Nocodazole-insensitive microtubules exhibit no orientation bias in urchin blastomeres. (A–C) Single sections, all expressing EMTB-3G; (A′–C′) 2× enlarged views of cells indicated by the white marks in A–C. (A) Eight-cell purple urchin embryo; 20 µM nocodazole was added at time 00:00. Within 2 min, most astral microtubules disassembled; remaining microtubules are randomly oriented and most end well short of the cortex. Even so, furrowing is accurate and timely. Arrowheads indicate surviving nonequatorial astral microtubules. (B) 16-cell purple urchin embryo; 10 µM nocodazole was added at time 00:00. The top left cell entered anaphase around time 0; none of the other cells in view left metaphase nor furrowed. This one cell cleaved despite nearly complete absence of astral microtubules, and the furrow crossed the spindle midzone. (C) 16-cell sand dollar embryo; 10 µM nocodazole was added at time 00:00, at which time three of four macromeres have entered anaphase; the east cell enters anaphase shortly thereafter (03:00). In all cells, numerous astral microtubules, pointing in all directions, persist >5 min after nocodazole addition (arrowheads). Stable microtubules may approach the cortex in west and south cells (03:00), but none can be seen to do so in north and east cells; all commence furrowing at a normal time and complete cytokinesis. Note that in all cases, stable microtubules become brighter because EMTB-3G liberated by disassembly becomes available for binding. Video 3 includes A–C. Time is indicated in minutes:seconds.