2o6 Vertebrate Embryology 



a similar closure of the oesophagus in the frog, 

 and the same phenomenon is seen in other 

 animals. In the chick the oesophagus gradu- 

 ally reopens, from behind forwards, at about 

 the ninth day. What the significance of this 

 curious fact may be is not known. 



The part of the digestive tract behind the 

 oesophagus becomes dilated, on the third day, 

 to form the beginning of the stomach (Fig. 67, 

 Si); and the short space between the pyloric 

 end of the stomach and the open mid-gut may 

 be recognized as the duodenum from the fact 

 that there are seen, in this region, the begin- 

 nings of the liver and pancreas. The devel- 

 opment of the latter two organs will be 

 described a little later. 



The posterior end of the digestive tract 

 may be, for a part of the third day, connected 

 with the neural tube by the narrow canal 

 which was described in connection with the 

 frog (p. 34), and was called the 7ieurenteric 

 canal. In front of the neurenteric canal is seen 

 the beginning of the cloaca, as a small pitting- 

 in of the external ectoblast to meet the ento- 

 blast. This invagination of the ectoblast is 

 known as the proctodceum, but it does not 

 open into the digestive tract until several days 



