Early development of Temnopleurus reevesii from primary mesenchyme cell ingression to cell mass formation. Micrographs show the initial left-right asymmetry of the primary pore canal (PPC) elongation and the initiation of adult rudiment formation from the cell mass (CM). (A) A blastula 9 h after fertilization (lateral view). (B) An early-gastrula 12 h after fertilization (lateral view). (C) A mid-gastrula 15 h after fertilization (lateral view). (D) A late-gastrula with a pair of coelomic sacs 18 h after fertilization (lateral view). (E) A late-gastrula 20 h after fertilization (presumptive oral view). A PPC is elongated asymmetrically towards the ectoderm from each coelomic sac (arrowheads show hydropores): the left PPC elongated to the left lateral side and the right PPC elongated more dorsally. (F) A prism embryo 20 h after fertilization (oral view). PPCs are symmetrically elongated towards the ectoderm. (G,H) Two-armed larvae 27 h after fertilization: oral (G) and aboral (H) views. A part of the ectoderm between the left post-oral arm and the left oral lobe has started to invaginate (G, arrow) and then pinched off (H, arrow). (I,J) Two-armed larvae with a CM 30.6 h after fertilization (oral view). The right PPC has started to degenerate (I) and then disappeared (J). (K,L) Four-armed larvae 4 days after fertilization: dorsal (K) and ventral (L) views. Only the left PPC is visible (K). The body rods possess barbs and the post-oral rods are fenestrated (L). Scale bars = 50 μm.