Internalization of the posterior pharyngeal arches in amniotes. (A) Expression of gcm2 in zebrafish and chicks at comparable stages. In fish, this transcription factor is expressed in the pharyngeal pouches and their derivatives, the gill buds, and is required for their development. In chicks, gcm2 is also expressed in the pharyngeal pouches, which subsequently give rise to the parathyroids (modified from ). (B) Schematic of a transverse section through a human embryo, showing the second arch growing caudally over the posterior pouches (arrows). Internally, the pouches form several structures (indicated in green) derived from the endoderm. Ectodermal derivatives are indicated in dark blue. The posterior end of the pharynx is closed. (C) Later in development, the expanding second arch fuses with the body wall, enclosing the posterior arches and pouches. A sinus is formed, which is later obliterated. (B and C) adapted from Larsen .