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THE DEVELOPING ENDOCRINE GLANDS 



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Fig. 366. Thyroid, parathyroid, and thymus glands in human embryo. (A) The 

 loci of origin of thyroid, parathyroid, thymus, and ultimobranchial bodies. (B) Late 

 stage (somewhat abnormal) of thyroid, parathyroid, and thymus gland development in 

 human. (C) Early stage of thyroid follicle differentiation. (D) Later stage of thyroid 

 follicle differentiation. 



In the embryos of all vertebrates the thyroid gland appears as a pharyngeal 

 derivative. In the human as in fishes and amphibia (Lynn and Wachowski, 

 '51), it arises as a midventral outpocketing of the anterior pharyngeal floor. 

 In the human embryo, this outpocketing occurs between the first and second 

 branchial pouches at about the end of the fourth week of development (fig, 

 366A). Its point of origin is observable during later development as a small 

 indentation, the foramen caecum, in the region between the root and body 

 of the tongue (fig. 285). It is a bilobed evagination which soon loses its con- 

 nection with the pharyngeal floor and migrates caudally to the laryngeal area 

 where it differentiates into a double-lobed structure, connected by a narrow 

 bridge of thyroid tissue, the isthmus. Occasionally, a persistent thyroglossal 

 duct, connecting the foramen caecum with the thyroid gland, remains (fig. 

 366B). While the thyroid rudiment migrates posteriad, the post-branchial 

 (ultimobranchial) bodies, which take their origin from the caudal margin of 

 the fourth branchial pouch, become incorporated within the thyroid tissue. 



