178 



THE ENTODERMAL CANAL AND THE BODY CAVITIES 



The lobules, or vascular units of the liver, are formed, according to Mall, by the pecuHar 

 and regular manner in which the veins of the liver branch. The primary branches of the 

 portal vein extend along the periphery of each primitive lobule, parallel to similar branches 

 of the hepatic veins which drain the blood from the center of each lobule (Fig. 186). 'As 

 development proceeds, each primary branch becomes a stem, giving off on either side second- 

 ary branches which bear the same relation to each other and to new lobules as did the primary 

 branches to the first lobule. This process is repeated until thousands of Hver lobules are 

 developed. _, 



Until the 20 mm. stage the portal vein alone supplies the liver. The hepatic artery, 

 from the cceliac axis, comes into relation first with the hepatic duct and gaU bladder. Later, 

 it grows into the connective tissue about the larger bile ducts and branches of the portal vein, 

 and also supplies the capsule of the liver. 



Anomalies. — A common anomaly of the liver consists in its subdivision into multi- 

 ple lobes. Absence or duplication of the gaU bladder and of the ducts may occur. In some 

 animals (horse, elephant) the gall bladder is normally absent. 



THE PANCREAS 

 Two pancreatic anlages are developed almost simultaneously in embryos of 

 3 to 4 mm. The dorsal pancreas arises as a hollow outpocketing of the dorsal 

 duodenal wall just cranial to the hepatic diverticulum (Fig. 177). At 7.5 mm. it is 



B 



j^stomoc^ Accessory pancreatic duct 

 Dorsal pancreas 



Ventral pancreas 

 I Pancreatic diict 



Accessory pancreatic duct 

 Dorsal pancreas 



Ventral pancreas 

 Bile duct Pancreatic duct 



Bile duct 



Fig. 187.— Two stages showing the development of the human pancreas: A, Embryo of 8 mm.; B, 



embryo of about 20 mm. (after KoIIman). 



separated from the duodenum by a slight constriction and extends into the dorsal 

 mesentery (Fig. 185 A). The ventral pancreas develops in the inferior angle be- 

 tween the hepatic diverticulum and the gut (Lewis) and its wall is at first continu- 

 ous with both. With the elongation of the ductus choledochus its origin is trans- 

 ferred to this portion of the diverticulum. 



Of the two pancreatic anlages, the dorsal grows more rapidly and in 10 mm. 

 embryos forms an elongated structure with a central duct and irregular nodules 

 upon its surface (Fig. 185 5). The ventral pancreas is smaller and develops 

 a short slender duct which opens into the ductus choledochus. When the stomach 

 and duodenum rotate the pancreatic ducts shift their positions as well. At the 



