Fig. 3. Gross morphology of E10.5 embryos demonstrate the first and second arch to be much smaller and fused in Fgf8 mutants (arrows in B, compare with A). Confocal optical sectioning through the arches revealed the absence of pharyngeal clefts 1 and 2 (compare D with C and arrowhead in E with F), while the upper pharyngeal pouches were roughly intact (compare pouches 1 and 2 in D and C). The caudal pharyngeal arches were often absent as well (compare 3 and 4 in E with F). Pax1, which is expressed in the pharyngeal pouch endoderm at E9.5 (G,H) and E10.5 (I,J), is abnormal in Fgf8 mutants (H,J) when compared with controls. Note gap in arch 2 (arrow, H) and loss of expression (arrow, J).