IntroFIG 7: Embryonic stages of development.

Numbers below the horizontal axis show approximate time in minutes after fertilization at 20-22^oC. First cleavage occurs approximately 40 minutes after fertilization. (Yellow-green bars) Period of time during which cells from a certain lineage migrate towards inside of the embryo. The first cells that move inwards from the ventral surface are gut precursors (E), followed by mesoderm (MS) and germline (P4) precursors. (Blue bar) Gastrulation. Gastrulation cleft is closed by short-range movement of ectodermal cells (neuroblasts, postmitotic neurons, glia and glia precursors) bewteen 270 and 330 minutes (Chin-Sang and Chisholm, 2000). (Red bar) Elongation of the embryo that occurs between 400 and 640 minutes due to circumferential contraction within the hypodermins. During elongation, the embryo becomes threefold thinner and its length increases about fourfold. Sexual dimorphism becomes visible for the first time at 510 min when the cephalic companion neurons (CEM) die in the hermaphrodite, whereas the hermaphrodite-specific neurons (HSN) die in the male. The stages, number of nuclei, marker events and DIC images of the embryos and a newly hatched larva are shown above the horizontal axis. (Based on von Ehrenstein and Schierenberg, 1980; Sulston et al., 1983; Wood, 1988b; Bucher and Seydoux, 1994; Chin-Sang and Chisholm, 2000.)

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