Fig. 7. Expression of lacZ in vehicle-treated and BMS453-treated embryos carrying the Rare-hsp68-lacZ transgene in a wild-type (WT; A,H) and in an Rarb-null (Rarb^–/–; I) genetic background, after 24 hours (A-D,G-I) or 30 hours (E,F) in culture. (A,B) Dorsal views of intact embryos. (C,D) Embryos cut sagittally into halves; the medial side of the right half is displayed. (E,F) Frontal histological sections through the pharynx. (G-I) Lateral views of intact embryos. Brackets (G-I) indicate the putative localization of migrating NCCs destined to the second BA. B1-B2 and M1-M2, branchial arches 1 and 2, and their mesenchyme, respectively; B1-2 and M1-2, fused first and second branchial arches and their mesenchyme, respectively; aE and pE, anterior and posterior regions of the pharyngeal endoderm, respectively; EC, ectoderm; F, frontonasal process; H, heart; O, otocyst; P1, first pharyngeal pouch; R2-R5, rhombomeres 2 to 5, respectively. Scale bar in F: 100 µm for E,F.