Fig. 6. Binucleate cells cleave from one to four. (A) A sequence of DIC images in which AB and P0 were fused in interphase. We fired several pulses from the UV laser at the cell-cell interface (`x' in frame 1). The hole rapidly widens as cytoplasm flows through; in frame 2 the arrowhead marks the edge of a curtain of withdrawing membrane. In this case, the nuclei remained separate during interphase (frame 3) and two independent spindles, oriented in parallel, developed at metaphase (frame 4). Furrows ingressed between each pair of spindle poles regardless of whether they shared a spindle (arrowheads in frame 5 indicate furrows; the fourth furrow, on the anterior, appeared later; `r' for Rappaport indicates furrows that apparently bisect unconnected spindle poles). (B) Identical case to (A) but conducted in an embryo expressing both GFP-tubulin and GFP-myosin (supplementary material Movie 2). In frame 1 `x' indicates the fusion site; arrowheads in frame 2 indicate margins of the spreading hole. Nuclear envelope breakdown and spindle assembly occur sooner in the anterior nucleus (frame 3); two mitotic spindles form in parallel with no apparent connection between them (frames 4 and 5). Furrows initiate between each pair of spindle poles (arrowheads in frame 6). All furrows ingress deeply, meet near the center, and complete (frames 7-9), creating a four-cell embryo with an abnormal arrangement of cells. Bars, 10 µm.