The embryology of the thyroid and parathyroid glands under- lies the anatomical position, anatomical variations and con- genital conditions of these structures and is therefore vital for surgery (Figure 50.1). The thyroglossal duct develops from the median bud of the pharynx. The foramen caecum at the junction of the anterior two-thirds and posterior one-third of the tongue is the vestigial remnant of the duct. This initially hollow structure migrates caudally and passes in close con- tinuity with, and sometimes through, the developing hyoid cartilage. The parathyroid glands develop from the third and fourth pharyngeal pouches. The thymus also develops from the third pouch. As it descends, the thymus takes the asso- ciated parathyroid gland with it, which explains why the inferior parathyroid, which arises from the third pharyngeal pouch, normally lies inferior to the superior gland. However, the inferior parathyroid may be found anywhere along this line of descent (see also Chapter 51). The developing thyroid lobes amalgamate with the structures that arise in the fourth pharyngeal pouch, i.e. the superior parathyroid gland and the ultimobranchial body. Parafollicular cells (C cells) from the neural crest reach the thyroid via the ultimobranchial body.