Receptor mediated endocytosis is a process in which extracellular molecules are taken into the cell via coated vesicles that pinch from the plasma membrane. In Paramecium the resulting vesicles fuse with flattened cisterna called early endosomes. Early endosomes accumulate just under the cortex of the cell. Sorting of proteins destined to be returned to the plasma membrane after separating from cargo occurs in the early endosomes and is facilitated by the formation of secondary coated pits and vesicles on the early endosomes that are smaller than the coated and uncoated preendosomal vesicles that arise from the plasma membrane. These small coated pits pinch off and subsequently lose their coats to form carrier vesicles that are roughly 100nm in diameter. These vesicles move deeper into the cell where they bind to microtubular ribbons. TEM taken on 11/4/68 by R. Allen with Philips 300 operating at 60kV. Neg. 20,500X.