Syncytial nuclear separation does not scale with available space. (A) Schematic of spatially confining nuclei and spindles in microchambers. (B) Image sequence of a dividing nucleus (cycles 7–8) inside a microchamber. The deforming central spindle (arrowheads) suggests unaltered separation activity. In the subsequent interphase, the distance between daughter nuclei is abnormally short, leading to spindle fusion during next mitosis. Time is shown in minutes/seconds. Bar, 5 µm. (C) Distance–time plot of DNA masses during chromosome and nuclear separation in a microchamber (black) compared with unconfined extract (white line on gray background; mean ± SD; from Fig. 1 D). The solid and dotted lines are the mean distance and SD, respectively, of five experiments. (D) Images of dividing nuclei at the cortex of an intact cycle 11 (blastoderm) embryo (top) and of nuclear divisions in unconstrained early (preblastoderm) embryo extract at cycle 7 (bottom). Time is shown in minutes/seconds. Bars, 5 µm. (E) Distance–time plot of daughter DNA masses during nuclear divisions in intact blastoderm embryos (blue, division 10; orange, division 11; see D). Solid and dotted lines are the mean and SD, respectively, of 15 independent extract experiments (gray) and 5 (blue) and 10 (orange) separation measurements in two intact embryos. nc, nuclear cycle.