engulfed through a process of endocytosis resulting in an accumulation of membranous fragments in the cytoplasm below the cytoproct. The endocytic forces probably bring about the restitution of the cytoproct ridge by pulling the lips back together as the membrane is engulfed. A filamentous meshwork underlying the food vacuole membrane may be active in this endocytic process.

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