THE LATER DEVELOPMENT OF THE CHICK 327 



to the anterior intestinal portal (Fig. 99). Later we have seen 

 that, through the process of folding the embryo off the yolk, 

 the splanchnopleural gut is rapidly extended posteriorly from 

 the fore-gut and anteriorly from the hind-gut, and is closed 

 in entirely, save where it communicates with the yolk-sac by 

 way of the splanchnic stalk. That part of the gut which is thus 

 formed primarily by the approach of the lateral embryonic folds 

 is distinguished as the mid-gut, the definitive fore- and hind- 

 gut being formed primarily by the head- and tail-folds. Em- 

 bryologically the fore- and hind-gut are more important than 

 the mid-gut, for in connection with these regions develop all 

 of the important appendages of the alimentary tract. 



In connection with the fore-gut we have to describe the for- 

 mation of the pharynx and visceral arches and pouches, the 

 thyroid, the thymus and post-branchial bodies, the whole res- 

 piratory tract, the oesophagus and stomach, the liver, and the 

 pancreas; in addition we must include the stomodaeum and the 

 structures derived from it, the hypophysis and the organs of the 

 buccal cavity. 



The stomodceum is an ectodermally lined depression on the 

 lower side of the head; the oral plate is at its bottom (Figs. 

 99, 123, A). The depth of the stomodaeum is increased by the 

 formation and growth of the jaws. As previously noted, the 

 oral plate becomes perforated during the* third day and then 

 gradually disappears, the stomodaeum itself, however, is the 

 seat of several important organs. The hypophysis appears 

 about the forty-fourth hour, as an elongated evagination from 

 its mid-dorsal wall, just in front of the oral plate (Fig. 123). 

 It grows directly toward the ventral surface of the. diencephalon, 

 in the region of the infundibulum, which it reaches at about 

 the beginning of the third day. Later it becomes glandular, 

 loses its connection with the epithelium of the stomodaeum and 

 joins with the infundibulum to form the glandular portion of 

 the pituitary body. 



The cavity of the stomodaeum and the future buccal cavity 

 are practically coincident, whether precisely or not can hardly 

 be said, on account of the disappearance of the oral plate 



