Furrow ingression and cytoplasmic isolation occur with normal timing in the absence of midbody microtubules. (A) Graph plotting mean furrow diameter versus time, for the first division of control (reproduced from Fig. 1 A for comparison) and PRC1^spd-1(RNAi) embryos (n = 10 embryos for each condition). Error bars are the SDs. Arrow denotes the last time point with a measurable opening (apparent closure). (B and C) Central plane confocal images of control and PRC1^spd-1(RNAi) embryos expressing Myosin II^NMY-2–GFP along with the mCherry-tagged plasma membrane probe (B, n = 5 for each condition) or GFP-septin^UNC-59 along with the mCherry-tagged plasma membrane probe and mCherry-histone (C; n = 5 for each condition). Times are seconds after furrow initiation. (D) Outline of the method used to compare cytoplasmic isolation kinetics in control and PRC1^spd-1(RNAi) embryos. Embryos expressing the GFP-tagged plasma membrane probe loaded with 10-kD caged carboxy-Q-rhodamine–labeled dextran were photoactivated on one side at different time points after furrow initiation, and images were collected at 5-s intervals to monitor probe diffusion. Kymographs are shown for embryos photoactivated early in cytokinesis, midcytokinesis, and at closure (the early and closure kymographs are reproduced from Fig. 1 B). Red arrows denote the point of photoactivation. The NID between the activated (A) and unactivated (U) halves of the embryo, calculated as shown, was plotted versus time, and the initial slope of the intensity difference, which reflects the rate of diffusion across the division plane, was calculated. (E) Graph plotting the mean initial slope of the NID versus time in seconds after furrow initiation for control and PRC1^spd-1(RNAi) embryos. Error bars are the 90% confidence interval; mean n = 10 slope measurements per time point. White boxes on the low magnification images in B and C mark the location of the region shown at higher magnification in the images at the bottom. Bars, 5 µm.