Fig. 9. Intermediate stages of notochord formation in embryos lacking the posterior neural plate. Posterior is left in all top and side views. Dorsal is up in all cross sections. (A-C) Approximately 1 hour PFD. (A) Top view. (B) Side view. (C) Cross section through the AP midline. Note that the anterior and lateral portions of the notochord remain a monolayer plate, but one that is oriented along the dorsal-ventral axis. The lateral epidermis and anterior endoderm now bound the basal surface of the notochord instead of the neural plate. (D-G) Approximately 2 hours PFD. (D) Dorsal top view. (E) Ventral top view. (F) Lateral section through one lateral arm of the notochord rudiment. (G) Cross section through the AP midline. Note the fusion of the two lateral notochord arms posteriorly (D) and the gradual posterior extension of the dorsal anterior boundary of the notochord plate (arrow in D) which is matched by a posteriorwards extension of the overlying epidermis (not shown). Note also the elongation of individual notochord cells perpendicular to the AP axis within the dorsal plate (D) and its lateral arms (F), and the constriction of cell apices within the lateral arms (arrows in G). (H-J) Approximately 3 hours PFD. (H) Top view. (I) Posterior cross section. (J) Anterior cross section. By this stage, the formation of a cylindrical intermediate is nearly complete. The organization of posterior muscle is essentially normal and the epidermis now bounds the notochord rod dorsally along its entire extent. Scale bar, 10 µm.