294 THE CHICK. 



especially the fact that it intercepts the blood returning from 

 the yolk-sac to the heart, suggest that its chief purpose is con- 

 nected with the elaboration of the food material which is obtained 

 from the yolk-sac, and at the expense of which the nutrition of 

 the embryo is effected. 



9. The Pancreas. 



The pancreas arises, rather Liter than the liver, as a tubular 

 outgrowth from the duodenum, just beyond the two liver diver- 

 ticula, from which secondary outgrowths arise in much the 

 same manner as in the liver itself. A second diverticulum 

 arises from the duodenum about the eighth day, and gives rise 

 to the greater part of the adult pancreas ; and at a later period 

 a third diverticulum is formed. The three diverticula persist as 

 the three pancreatic ducts of the adult bird, while the three lobes 

 of the pancreas, with which they are connected, soon fuse indis- 

 tinguishably with one another. 



10. The AUantois. 



The allantois is really an appendage of the alimeutaiy canal, 

 arising as an outgrowth of its ventral wall, in front of the 

 cloaca ; it is therefore lined by hypoblast, like all other out- 

 growths of the mesenteron, while the rest of the thickness of its 

 wall is formed by the splanchnopleuric mesoblast. 



The allantois of the chick is homologous with the bladder of 

 the frog (Fig. 89, TB). It differs mainly from this in the fact 

 that, while arising in the same manner, it is not confined within 

 the body of the embryo, but, growing rapidly, passes out beyond 

 this as a thin- walled vascular sac (Figs. 100 and 101, TA), which 

 spreads out in close contact with the inner surface of the egg- 

 shell, and acts as the respiratory organ of the embryo during 

 the greater part of its development. 



In the chick the allantois commences to form about the 

 middle of the second day. At this time the tail fold is not yet 

 established, so that the allantois (Fig. 112, TA) appears at first 

 as a pocket-like fold of the splanchnopleure, lying a short way 

 behind the embryo, and with its cavity opening ventralwards. 



On the formation of the tail fold, early on the third day, the 

 part of the splanchnopleure from which the allantois arises 

 becomes doubled forwards under the embryo to form the ventral 



