274 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



and formation of the cord, comes to lie on the placenta (Fig. 222). 

 The neck of the yolk sac, the vitello-intestinal duct, lies within 

 the umbilical cord. By the sixth week the sac is in a state of 



coeliao axis 



left colic 

 inf. mes. 



sup. haem. 



intest. loop 



placenta 

 yolk sac 



Flo. 222. — The Form of the Alimentary Canal during the 5th week. 



retrogression. The U-shaped intestinal loop, which is formed 

 from the mid-gut, is at first really extra-abdominal, being situated 

 within a funnel-shaped cavity formed by the somatopleure at the 

 umbilicus. The vitelline artery, afterwards the superior mesenteric, 

 is the artery of the TJ-shaped loop ; it terminates at the vitello- 

 intestinal canal — the elongated neck of the yolk sac (Fig. 2 2 2). 



Persistence of Certain Embryonic Structures. — Many of 

 the features seen in the human embryo at the stage of 

 development reached during the fifth or sixth weeks may persist. 



1. The most common structure to remain is the intestinal 

 end of the neck of the yolk-sac — Meckel's diverticulum. It 

 occurs in 2 per cent, of subjects, and commonly forms a finger- 

 glove-like sac on the free border of the ileum about four feet 

 above the ileo-caecal valve. Hence we know that this part of 

 the ileum forms the apex of the U-shaped loop of intestine. 



2. The neck of the yolk sac, instead of closing, may remain 

 open at the umbilicus and form a faecal fistula at birth. 



