286 



TEXT-BOOK OF EMBRYOLOGY. 



it opens widely into the yolk sac (Figs. 244 and 245). 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. 76; 

 compare with 77). 



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 



m . 



|_ Oral fossa 



jr "Branchial arch I 



a Branchial arch II 



" Body wall 

 Ccelom 





Coelom 



Hind-gut 



Belly stalk 



FlG. 245. 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. 244. 



to and continuous with this, is the short, narrow cesophageal 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. 246 and 247). 



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. 



