The                   is bounded laterally by pharyngeal arhes.

Each arch consists of a core of mesenchyme covered externally by ectoderm and internally by endoderm.

The original mesenchyme of each arch is derived from                .

Later,                    migrate into the arches and are the major source of the connective tissue components, including               ,            , and                  in the oral and facial regions.

Each arch contains an            ,                    ,            , and a                    .

The                        disappear except for the first pair, which persists as the external acoustic meatus. The                        also disappear, except for the first pair, which becomes the tympanic membranes. The                        forms the tympanic cavity, mastoid antrum, and pharyngotympanic tube.

The                        is associated with the development of the palatine tonsil.

Externally,                        are separated by pharyngeal grooves.

Internally, the arches are separated by                    of the pharynx (                      ).

Where the ectoderm of a groove contacts the endoderm of a pouch,                        are formed.

The thymus is derived from the          pair of                 , and the parathyroid glands are formed from the           and           pairs of pouches.

The                 develops from a down growth from the floor of the primordial pharynx in the region where the tongue develops.

The parafollicular cells(C cell) in the thyroid gland are derived from the                       , which are derived mainly from the fourth pair of pharyngeal pouches.

Cervical cysts, sinuses, and fistulas may develop from parts of the second pharyngeal groove, the                                  , or the second pharyngeal pouch that fail to obliterate.

An                       results when the gland fails to descend completely from its site of origin in the tongue.

The                  may persist, or remnants of it may form                   and ectopic thyroid tissue masses.

Infected cysts may perforate the skin and form                                      that open anteriorly in the              plane of the neck.

                       is a common birth defect.

Although frequently associated with cleft palate, cleft lip and cleft palate are etiologically distinct defects that involve different developmental processes occurring at different times.

Cleft of the upper lip results from failure of mesenchymal masses in the                     and                                         to merge, whereas cleft palate results from failure of mesenchymal masses in the                     to meet and fuse.

Most cases of cleft lip, with or without cleft palate, are caused by a combination of genetic and environmental factors　(multifactorial inheritance; see Chapter 20)

Cleft of the upper lip results from failure of mesenchymal masses in the                       and                             to merge, whereas cleft palate results from failure of                              in the palatal processes to meet and fuse.