Asters alone induce furrows only if they are far enough apart. (A–C) Single-plane sequences of anucleate, centrosome-containing cytoplasts (sand dollars). (A) Anucleate cytoplast with four centrosomes, two in focus. In normal cells this size, furrowing would begin between 03:00 and 07:00, but no furrow occurs even 20 min later. Centrosomes are closer than the distance to the equator. (B) Anucleate cytoplast with two centrosomes, farther apart than the distance to the equator. A furrow initiates at the expected time and place, and proceeds to completion. (C) An anucleate cytoplast with four centrosomes variously spaced. A deeply ingressing furrow bisects the well-spaced centrosomes; shallow furrows form over closely spaced centrosomes. Time is indicated in minutes:seconds. (D) Long-term differential interference contrast sequence of two halves of a bisected sand dollar zygote. The film began shortly after the unbisected zygote would have divided; because each half has only one centrosome, neither divided at first mitosis. All subsequent divisions proceeded normally in the nucleated half (right), creating a perfect blastula. The anucleate half cleaved abortively in its first attempt. After that, four centrosomes (dots in the 1:51:40 frame) were variously spaced. The second attempt made deep furrows between the widely separated asters, but none between the closely spaced pair. Subsequent divisions were variously successful. Time is indicated in hours:minutes:seconds. (E) Aster separation strongly correlates to the furrow extent. See Materials and methods for measurement. Video 6 corresponds to A–C.