THE ENDOCRINAL ORGANS 



457 



level of the first visceral pouch. For a while the anlage of the thyroid 

 retains connexion with the floor of the pharynx by means of the thyro- 

 glossal duct. When, at a considerably later stage of the development, 

 the tongue forms, the point of connexion of the thyroglossal duct is indi- 

 cated by a pit, the foramen cecum, near its posterior border. The thyro- 

 glossal duct persists for some time in the embryo, and becomes gradually 

 elongated as the gland anlage grows backward to take its definitive 

 position on the sides of the trachea. Eventually, however, connexion 

 with the tongue is lost. The median lobe of the gland, if present, develops 

 from the thyroglossal duct, and the accessory thyroids frequently come 

 from the same source. The lateral lobes increase in size more rapidly 

 than the median portion which persists as the isthmus. (Fig. 380) 



aaaauatffi ^^B^S^^i^^ 



Fig. 380. — Longitudinal section of head of 19-day Petromyzon embryo, ch, 

 optic chiasma; ep, epiphysial outgrowth; h, hypophysial ingrowth; mes, mesenteron; n, 

 nasal epithelium; nc, notochord; oc, oral cavity; op, oral plate; sc, canal of spinal cord; 

 th, thyroid. (From Kingsley's "Comparative Anatomy of Vertebrates.") 



Evolution. The thyroid gland is a peculiarity of chordates, and is 

 found in all classes of this phylum. Since no homologous structure is 

 known in non-chordates, we must assume that the gland, like the noto- 

 chord, emerged with the phylum from unknown non-chordate ancestors. 

 In the hemichordates, the only possible homologue of the thyroid is a 

 groove apparently functionless in the floor of the pharynx. The uro- 

 chordates and cephalochordates have a ciliated groove, the endostyle, 

 in the floor of the pharynx. This groove is morphologically, if not 

 physiologically, comparable with the thyroid. 



In Amphioxus, a typical cephalochordate, the endostyle is lined by 

 columnar epithelial cells of two sorts, glandular and ciliated. Four rows 

 of gland cells alternate with rows of ciliated cells which extend the entire 

 length of the pharyngeal floor. Particles of food caught up in the mucus 

 are swept forward towards the mouth, and are carried by a ciliated cir- 

 cumpharyngeal groove to a median epibranchial groove, which carries 

 them posteriorly to the intestine. It has been suggested that the epi- 

 branchial groove is represented in vertebrate embryos by the hjrpochorda, 

 a transient embryonic structure below the notochord, which disappears 

 during ontogenesis. 



