334 



TEXT-BOOK OF EMBRYOLOGY. 



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 



Lateral lobe 



Fig. 297. — Section of the right half of the thyreoid gland of a pig embryo of 22.5 mm. Born, 



Accessory thyroeids 

 (thyreoglossal duct) 



... i Pyrami 



Carotid artery — f» ^ | P -" th 7 / 



P.-th. 

 Lat. thyreoid 



(postbr. body) 



Rignt subclavian artery — i 



ial process 



Thymus 



Carotid artery 



Lateral thyreoid 

 I thmus 



Lumen in thymus 



» 



Left subclavian artery 



Arch of aorta 



Fig. 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 



