(A) At developmental stage 30 (based on Wise et al., 2009, that is E6-7 in Chlamydosaurus) the BA2, BA3, and BA4 are situated behind the hearth cavity and (B) the BA6 is only visible on parasagittal sections. Inset of B: schematic diagram of branchial arches organisation in land vertebrate embryos. (C,D) At stages 30–31 (about E11), the BA2 is still externally visible while the more posterior branchial arches are internalised. (E,F) At stage 32, (i.e., about E15-16), a portion of BA2 (arrowhead) does not fuse with the cardiac eminence. (G,H,I) At later developmental stages (stages 32–33, 34, and 36, that is about E18, E24, and E29, respectively), the unfused portion of BA2 (arrowheads) intensively grows and generates the frill. ‘BA1’ to ‘BA6'=branchial arches 1 to 6; ‘heart’=heart cavity; 'max'=maxillary process; ‘ov’=otic vesicle. Note that the first branchial arch (BA1) generates the mandibular process (‘mand’). Scale bars = 500 μm.