INTESTINE 



163 



forms a U-shaped loop surrounding the coils of the small intestine (fig. 206). 

 The small intestine is next withdrawn into the abdominal cavity, and the 

 umbilicus is closed. The U-shaped loop of the large intestine is produced by its 

 proximal portion being carried to the right, so that it comes to be transversely 

 disposed, passing from right to left. As it lengthens, the caecum descends to 

 assume its definitive position in the right iliac fossa, and thus we have laid down 

 an ascending and a transverse colon. The distal portion of the large intestine, at 

 first in the mesial plane, becomes, on the other hand, displaced to the left, as the 



1st thoracic nerve 



pericardium 



lung 



appendix 



small intestine 



urogenital sinus 



xiall intestine 



FIG. 208. RECONSTRUCTION OF A HUMAN EMBRYO OF 24 MM. SOMEWHAT MORE ADVANCED IN 



DEVELOPMENT THAN THAT SHOWN IN FIG. 205. (After Mall.) 



coils of the small intestine increase in number until it ultimately assumes the 

 position of the adult descending colon. The primary coils of the small intestine 

 become progressively more complex by the formation of secondary coils, which are 

 arranged in definite groups so that they can be followed through all the phases 

 of development (Mall). 



The epithelium in the duodenum during the second month increases greatly in thickness by 

 the active multiplication of the cells (fig. 170, p. 126) until the narrow lumen is nearly or quite 

 -closed (Tandler). 1 As the calibre of the gut increases in the third month the lumen is re- 



1 Anat. Anzeiger (Ergiinzangsheft), xviii. 1900. 



