Fig. 1. Postembryonic development of the ventral nerve cord in C. elegans. (A) X: the positions of the ten cells that undergo programmed cell death. Filled circles indicate the six cells in the midbody that survive and differentiate to become VC motoneurons. Anterior is leftwards and ventral is downwards. (B) The postembryonic cell lineages are shown schematically, with vertical lines representing time and horizontal lines representing a cell division along the anteroposterior body axis. Each P cell divides to produce an anterior daughter, Pn.a, which is a neuroblast, and a posterior daughter, Pn.p, which is hypodermal. W, a neuroblast in the retrovesicular ganglion, follows a division pattern identical to that of Pn.a cells. (C) The nomenclature used to identify cells in the P cell lineages. Cell lineages are from Sulston and Horvitz (Sulston and Horvitz, 1977).