Fig. 1. Schematic depiction of relevant stages of Helobdella development. (A-E) Lateral views with animal pole at top and anterior to left; (F) dorsal view. (A) Stage 4b (10 hours AZD); cells DNOPQ and DM arise from macromere D' at third cleavage and give rise to the teloblasts that generate segmental mesoderm and ectoderm. Macromeres A''', B''' and C''' are endodermal precursor cells. Micromeres are evident at the animal pole. (B) Early stage 8 (61 hours AZD); DM and DNOPQ have cleaved to generate the full complement of 10 teloblasts (circles, only 8 are shown in the drawing) plus additional micromeres. Each teloblast produces a column of segmental founder cells (blast cells); ipsilateral bandlets merge, forming left and right germinal bands (gb; shaded; only left gb is visible in this lateral view), which are covered by micromere-derived epithelium stretching over the animal pole (net pattern). As blast cells are added to their posterior (pos) ends, the germinal bands elongate and move ventrovegetally (arrows) and coalesce from anterior (ant) to posterior (pos) along the ventral midline, forming the germinal plate (gp), accompanied by the expansion of the micromere-derived epithelium. (C) Late stage 8 (94 hours AZD); the completed germinal plate (shaded) extends from anterior to posterior, defining the ventral territory of the embryo. (D) Mid stage 10 (155 hours AZD); during stage 10, segmental tissues arise from the proliferation and differentiation of cells within the germinal plate (shaded), the edges of which gradually expand dorsally and meet at the dorsal midline, closing the definitive body tube. Macromeres, teloblasts and supernumerary blast cells have fused into a syncytial yolk cell (hatched) that constitutes the midgut; foregut arises largely from micromere progeny (the micromere-derived epithelium is omitted for clarity). (E) Early stage 11 (172 hours AZD). By this stage, the foregut has generated distinct proboscis and esophagus, while the midgut has given rise to crop, intestine and rectum; segmental tissues are well-differentiated, including ganglia within the ventral nerve cord (vnc). (F) Late stage 11 (195 hours AZD); crop caeca are well-differentiated [adapted from Kang et al. (Kang et al., 2002)].

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