pRLC elevation at the equatorial cortex correlates with proximity to ncdz-resistant astral microtubule tips. Medial single-section confocal images of S. purpuratus zygotes, from one female, synchronously fertilized and allowed to develop to the stage indicated, then fixed and stained for pRLC (lavender) and tubulin (white). In each two-cell panel, the bottom cell was treated with 20 μM ncdz for 5 min before fixation (11°C), whereas the top cell was fixed without drug treatment. Arrows indicate the ncdz-resistant astral microtubules that appear during anaphase and elongate; pRLC accumulates to high levels near their tips (G). Cyan arrowheads mark bundled kinetochore microtubules (C–E), red arrowheads indicate interpolar microtubules (E–G and J), and green arrowheads indicate reforming telophase nuclei (H–K). Identical microscope settings were used throughout for pRLC. Laser intensity was adjusted to ensure visibility of microtubule tips, and higher laser intensity was used for the ncdz-treated cells (thus, e.g., stable kinetochore microtubules, represented by cyan arrowheads in C, appear brighter in ncdz-treated than in control cells). Bar, 20 μm.