Fig. 1. Summary of Helobdella development. (A) Timeline (not to scale) showing selected developmental stages; all views are from the animal pole (prospective dorsal) unless indicated otherwise. Times given indicate the approximate age in hours after zygote deposition at 23°C. tb, teloblast; gb, germinal band; gp, germinal plate. (B) The formation of segmental ectoderm, focussing on the N lineage. Each N teloblast undergoes stem cell divisions to produce primary ns (red) and nf (dark blue) blast cells in exact alternation. The timing of subsequent events in each n blast cell clone is given in terms of the time elapsed since the birth of the primary blast cell (hours clonal age; timeline not to scale). Ipsilateral columns (bandlets) of primary blast cells merge to form germinal bands, which coalesce in anteroposterior progression during stage 8 into the germinal plate, from which segments arise (see A). The ns and nf blast cells undergo distinct and stereotyped lineages, beginning with unequal, obliquely anteroposterior divisions producing progeny called ns.a (red), ns.p (pink), nf.a (dark blue) and nf.p (light blue) at 28 and 26 hours clonal age, respectively. During subsequent development, these subclones generate approx. two-thirds of the 200 identified neurons and glia in each hemiganglion, plus three peripheral neurons (nz1-3) and a few epidermal cells (not shown) (Bissen and Weisblat, 1987; Bissen and Weisblat, 1989; Kramer and Weisblat, 1985; Ramirez et al., 1995; Shain et al., 1998; Shain et al., 2000; Weisblat et al., 1984; Zackson, 1984).