THE FACE. 289 



converted by fusion of its lips, as described on p. 275, into the 

 posterior narial passage. 



By the seventh day (Fig. 126) the parts of the face begin 

 to assume more definite form. The mouth opening, DS, is 

 more slit-like, and its boundaries are more clearly defined. The 

 fronto-nasal process is narrower, and has begun to grow forwards 

 as the upper beak, on the tip of which the small epithelial knob, 

 OJ, which is used for breaking the egg-shell at the time of 

 hatching, is already present. The maxillary arches have fused 

 with the sides of the fronto-nasal process ; the nasal grooves are 

 converted into the narial passages, and the lacrymal grooves 

 have disappeared. 



The two mandibular arches, MN, have fused in the median 

 plane to complete the lower jaw, which is already beginning to 

 grow forwards as the lower beak. Finally, the external nostrils, 

 OK, have narrowed very considerably, and have acquired the slit- 

 like form characteristic of them in the adult. 



The pituitary body (Fig. 114, PT) is a pocket-like diverticulum 

 from the anterior angle of the stomatodasum, which appears 

 towards the end of the second day, and which early acquires its 

 characteristic relations with the infundibulum, and with the 

 anterior end of the notochord. Its development has already 

 been described in the section dealing with the brain (p. 259). 



4. The (Esophagus. 



Immediately behind the pharynx the alimentary canal sud- 

 denly narrows, becoming a very slender tube, the oesophagus, 

 which runs back in a perfectly straight course through the neck 

 (Figs. 114, TO, and 123). 



The oesophagus is at first very short ; but, as the neck 

 lengthens, the oesophagus grows rapidly, to keep pace with this. 

 A curious point with regard to the oesophagus is that for a 

 time, commencing about the middle of the sixth day, and 

 lasting for two or three days, the lumen is completely lost, the 

 oesophagus becoming solid along the greater part of its length. 

 A little later, about the ninth day, the lumen is gradually 

 re-established, from below upwards. 



This temporary obliteration of the cavity of the oesophagus 

 in the chick is perhaps to be associated with the rapid lengthening 

 which the neck and the oesophagus are undergoing at this period ; 



U 



