The furrow forms in a microtubule-poor region. (A) Single superficial confocal sections of a 16-cell sand dollar embryo, slightly compressed (EMTB-3G). (02:00) All three cells in metaphase; few microtubule ends are visible. (07:00) Left cell begins cleavage, right cell is still in metaphase, and the top cell has likely entered anaphase. (13:00) Right cell begins to cleave; fewer microtubules approach the cell surface (bright dots) in the incipient furrow than outside it. (A′) “Kymocubes” made by 3D rendering sequence in A; white lines denote frames in A. The whole of each of three cells is shown in the kymocube. Bright dots (open arrowheads in A–C) indicate brief cortical visits by microtubule ends; these predominate in metaphase. Vertical streaks (closed arrowheads in A–C) indicate kinematically stable microtubule ends, which appear before furrowing, and are scarce in the equatorial zone. (B) Kymograph of 5-µm medial strip (indicated by box) ∼1 µm beneath the surface of a slightly flattened eight-cell sand dollar embryo. The arrow indicates furrow initiation time; vertical streaks above this point indicate stable microtubules at the cortex before furrowing. These are notably fewer in the equator. (C) Surface rendering of single medial sections of uncompressed sand dollar blastomeres, each covering anaphase through cytokinesis. Long streaks (stable microtubules at the cell surface) are evident well before furrowing. Broad smears appear during furrowing, notably in the area around the crotch; these reflect microtubules growing along the cortex (see also A′ and B). (D) Single superficial sections from a 6.5-h X. laevis embryo mosaically expressing GFP-rGBD (red) and 3C-EMTB (cyan). Surface microtubules disappear in metaphase, then reappear (16:52) ∼90 s before active Rho appears in the equator (18:28). Microtubules are largely absent from a >20-µm-wide band inhabited by the Rho zone; arrows indicate equatorial microtubules that disappear during furrowing. Time is indicated in minutes:seconds.