Coronal views of pharyngeal arch transformations

The first pouch hardly changes (A). The tympanic membrane of the ear remains as a thin barrier between outside and inside. On the inner side is the auditory tube, which is open to the throat, and on the outer side is the ear canal. The remaining clefts smooth over as a result of expansion and descent of the second arch, leaving the possibility of a trapped cyst or fistula in the connective tissue of the neck (B). The second pouch harbors a condensation of lymphatic tissue – the future tonsil. Parathyroid glands and critical immunologic tissue (the thymus gland) develop from involutions of the third and fourth pouches. From the midline of the ventral pharynx, the thyroid gland originates from the lining of the foramen cecum and descends external to the gut tube (B).