Small, enigmatic structures which originate as terminal outpocketings from each side of the embryonic pharynx. They occur only in vertebrates, where they are almost universal but difficult to homologize. They probably represent an expression of continued growth activity caudally, associated with pouch- or gill-forming potentialities of foregut entoderm. They are usually bilateral in mammals. The last (and sometimes next to last) “true” pharyngeal pouch and ultimobranchial primordium often unite to form a combined entity known as a caudal pharyngeal complex. During development in humans, the ultimobranchial bodies may be intimately related to the third, as well as the fourth, pharyngeal pouch. Here, as in most mammals, this complex would be the fourth or last.