Fig. 1.

Trunk and segment morphology of a developing zebrafish embryo. (A–C) Lateral views of a developing zebrafish embryo from a representative time-lapse movie (supplementary material Movie 1). Anterior is to the left. Arrowheads mark segments 1, 7 and 12. Insets show the entire embryo. Scale bar, 100 μm. (A) Seven cuboidal segments have formed. (B) Twelve segments have formed. Segment 1 is cuboidal, segment 7 already has a chevron shape, the most recently formed segment 12 still is cuboidal. (C) Eighteen segments have formed. Segment 12 now is chevron shaped. (D,E) Illustrations of transversal sections of the anterior trunk at the level of segment 7. [Copyright 2000 Wiley. Modified and used with permission from Stickney et al. (Stickney et al., 2000).] (D) At the time of somite formation (segment 7 as representative), the trunk is initially flattened and mediolaterally extended, with the notochord (nc) in the middle. (E) Later, when the segment has completed the rapid morphogenesis of the chevron, the trunk is dorsoventrally extended. The shaded regions in the middle of the segment indicate the position of the muscle pioneers. (F) Sketch of C. Visible segment boundaries are shown as solid lines and the poorly visible boundaries of the first four segments as dotted lines. One chevron-shaped segment is marked in grey. The notochord (dashed line) intersects the apexes of the chevrons.