DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 337 



It is obvious that the lower end of the oesophagus is carried toward the left 

 side of the body with the cardiac end of the stomach, and at the same time 

 twisted so that the side which originally faced the left comes to face ventrally. 

 The changes in the mesentery which accompany the changes in the stomach 

 are described elsewhere (p. 378). 



The torsion of the stomach also produces an asymmetrical condition of the 

 vagi nerves. The latter reach the stomach before it changes its position. As 

 the changes take place, the left nerve is carried around to the left and ventrally 

 so that in the adult it passes through the diaphragm ventral to the oesophagus 

 and extends over the ventral surface of the stomach. The right nerve passes 

 over the dorsal surface of the stomach. 



The Intestine. 



When the primitive gut is differentiated into recognizable regions (p. 317) 

 the intestinal region forms a simple tube, of uniform diameter, extending from 

 the stomach to the caudal end of the embryo where it ends blindly. The yolk 

 stalk is attached to the intestine a short distance from the stomach. Near the 

 caudal end the allantoic duct arises (p. 114). The lumen of the yolk stalk and 

 of the allantoic duct is continuous with that of the intestine (Fig. 285). In 

 embryos of 2 to 3 mm. the liver anlage arises from the ventral side of the 

 intestine near the stomach, that is, from that. part of the intestine which is to 

 become the duodenum. In embryos of 3 to 4 mm. the pancreas anlage arises 

 in the same region, in part from the liver evagination and in part from the dorsal 

 side of the intestine (Fig. 285). 



The intestine as a whole is suspended in the abdominal cavity by the dorsal 

 mesentery which is attached to the dorsal body \vall and which is continuous 

 with the dorsal mesogastrium. A ventral mesentery, continuous with the 

 ventral mesogastrium, is present only at the cephalic end of the duodenum 

 (Fig. 301). 



The further development of the intestine, apart from histogenesis, consists 

 very largely of the formation of loops and coils, due to an enormous increase in 

 the length of the tube. The abdominal cavity at the same time enlarges to 

 accommodate the increased bulk. As the stomach changes its position (p. 336), 

 the duodenum comes to lie obliquely across the body and forms a curve with the 

 concavity directed dorsally (Fig. 301). The rest of the intestine forms a loop 

 which extends ventrally and caudally as far as the umbilicus. The arms of the 

 loop are almost parallel and the cephalic arm lies a little to the left of the caudal. 

 The apex of the loop extends into the umbilical coelom and is attached to the yolk 

 stalk. From the dorsal end of the caudal arm the intestine extends directly 

 to the caudal end of the body (Fig. 301). 



Soon after the loop is formed a small evagination appears on its caudal arm, 

 not far from the apex. This is the anlage of the cacum and marks the bound- 



