DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 333 



and fourth grooves, dorsal to the thymus and the lateral thyreoid evaginations 

 (Figs. 296 and 299). As the thyreoid grows caudally from its point of origin, 

 these bodies come to lie close to it or may even become embedded in it (Fig. 298). 

 They acquire a structure which resembles that of the suprarenal gland and not 



Trachea 



Lateral lobe 



Capillaries 

 Isthmus 



FIG. 297. Section of the right half of the thyreoid gland of a pig embryo of 22.5 mm. Born. 



Accessory thyroeids 



Accessory tnyroeids 

 (thyreoglossal duct; 



I 



Carotid artery 



P.-th. 



Lat. thyreoid 

 (postbr. body) 



Rignt subclavian artery 



Thymus 



Pyramidal process 



Carotid artery 

 Lateral thyreoid 

 Isthmus 



Lumen in thymus 

 subclavian artery 



Arch of aorta 



FlG. 298. Branchial groove derivatives of a rabbit embryo of 16 mm. P.-th., parathyreoid 

 or epithelial body. Verdun, Bonnet. 



that of the thyreoid. Their relation to the latter organ seems to be purely 

 topographical. 



THE THYMUS. The thymus appears in embryos of about 6 mm. as an 

 entodermal evagination from the ventral part of the third branchial groove on 



