Typically, five paired structures associated with the pharynx in humans that evolved from gills, with four being externally visible. Initially, each arch has identical structures. These are an internal ectodermal pouch, a core of mesenchyme, a membrane (consisting of both endoderm and ectoderm), and a external cleft (ectoderm). In humans and other vertebrates, they appear transiently and give rise to specialized structures in the face, jaw, ear and neck. The first pharyngeal arch is evident by four weeks gestation, with others appearing in a caudal direction such that all five arches are present by the end of the fourth week. The fifth arch, however, does not fully form and so the arches are numbered 1, 2,3 4, and 6. Each of these arches has a cranial nerve associated with it.
See Cranial nerves, Ectoderm, Entoderm (or endoderm), Mesenchyme, Ontogenetic adaptation, Pharynx