High resolution micrograph of the cortex of Tetrahymena. One parasomal sac lies anterior to each basal body of the somatic ciliature. A parasomal sac is an indentation of the plasma membrane that penetrates into the cytosol past the alveolus. Most sacs end as a coated pit. However, as seen in this micrograph, they can also give rise to more extensive tubules and vesicles. These vesicles pinch off into the cytosol to become early endosomes in a process called endocytosis where they lose their coat and fuse with a flattened smooth-membraned cisterna that is the early endosome. lip, lipid droplet. TEM taken on 7/17/67 by R. Allen with Philips 200 operating at 60kV. Neg. 9,000X. The raw film was scanned with an Epson Perfection V750 Pro. This image is best used for quantitative analysis.