Most midline cells divide 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 minute. The movie consists of 240 frames covering 4 hr of development (stage 9 to late stage 11). Ventral view; anterior up. After approximately 4 hr, the upper midline cells divide again, whereas the cells at the bottom do not divide a second time. The upper cells differentiate into four VUM neurons, and the bottom cells develop into two midline glia (data not shown). We recorded the division of midline precursors in six embryos and never detected additional divisions before late stage 11.