File:Gray1175.jpg

Scheme showing development of branchial epithelial bodies

(Modified from Koh.) I, II, III, IV. Branchial pouches.

Development.—The thyroid gland is developed from a median diverticulum (Fig. 1175), which appears about the fourth week on the summit of the tuberculum impar, but later is found in the furrow immediately behind the tuberculum (Fig. 979). It grows downward and backward as a tubular duct, which bifurcates and subsequently subdivides into a series of cellular cords, from which the isthmus and lateral lobes of the thyroid gland are developed. The ultimo-branchial bodies from the fifth pharyngeal pouches are enveloped by the lateral lobes of the thyroid gland; they undergo atrophy and do not form true thyroid tissue. The connection of the diverticulum with the pharynx is termed the thyroglossal duct; its continuity is subsequently interrupted, and it undergoes degeneration, its upper end being represented by the foramen cecum of the tongue, and its lower by the pyramidal lobe of the thyroid gland.