ABSTRACT
Pharyngeal arches are paired bars of mesoderm in the side walls of the embryonic pharynx, numbered I, II, III, IV, and VI in the human embryo. Pharyngeal septa, comprising ectodermal pharyngeal clefts and endodermal pharyngeal pouches separate the arches (Figure 2). Pharyngeal arches are no longer respiratory in function. The full ancestral complement of seven pairs of branchial (gill) arches is found in modern elasmobranchs, such as the shark and dogfish (Figure 1). In the fossil ostrachoderms of the Silurian period there are two arches cranial to arch I of modern vertebrates i.e., pharyngeal arch I of human anatomy would be arch III in ancestral vertebrates living 250,000,000 years ago.
