ABSTRACT
Development The lingual swellings enlarge (5th and 6th weeks), and for a time the apex of the tongue is bifid. Because the myoblasts of the hyoid arches largely migrate to the head and hyoid operculum, arches II lose much of their substance (Figure lb). Hence, arches III grow forwards sub-endodermally into 2nd arch space, and fuse with the lingual swellings and tuberculum impar. The fusion line is V-shaped, pointing caudally. As the lingual swellings advance to form the tongue (7 to 8 weeks), tubercle and copula disappear. At 10 weeks, the sulcus terminalis separates the palatal part of the tongue, i.e. the anterior two thirds, from the pharyngeal posterior third (Figure lc). The foramen cecum (origin of the thyroid gland) lies just caudal to the apex of the sulcus.
