The majority of midline precursors divide only once during early embryogenesis. Two midline precursors are labeled with the membrane dye DiI (red). The Jupiter::GFP fusion protein (green) labels microtubules (Morin et al., 2001). One frame documenting one Z level was recorded every 3 min. The movie consists of 107 frames covering 5 hr and 18 min of development (stage 9 to late stage 11). Ventral view; anterior up. The recording started at the end of the initial midline division. The upper midline cells disappear from focus. The cells at the bottom do not divide again. Both sibling pairs developed into two cell VUM clones (see Movie S3).