Fig. 4. SEM views of Boltenia villosa embryos fractured to reveal the apical (ventral) surface of the notochord plate and the ventral folds at: (A) approx. 16 hours AF; (B) approx. 17 hours AF and (C) approx. 18 hours AF. Folds are present before cell rearrangement and notochord extension occurs (arrows in A), and then move towards the midline, meeting first at the posterior (asterisk in B) where they begin to fuse. Fusion then progresses posterior to anterior (fusion ‘front’ indicated by arrow in C). Posterior is top and left in all panels. Scale bar, 10 µm.
