308 



TEXT-BOOK OF EMBRYOLOGY. 



where they remain until the embryo reaches a length of 40 mm. (compare Figs, 

 265 and 266). Then they return very quickly to the abdominal cavity proper. 



After their return, the primary loops, with the secondary loops derived from 

 them, come to occupy fairly constant positions. The second and third move 

 to the left upper part of the abdominal cavity; the fourth crosses the medial 

 line and occupies the right upper part. The fifth crosses back and lies in the 

 left iliac fossa; the sixth lies in the pelvis and lower part of the abdominal 

 cavity (Fig. 267). 



Certain variations may occur but are usually not considered as abnormal. 

 The most frequent variation is one in which the fourth coil, along with the 



FIG. 265. Reconstruction of the stomach and intestine of a human embryo of 28 mm. Matt. 



The numbers are placed on the coils derived from the primary bends as shown in 



Fig. 302; i represents the duodenum. 



second and third, lies on the left side, its usual position on the right being oc- 

 cupied by the ascending colon. Not uncommonly the positions of the fourth 

 and the second and third are reversed. Less commonly extra loops are formed. 



Usually the proximal part of the yolk stalk disappears during foetal life. In 

 a few cases, however, it persists as a blind sac of variable length, known as 

 Meckel's diverticulum (see also p. 581). 



Even before the loops return to the abdominal cavity the colon or large 

 intestine increases in diameter more rapidly than the small intestine. After 

 the return, the caecum is carried across to the right side and comes to lie just 

 caudal to the liver. From the caecum the colon extends across the abdominal 



