334 EMBRYOLOGY. 



anus then marks the termination, as the mouth does the beginning,, 

 of the alimentary canal. 



B. Separation of the Alimentary Tube and its Mesentery into 

 Distinct Regions. 



1. The alimentary canal is originally a tube running straight front 

 mouth to anus, near the middle of which the yolk-sac (umbilical 

 vesicle) is attached by means of the vitelline duct (stalk of the 

 intestine). 



2. The alimentary tube is attached throughout its whole length to- 

 the vertebral column by means of a narrow dorsal mesentery ; it is 

 also connected with the anterior wall of the trunk, as far back as the 

 umbilicus, by means of a ventral mesentery (mesocardium anterius 

 and posterius, anterior [ventral] gastric and duodenal mesentery). 

 (Yorleber.) 



3. At some distance behind the visceral clefts, the stomach arises- 

 as a spindle-shaped enlargement of the alimentary tube ; its dorsal 

 mesentery is designated as mesogastrium. 



4. The portion which follows the stomach grows more rapidly in 

 length than the trunk, and therefore forms in the body-cavity a 

 loop with an upper [anterior], descending narrower arm, which be- 

 comes the small intestine, and a lower [posterior], ascending more 

 capacious arm, which produces the large intestine. 



5. The stomach takes on the form of a sac, and becomes so turned 

 that its long axis coincides with the transverse axis of the body, and 

 that the line of attachment of the mesogastrium, or its greater 

 curvature, which was at first dorsal, comes to lie below, cr caudad. 



6. The intestinal loop undergoes such a twisting that its lower,, 

 ascending arm (large intestine) is laid over [ventral to] the upper, 

 descending arm (small intestine) from right to left, and crosses 

 it near its origin from the st.omach. 



7. The twisting of the intestinal loop explains why in the- 

 adult the duodenum, as it merges into the jejunum, passes under 

 the transverse colon and through its mesocolon. (Crossing and 

 crossed parts of the intestine.) 



8. The lower arm of the loop, during and after its twisting and 

 crossing of the upper arm, assumes the form of a horseshoe and. 

 permits one to distinguish the coacum, the colon ascendens, c. trans- 

 versum, and c. descendens. 



9. Within the space bounded by the horseshoe, the upper arm. 



