This time-lapse confocal movie shows vesicles exocytosing onto the surface of a cytochalasin-B-treated Xenopus embryo (same individual as shown in Figs 6 and 7). Vesicles are about 2 mm in diameter, and appear as small spheroids grouped in clusters just under the surface. Frames were 1.8 seconds apart, and so some of these vesicles apparently were open as stable fusion pores for as much as 150 seconds before completing exocytosis. As individual vesicles flatten during exocytosis, they each abruptly add about 20 mm² of unlabeled membrane to the surface, as indicated by the dark irregular patches quickly diffusing into the surrounding labeled membrane. Notice many of the exocytotic events are clustered (particularly near the top-left corner), even though the surface itself flows rapidly past the field in view.