111 1'ANCKEAS 189 



commiiiiiejite with the just - mentioned vessel. This spongy mass, 

 the trabeculae of \\hirh ;nc ;it lirst solid and only secondarily 

 develop a lumen, forms the secretory portion of the liver, while 

 the proximal portions of the outgrowths persist as the two con- 

 spicuous bile-ducts of tin- adult bird (Fig. 108, D, E, Id. I and Id. 2). 

 In such birds as possess a gall-bladder this is formed by a dilatation 

 close to the point of junction of the posterior bile-duct with the 

 gut-wall (Fig. 108, 1), E, gV). 



PANCREAS. The pancreas, though in the adult a single structure, 

 arises typically from three distinct rudiments, each of which is at 

 first a simple pocket-like outgrowth of the splanchnopleure. One of 

 the rudiments (cf. Fig. 80, H) is situated dorsaily a little posterior 

 to the stomach, the other two, which appear somewhat later, are 

 ventral and arise as outpushings of the hepatic diverticulum in the 

 region of the bile-duct. The ventral pancreatic rudiments are 

 commonly paired, arising one on the right and one on the left of 

 the bile-duct. 



The three rudiments increase in size, secretory tubules sprout 

 out from them and the two ventral rudiments become carried in a 

 dorsalward direction, up the right side, by the rotation which the gut 

 undergoes in this region (see p. 168). -The right ventral rudiment 

 comes in contact with the dorsal rudiment and fusion takes place 

 all three rudiments forming a single organ the three-fold origin of 

 which is indicated by its three communications with the alimentary 

 canal. 



Such may be considered the typical mode of development of the 

 pancreas, but important variations in detail occur in the different 

 groups. In Cyclostomes and Elasniobranchs only the dorsal pancreas 

 is known to occur. Its development in the former group requires 

 further investigation. In Elasmobranchs it arises as a longitudinal 

 groove of the enteric wall dorsaily and a little posterior to the open- 

 ing of the bile-duct. It becomes constricted off from before backwards 

 and in accordance with the rotation of the alimentary canal it becomes 

 shifted to the left side and ends up by being ventral. 



In Crossopterygians the three typical rudiments appear (Fig. 80, H) 

 but their development has not been followed in detail. Eventually 

 the pancreatic complex extends forwards beneath the liver and com- 

 pletely fuses with it forming a thick layer over its ventral surface 

 in the region near the opening of the bile-duct. 



In Actinopterygian Ganoids also (Piper, 1902 ; Nicolas, 1904), 

 the pancreatic complex derived from the original three rudiments 

 becomes fused with the substance of the liver, only its posterior dorsal 

 portion remaining extrahepatic. The main duct of the pancreas is 

 1 he persistent stalk of the right ventral rudiment which opens into 

 the gall-bladder formed by the dilated terminal part of the bile-duct. 

 Of the two other pancreatic ducts the left ventral apparently 

 atrophies entirely, while the dorsal is said in the case of Amia to 

 disappear but in the Sterlet (Acipenser ruthenus) to persist. 



