298 EMBRYOLOGY. 



running near each other, between which is stretched the mesentery 

 (ms), which is likewise drawn out with the loop. One arm (fZ 1 ) lies 

 in front and is directed backward, the other (d 2 ) lies behind it and 

 runs upward, to be again bent near the vertebral column ; thence, 

 supported by a narrow mesentery, it pursues a straight course (r] 

 backward to the anus. The transition from the first to the second 

 arm, or the apex of the loop, is imbedded in an excavation in the 

 fcetal end of the umbilical cord, and it is there in communication 

 with the umbilical vesicle by means of the vitelline duct (d ?> ], now 

 in process of degeneration. At some distance from the origin of 

 the vitelline duct there is to be seen in the second arm of the loop 

 a small enlargement and evagination (cZ 2 ). This is afterwards de- 

 veloped into the coecum, and it therefore indicates the important 

 boundary between the small and large intestine. 



In consequence of these first foldings four regions of the intestine 

 can be distinguished even now ; these are more sharply separated 

 later. The short portion, running from the stomach to the back- 

 bone and provided with a small mesentery, becomes the duodenum 

 (du) ; the anterior [ventral], descending arm (d l \ together with the 

 bend in the loop, furnishes the small intestine ; the posterior [dorsal], 

 ascending arm is developed into the colon (<rZ 2 ), and the terminal 

 part, embracing the last bend, into the sigrnoid flexure and the 

 rectum (r). 



In embryos of the third and following months there occur, in con- 

 nection with a further increase in length, important changes in the 

 position of the stomach and the intestinal loops. 



The stomach undergoes a double twisting, about two different axes, 

 and thereby early acquires a form and position (figs. 165 A and 7>) 

 which correspond approximately to the permanent condition. First 

 its longitudinal axis, which unites cardia and pylorus and is in the 

 beginning parallel with the vertebral column, takes an oblique and 

 finally an almost transverse position, in consequence of a rotation 

 around the dorso- ventral axis. Thereby the cardia moves to the left 

 half of the body and downwards, but the pylorus more to the right 

 side and somewhat higher. Secondly, at the same time the stomach 

 experiences a torsion around its longitudinal axis, by which the 

 originally left side becomes the front [ventral] and the right the back 

 [dorsal]. Consequently the greater curvature comes to lie below 

 [posterior], the lesser above [anterior]. The terminal part of the 

 esophagus is also affected by the torsion ; it undergoes a spiral 

 twisting, by which its left side becomes the front. 



