Derivatives of the third and fourth pharyngeal arches
The outwardly visible 3rd and 4th pharyngeal arches are never as effectual as the 1st and 2nd ones. Normally the hyoid arch (2nd pharyngeal arch) grows over the 3rd and 4th ones (operculum).
The thymus arises from the 3rd and 4th pharyngeal pouches that are found on both sides at the base of the cervical sinus.
The endodermal portions of the thymus anlagen grow on both sides as ventrally- oriented sacculations of the pharyngeal pouches in the neck mesenchyma (for comparison, the parathyroid arises from dorsal solidifications). In the segmentation of the pharyngeal pouches, the neighboring ectoderm of the cervical sinus is also incorporated and surrounds the endodermal epithelial cords. Through fusion of the two anlagen of the 3rd and 4th pharyngeal pouches and the derivative of the cervical sinus the thymus arises as a bilobular organ in the anterior upper mediastinum.
Parathyroid glands and ultimopharyngeal bodies
The parathyroid gland has its origin initially in one of the two dorsal thickenings of the epithelium of the 3rd and 4th pharyngeal pouches. Soon afterwards arise the solid epithelium cords, the epithelial bodies, which with the descent of the heart are shifted caudally.
The anlagen from the 3rd pharyngeal pouches (larger portion of the thymus and inferior parathyroid gland) grow faster and get shifted further caudally than those of the 4th pharyngeal pouches, (smaller portion of the thymus and superior parathyroid gland)]. Thus the superior parathyroid gland gets only to the upper pole of the median-lying thyroid (stage 17, ca. 41 days), while the inferior parathyroid glands of the 3rd pharyngeal pouches reach its lower pole (stage 19, ca. 46 days).
The ultimopharyngeal body arises from bilateral ventral swellings of the 4th pharyngeal pouches and gets integrated into the thyroid where it forms the parafollicular cells or C-cells for the production of calcitonin.