Surgery for thyroid or parathyroid glands has to be logical. It is based upon a precise knowledge of the development of the 3rd and 4th pharyngeal pouches and aortic arches. Pharyngeal pouches are endodermal derivatives of the lateral walls of the early foregut. Parathyroid glands have an ectodermal origin in concert with inductive forces of cells derived from the neural crest (which also creates parafollicular cells secreting calcitonin). From the third pouch thymus and parathyroid III move down together because of the cervical flexure and heart's "descent" (stages 14 to 20 mm). The fourth pouch gives rise to the so called "caudal pharyngeal complex" constituted by parathyroid IV dorsally, a "lateral thyroid" ventrally and an ultimo-branchial body derived from the fifth pouch. The thyroid gland first appears at the 4th week as a thickening of the endoderm of the floor of the pharynx which evaginates as a diverticulum in front of the trachea below. This medial component of the thyroid joins both lateral thyroids derived from the caudal pharyngeal complexes to form a bi-lobed structure with an isthmus. The lateral edge of the adult thyroid lobe has a thickening which is called Zuckerkandl's tuberculum because of the fusion of the ultimobranchial body into the principal médial thyroid process. This tuberculum lies in front of the caudal thyroid artery born from the 4th aortic arch. The recurrent laryngeal nerve courses round the 4th aortic arch: therefore parathyroids IV have to be above and behind the nerve and parathyroids III below and in front of the nerve.(ABSTRACT TRUNCATED AT 250 WORDS)