318 TEXT-BOOK OF EMBRYOLOGY. 



it opens widely into the yolk sac (Figs. 282 and 283) . The primitive gut, there- 

 fore has no communication with the exterior. It communicates at its caudal 

 end with the central canal of the spinal cord through the neurenteric canal (Fig. 84 ; 

 compare with 85). 



As development proceeds, this simple tube elongates rapidly and becomes 

 differentiated into distinct regions. The cephalic end, in connection with the 

 branchial arches and grooves, becomes the dilated pharyngeal region. Caudal 



Oral fossa 

 Branchial arch I 

 Branchial arch II 



Belly stalk 



Fig. 283. — Ventral view of human embryo of 2.4 mm. His, Kollmann. 



Note the opening in the ventral wall of the gut. This indicates the communication between the 



gut and the yolk sac. The latter has been removed. Compare with Fig. 282. 



to and continuous with this, is the short, narrow (esophageal region which in 

 turn passes over into the slightly dilated stomach region. The portion of the 

 gut caudal to the stomach is the intestinal region. During the differential 

 changes, the communication with the yolk sac becomes relatively smaller, form- 

 ing the yolk stalk which joins the intestinal portion a short distance caudal to the 

 stomach (Figs. 284 and 285). 



The Mouth. 



At a very early period the primary fore-brain region bends ventrally almost 

 at a right angle to the long axis of the body to form the naso-frontal process. 



