THE PANCREAS. 



107 



The Liver. 



When the omphalo-mesaraic veins, the first large veins to appear, are developed, 

 they are situated in the splanchnopleure and join the heart. They are of such 

 large size as to cause a projection into the coelom. This projection is the septum 

 transversum (p. 87). As shown in the diagram (Fig. 16), the entoderm of the 

 digestive canal of the head of the embryo passes over behind the pericardial cavity 

 and behind the septum transversum into the yolk-sac. Out of the entoderm cover- 

 ing the septum transversum on its caudal side, the anlage of the liver is developed 

 (Fig. 25, fo). This anlage is produced by a rapid proliferation of the entodermal 

 cells, and they grow toward the space occupied by the omphalo-mesaraic veins 

 (Fig. 157). An intergrowth of the liver cells and of the endothelium of the veins 

 takes place. The cavity of the veins becomes subdivided into smaller blood chan- 

 nels, which we call sinusoids to distinguish them from capillary vessels. The liver 

 cells arrange themselves in the form of cords which are termed the hepatic cylinders. 

 Each hepatic cylinder is closely invested by the venous endothelium. The liver 

 consists at first only of hepatic and endothelial cells and is situated in the septum 

 transversum. 



When the liver becomes larger, it protrudes 

 from the septum transversum, but does not 

 separate from it, so that in the adult the liver is 

 always found attached to the diaphragm, which 

 is merely the modified septum transversum. 



The Pancreas. 



The pancreas is a double organ, for it arises 

 from two distinct anlages: first, the dorsal pan- 

 creas, which appears as an entodermal evagination 

 on the dorsal side of the duodenum (Fig. 61, 

 Panc.d), soon branches, and, continuing to grow, 

 rapidly develops into the body and tail of the 

 adult organ. Its connection with the intestine 

 becomes the dorsal pancreatic duct (ductus San- M, ', 4 Cords of hepatic cells. Panc.d, 



- 



torini). The ventral pancreas appears a little 



later and grows more slowly than the dorsal. It 

 arises as an Outgrowth (Fig. 6l, Panc.v} from 



the entodermal ductus choledochus. It develops 

 into the head of the pancreas and dorsal pancreatic duct (ductus Wirsungi). The 

 two anlages expand, meet (Fig. 62), and unite. In the pig, the dorsal anlage is 

 farther from the stomach than the ventral, but in man it is nearer the stomach. 



In the pig, the ventral pancreatic duct is obliterated and the dorsal duct alone 

 is normally persistent in the adult. In man, on the contrary, the dorsal duct is 

 normally obliterated and the ventral duct persists. Occasionally both ducts are 



Ves-fel* 



FIG. 61. PIG EMBRYO OF 5.5 MM. SERIES 

 915. WAX RECONSTRUCTION OF THE 

 STOMACH AND PANCREATIC ANLAGES 

 BY F. W. THYNG. 



Dorsal pancreas. Panc.v, Ventral pan- 

 creas. St, Stomach. Ves.fel, Gall- 

 bladder, x, Ventral process of the 

 dorsal pancreas (situated on the right of 

 the portal vein). X 55 diams. 



