(A–G) In situ hybridization with dlx-2, a gene which is expressed in neural crest cells, nuclei in the forebrain, and pectoral fins. At 18 somites dlx-2 is expressed in three neural crest streams (A, s1, s2, s3, dorsal view). In 33-hpf wild-type larvae (B, lateral view; D, dorsal view) neural crest cells have populated the arches, which are separated from each other by the endodermal pouches (nonstaining tissue between the arches). Arches p5–p7 have not been divided by the endoderm yet. At this stage the endodermal pouches consist of two rows of cells which have not formed a cavity yet. In vgo (C) only the first endodermal pouch is present. In the absence of posterior endodermal pouches the neural crest cells of the posterior pharyngeal arches fuse (C, E). (F and G) Dorsal views of dlx-2 expression in 28-hpf wild-type and oep mutant embryos, respectively. (H) dlx-2 expression pattern in endodermal-deficient cas mutants. No distinct pharyngeal arches form and the neural crest cells migrate to ectopic locations (arrows). In oep (G) and cas (H), neural crest cells of the anterior arches fuse with each other and expression is reduced in the posterior arches. (I, J) Alcian blue-stained cartilage preparations of 3-dpf wt (I) and cas (J) larvae. In cas larvae the cranial cartilages are severely reduced. The only elements present are the anterior parts of the neurocranium (reduced trabeculae (tr) and ethmoid plate (etp)) and remnants of the parachordals (pc) or possibly two pharyngeal cartilages of unknown identity. Abbreviations: s1–s3, neural crest streams 1–3; p1–p7, pharyngeal arches 1–7.
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