The position of the spindle poles during anaphase defines a local minimum in the density of astral microtubules that reach the cell cortex at the future cleavage site. (A and B) Density of microtubules near the cell cortex after anaphase onset. (A) Stills from a time-lapse recording of embryos expressing tubulin-YFP. (B) The density of the microtubules (MT) along the cell boundary from the anterior (A) tip (0 as the relative position) to the posterior (P) tip (1 as the relative position) in the indicated time window (seconds after anaphase onset). The tubulin/microtubule signals were measured along a line of 1 µm width placed 1.5 µm inside the cell boundary. The intensity above the local background defined by a threshold that corresponds to the top 5% level within a local spatial window (0.2 relative length) was scored as the density of microtubules (see Materials and methods and Fig. S2 for more details) and averaged across the embryos (n = 9, mean ± SEM). (C and D) Distance from the cell cortex to the nearest spindle pole. (C) The positions of the spindle poles (yellow arrowheads) and the chromosomes (magenta arrows) can be determined in the myosin II-GFP videos. (D) The distance from each point on the cell periphery to the nearest spindle pole was measured and presented with color coding (left) and by line plotting (right). The equatorial maximum became prominent after ∼40 s and went above a virtual level that linearly links the anterior and posterior tips (gray dotted line). Green arrows indicate the positions of the local minima in the microtubule density (A and B) or the local maxima in the distance to the nearby pole (A and D).