A LI MEN T A R Y QA NA L. 



633 



It is still doubtful whether the hepatic cylinders are as a rule 

 liollow or solid. \i\ Elasinobrauchii they are at first provided with a 

 large lumen, which though it becomes gradually smaller never entirely 

 vanishes. The same seems to hold good for Amphibia and some Mam- 

 malia. In Aves the lumen of the cylinders is even from the first 

 much more difficult to see, and the cylinders are stated by Kemak to be 

 solid, and he has been followed in this matter by Kolliker. In the Rabbit 

 also Kolliker finds the cylinders to be solid. 



The embryonic hepatic network gives rise to the parenchyma of the 

 adult liver, with which in its general 

 arrangement it closely agrees. The 

 blood -channels are at first very large, 

 and have a very irregular arrange- 

 ment ; and it is not till compara- 

 tively late that the hepatic lobules 

 with their characteristic vascular 

 structures become established. 



The biliary ducts are formed 

 cither from some of the primitive 

 hepatic cylinders, or, as would seem 

 to be the case in Elasmobranchii 

 and Birds (tig. 422), from the larger 

 diverticula of the two primitive out- 

 growths. 



The gall-bladder is so inconstant, 

 and the arrangement of the ducts 

 opening into the intestine so varia- 

 ble, that no general statements can 

 be made about them. In Elasmo- 

 branchii the primitive median di- 

 verticulum (fig. 421) gives rise to the ductus choledochus. Its 

 anterior end dilates to form a gall-bladder. 



In the Rabbit a ductus choledochus is formed by a diverticulum 

 from the intestine at the point of insertion of the two primitive 

 lobes. The gall-bladder arises as a diverticulum of the right primitive 

 lobe. 



The liver is relatively very large during embryonic life and has, 

 no doubt, important functions in connection with the circulation. 



The pancreas. So far as is known the development of the pan- 

 creas takes place on a very constant type throughout the series of 

 craniate Vertebrata, though absent in some of the Teleostean fishes 

 and Cyclostomata, and very much reduced in most Teleostei and in 

 Petromj'zon. 



It arises nearly at the same time as the liver in the form of a 

 hollow outgrowth from ti)e dorsal sitle of the intestine nearly oppo- 

 site but slightly behind the hepatic outgrowth (fig. 422, jj). It soon 

 assumes, in Elasmobranchii and Mammalia, somewhat the form of 

 an inverted funnel, and from the expanded dorsal part of the funnel 



Fig. 422. Diagram of the diges- 

 tive TKACT OF A ChICK UPON THE 



FOURTH DAY. (After Gotte ) 



The black line indicates the hypo- 

 blast. The shaded jjart around it is 

 the splanchnic mesoblast. 



Ig. lung; st. stomach; j). joancreas; 

 I. liver. 



