STRUCTURE OF FOUR-DAY CHICKS 127 



mesenteric veins which, when they first appear, lie in the lateral 

 folds of the anterior intestinal portal (Fig. 35). As the intes- 

 tinal portal moves caudad in the lengthening of the fore-gut, 

 the proximal portions of the omphalomesenteric veins are 

 brought together in the mid-line and become fused. The fusion 

 extends caudad nearly to the level of the yolk stalk (Fig. 47). 

 Beyond this point they retain their original paired condition. 

 In its growth the liver surrounds the fused portion of the om- 

 phalomesenteric veins (Figs. 43 and 46, D, and E). This early 

 association of the omphalomesenteric veins with the liver 

 fore-shadows the way in which the proximal part of the afferent 

 vitelline circulation is to be involved in the establishment of the 

 hepatic-portal circulation of the adult. 



The Pancreas. The pancreas is derived from evaginations 

 appearing in the walls of the intestine at the same level as the 

 liver diverticulum. There are three pancreatic buds, a median 

 dorsal, and a pair of ventro-lateral buds. The dorsal evagina- 

 tion appears at about 72 hours, the ventro-lateral evaginations 

 toward the end of the fourth day. The dorsal pancreatic bud 

 arises directly opposite the liver diverticulum and grows into 

 the dorsal mesentery (Fig. 43). The ventro-lateral buds arise 

 where the duct of the liver connects with the intestine so that 

 the ducts of the liver and the ventral pancreatic ducts open 

 into the intestine by a common duct (ductus choledochus). 

 Later in development the masses of cellular cords derived 

 from the three pancreatic primordia grow together and 

 become fused into a single glandular mass, but usually two 

 and in rare cases all three of the original ducts persist in the 

 adult. 



The Mid-gut Region. In chicks of four days the enteric 

 tract shows no local differentiation from the level of the liver 

 to the cloaca except where the yolk-sac is attached. All of the 

 gut tract between the stomach and the yolk-stalk, and the 

 anterior third of the gut lying caudal to the yolk-stalk is des- 

 tined to become the small intestine. The posterior two-thirds 

 of the hind-gut becomes large intestine and cloaca. 



The Cloaca. The beginning of the formation of the cloaca 

 is indicated in chicks of four days incubation, by a dilation of 

 the posterior portion of the hind-gut (Fig. 43). Although ex- 

 tensive differentiations in the cloacal region do not appear 



