Fig. 6

23-h larva. 3-D reconstructions based on SBSEM sequences. a Left side view of the posterior part of the first left somite (light pink) and part of Hatschek’s left diverticulum (light blue). Nuclei of the more anterior mesoderm cells are dark pink, and nuclei of the 48 compact mesoderm cells making up the posterior wall of the first left somite are dark blue. b Viewpoint of a with notochord orange and the first left somite light pink; the compact mesoderm cells of the posterior wall of the somite are shown in semitransparent yellow. c Similar to b, but turned to be viewed from the anterior. d View from posterior showing the compact mesoderm cells (in various colors) of the posterior wall of the first left somite; the notochord is orange, the nuclei of the endoderm cells are semitransparent purple, and the ectoderm cells (semitransparent brown) are locally compressed (asterisk) by the lateral protrusion of a few of the mesoderm cells; basal laminae are indicated by the parallel black traces. e View comparable to a, showing the nuclei of the more anterior mesoderm cells in dark pink, and the compact cells of the posterior wall in various colors; the basiepithelial basal lamina is indicated by the parallel black traces; a basal lamina-free area (the lateral window) is visible near the lower right. f The reconstruction in Fig. 5e turned to be viewed from the anterior left. g The compact mesoderm cells comprising the posterior wall of the first left somite viewed from the medial side; the subendodermal basal lamina (parallel black traces) is absent from an area (the medial window) near the bottom left. h The notochord and basal laminae (parallel black traces) of the left side from a dorso-anterior vantage point. The basal lamina ventral to the compact mesoderm cells of the first left somite is indicated by an asterisk; 1 and 2 are, respectively, the medial and lateral windows in the basal laminae, while 3 indicates the posterior communication of the first left somite with the lateral plate mesoderm. i Anterior view comparable to Fig. 5c; notochord is orange; compact mesoderm cells of the posterior wall of the first left somite in various colors, and basal lamina indicated by parallel black traces. All scale bars = 5 μm. ect, ectoderm; fls, first left somite, Hld, Hatschek’s left diverticulum, no, notochord; pl, pharyngeal lumen