The cortex reacts differently to the proximity of stable vs. dynamic microtubules. S. purpuratus zygotes: (A–C and G–I) Projections of 71 serial confocal sections (0.5 μm apart) revealing surface views of pRLC (white). Immediately below each surface projection are medial sectional views of the same cell (D–F and J–L) showing microtubules (white) and pRLC (lavender). (A–C) Telophase zygotes after 0, 4, and 8 min in ncdz (12°C) from the same preparations as Fig. S3, Q, S, and U, respectively . (G) An anaphase zygote, developing normally without ncdz, fixed just before elongating microtubules contacted the equator. (H and I) Late anaphase zygotes cultured in 1 mM colchicine. The same confocal microscope settings were used throughout for pRLC, but adjusted to optimize microtubule images. Bars, 25 μm; Surface projections are shown at 1.3× higher magnification than midsection micrographs.