THE ORGANS OF DIGESTION. 



273 



3. The mid-gut or roof of the yolk sac, from which is formed 

 that part of the gut between the opening of the common bile duct 

 in front to the splenic flexure of the colon behind. Only the 

 fore-gut has a ventral mesentery; the mid-gut and hind-gut 

 (excepting the cloaca) are suspended by a dorsal mesentery only. 



■fore-gut mid-gut 



hind gut 



cord 

 allantois 

 umb. art. 



art. of yolk sac (sup. mesent.) 

 position Oj 



yolk sac 



Fig. 221. — The Form of the Alimentary Canal in a human embryo of the 3rd week. 



The Yolk Sac and Meckel's Diverticulum. — The fore and hind 

 gut may be regarded as diverticula of the yolk sac. The yolk 

 sac reaches its maximum size in the 4th week. In the 3rd 

 week the umbilicus extends along the whole length of the 

 abdomen, from the septum transversum to the allantois. The 

 neck of the yolk sac completely fills it. The vitelline arteries, 

 which afterwards form the superior mesenteric, end on its walls, 

 the vitelline veins commence on them (Fig. 221). 



In the fifth week the form of the alimentary tract is that 

 shown in Fig. 222. The condition then differs from that shown 

 in the 3rd week in the following points : 



1. The production of the mid -gut as a U-shaped loop from the 

 roof of the yolk sac ; 



2. The formation of a long neck to the yolk sac — the vitello- 

 intestinal canal ; if it persists it forms a Meckel's diverticulum ;■ 



3. The yolk sac, by the constriction of the umbilical orifice 



s 



