The thyroid gland appears during the fourth week as a median endodermal thickenning in the floor of the pharynx, between the tuberculum impar and copula. The thyroid gland descends in front of the pharyngeal gut, hyoid bone and laryngeal cartilages and reaches its final position in front of the trachea in the 7th week. During migration, the gland remains connected to the tongue by the thyroglossal duct, which opens in the tongue at the point of the foramen cecum. The thyroid follicular epithelial cells derive from the endoderm, while the connective tissue and blood vessels derive from splanchnic mesoderm. The ultimobranchial bodies fuse with the thyroid and become infiltrated with the cells derived from neural crest. These cells give rise to parafollicular cells of the thyroid.