STRUCTURE OF POUR-DAY CHICKS 125 



system. Nevertheless the origin of their epithelial portions 

 from fore-gut entoderm and their early association with this 

 part of the gut tract makes it convenient to take them up in 

 connection with the digestive system. 



The thyroid gland arises as a median diverticulum from the 

 floor of the pharynx which makes its appearance at the level of 

 the second pair of pharyngeal pouches. Toward the end of 

 the fourth day the thyroid evagination has become saccular and 

 retains its connection with the pharynx only by a narrow open- 

 ing at the root of the tongue known as the thyro-glossal duct 

 (Fig. 43). In mammalia the thyroid is contributed to by pri- 

 mordia which arise laterally from the fourth pharyngeal pouches 

 as well as by a median evagination from the floor of the 

 pharynx. It is possible that evaginations which in the chick 

 arise from the fourth pharyngeal pouches are homologous with 

 the lateral thyroid primordia of mammals. In the chick, how- 

 ever, these evaginations do not form typical thyroid tissue. 



The thymus of the chick does not appear until after the fourth 

 day of incubation. It takes its origin primarily from divertic- 

 ula arising from the posterior faces of the third and fourth 

 pharyngeal pouches. The original epithelial character of the 

 thymus is soon largely lost in an extensive ingrowth of mesen- 

 chyme and the organ becomes chiefly lymphoid in its histolog- 

 ical characteristics. 



The Trachea.- The first indication of the formation of the 

 respiratory system- is an outgrowth from the pharynx. In 

 chicks of 3 days a mid- ventral groove is formed in the pharynx, 

 beginning just posterior to the level of the fourth pharyngeal 

 pouches and extending caudad. This groove deepens rapidly 

 and by closure of its dorsal margins becomes separated from the 

 pharynx except at its cephalic end. The tube thus formed is 

 the trachea, and the opening which persists between the cephal- 

 ic end of the trachea and the pharynx is the glottis (Fig. 43) . 

 The original entodermal evagination gives rise only to the 

 epithelial lining of the trachea, the supporting structures of the 

 tracheal walls being derived from the surrounding mesenchyme. 



The Lung-buds. The tracheal evagination grows caudad 

 and bifurcates to form a pair of lung-buds. As the lung-buds 

 develop they grow into the loose mesenchyme on either side of 

 the mid-line. The adjacent splanchnic mesoderm is pushed 



