Cleavage occurs despite diminution of the aster and cortical microtubules by TSA. All panels are single confocal sections. (A) 16-cell purple urchin embryo (EMTB-3G), treated with 20 µM TSA at time 00:00. A′ shows a 2× enlarged view of the indicated cell (white mark in A). Within minutes of TSA addition in metaphase, asters were reduced to nearly nothing (04:00). Despite reduction in spindle length, cells enter anaphase (07:20) and complete cytokinesis (11:20–16:10). No microtubules connecting the mitotic apparatus to the cortex are visible (see Video 4). (B) 16-cell purple urchin embryo (cyan, EMTB-3G; yellow, mC-H2B); 15 µM TSA was added at time 00:00. After a delay in metaphase, five of six cells in focus initiate cytokinesis. B′ shows a 2× enlarged view of the indicated cell (white mark in B). (C) One cell within a 16-cell sand dollar embryo (EMTB-3G), treated with 25 µM TSA ∼10 min before time 00:00. Although the aster regrows slightly, the cortex is ∼20 µm away from the nearest spindle microtubule ends; nevertheless, cytokinesis completes accurately (see Video 4). Arrowheads in A and C indicate metaphase plate before (04:00 and 04:48), and the gap after (07:20 and 10:48), anaphase onset. Time is indicated in minutes:seconds.