DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 333 



of the thyreoglossal duct. The duct usually disappears for the most part, but 

 certain structures sometimes found in the adult in the line of the duct are 

 possibly remnants of it. They have been variously named, according to their 

 position, accessory thyreoid, suprahyoid, and prehyoid glands (Fig. 298). 



A pair of structures, appearing first in embryos of 8 to 10 mm., arise as 

 evaginations from the ventral ends of the fourth inner branchial grooves. They 

 grow into the mesodermal tissue and then caudally along the ventro-lateral side 



Notochord 



Thymus 



Thy 



Jugular vein 

 Vagus nerve 



Carotid artery 



reoid (epith. body) 



r. groove III) 



Parathy 



Heart 



FIG. 296. Transverse section through the region of the 3d branchial groove 



of an Echidna embryo. Maurer.. 

 i. = Pharynx, below which are the paired analgen of the tongue. 



of the larynx, where they come into close relation with the lateral lobes of the 

 thyreoid (Fig. 298). They have been called the lateral thyreoids, and acquire 

 the thyreoid structure. 



Considerable confusion has arisen in regard to the lateral thyreoids. The earlier investi- 

 gators held that they were derived from the fourth groove and united with the medial portion, 

 which appeared at the foramen caecum, to become integral parts of the thyreoid. Further 

 researches among the lower Vertebrates led others to deny that the thyreoid arose other 

 than as a medial anlage, and that the so-called lateral thyreoids in the embryo were the 

 postbranchial bodies which never assumed the thyreoid structure, but atrophied and dis- 

 appeared. More recently it has been thought that, although the postbranchial bodies do 

 not function in the lower Vertebrates, they may in the higher Mammals and man unite with 

 the medial thyreoid and secrete colloid. 



The parathyreoids or epithelial bodies also come into close relation with the 

 thyreoid. They arise as paired evaginations from the cephalic sides of the third 



