Figure 6. Transmission electron micrograph of endocytotic vesicles in the cortex of a vitellogenic Hyalophora follicle. The membrane enclosing each vesicle appears as a dark line (arrow). The structures radiating outward from the membrane are sections of the clathrin coat, a protein complex that mechanically converts a region of planar cell surface membrane into a spherical vesicle with a diameter of about 150 nm. The inner surface of each vesicle is lined by receptor-bound proteins derived from extracellular fluids. The non-staining lumen is believed to contain unbound solutes.