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Fig. 8 Pouch–cleft contacts and tooth formation. (A) Diagrammatic ventral view of dissected branchial region of an ‘idealised' primitive craniate. c, coelom; cl, cleft; n, notochord; p, pouch [after Bjerring, 1977, with permission from the author and publisher]. (B) Danio rerio; sagittal section showing the open pouches (gill slits) in a 96 h post-fertilisation (hpf) specimen (arrowheads); pharyngeal teeth (arrow) are associated with the last pouch (pouch 6, double arrowhead). Scale bar = 100 μm. (C) Danio rerio; cross section at the level of the forming pharyngeal dentition in a 50 hpf specimen. Note connection of the tooth-forming epithelium (arrowhead) with that lining pouch 6 (P6). Dotted line follows basal lamina, delimiting the pharyngeal epithelium. Scale bar = 10 μm. (D) Latimeria chalumnae; teeth formed in the spiracular pouch (sp); based on a photograph of a transverse section at the level of the intracranial articulation (modified after Millot & Anthony, 1958<, with permission from the publisher). (E) Oryzias latipes; transmission electron microscopy (TEM) picture of a cross section of the pharyngeal epithelium in a 7 days post-fertilisation (dpf) embryo, showing that a flattened cell layer (arrowhead) covers the hatching gland cells (hg) and basal endodermal layer (asterisk) of the pharyngeal epithelium. Scale bar = 5 μm. (F) Danio rerio; sagittal section through the mouth entrance of a 55 hpf Tg(krt4:tomatoCAAX;sox17:egfp) embryo, periderm (red) has partially invaded the mouth (boundary indicated by arrowheads); endoderm is green. Scale bar = 50 μm.