THE LIVER AND PANCREAS. 411 



in the chick, especially close relations with the vitelline veins, 

 through which the blood is returned to the embryo from the 

 yolk-sac. 



From the twelfth day onwards, the common part of the bile 

 duct, where the right and left bile ducts join to enter the 

 duodenum, lengthens rapidly, thus giving rise to the single bile 

 duct of the adult rabbit (cf. Fig. 160, 11). 



C. The Pancreas. 



The pancreas arises, on the twelfth day, as a swelling or 

 bulging of the dorsal wall of the intestine, slightly further back 

 than the bile duct, and opposite the yolk-stalk. On the eleventh 

 day it becomes much more sharply defined, and constricted off 

 from the gut as a somewhat pyriform, hollow sac, which opens 

 into the dorsal wall of the intestine, and gives off small buds from 

 its surface. In the succeeding days these buds enlarge, and 

 give off other similar buds from their sides, and so form a com- 

 pound gland of the racemose type. The pancreas, therefore, in 

 its mode of development agrees closely with the salivary or other 

 ordinary glands, and differs markedly from the liver. 



As the alimentary canal is at first straight, and lies but a 

 little distance ventral to the notochord (Fig. 150), the pancreas, 

 in its early stages, is embedded in the dorsal wall of the body 

 cavity, and wedged in between the intestine and the dorsal aorta. 

 As the intestine lengthens, to form the duodenal loop, the 

 pancreas is drawn down- with the mesentery, between the 

 two limbs of the loop, and so attains its adult position (Fig. 

 160, f). 



7. The Cloaca. 



On the twelfth clay (Fig. 150), the rectum opens into a 

 dilated cloacal chamber, tc, from which the stalk of the allantois, 

 ta, opens, and into which the Wolffian ducts, KC, and ureters, 

 kd, discharge. This cloacal dilatation lies in a rounded cloacal 

 papilla, which forms a well-marked projection on the ventral 

 surface of the embryo, behind the allantoic and yolk stalks,- and 

 in front of the root of the tail (Fig. 150). 



Just before entering the cloacal dilatation, the intestine makes 

 a rather sharp bend ventralwards, the distinctness of which is 

 slightly exaggerated in Fig. 150. Between the intestine and the 



