THE PANCREAS 



I 79 



During the development of the liver the endothelial cells of the sinusoids become 

 stellate in outline, and thus form an incomplete layer. From the second month of fetal 

 life to some time after birth, blood cells are actively developed between the hepatic cells 

 and the endothelium of the sinusoids. At 22 mm. hollow interlobular ducts develop, 

 spreading inward from the hepatic duct along the larger branches of the portal vein. In 

 44 mm. (C R) fetuses, bile capillaries with cuticular borders are present, most numerous 

 near the interlobular ducts with which some of them connect. At birth, or shortly after, 

 the number of liver cells surrounding a bile capillary is reduced to two, three, or four. 

 Secretion of the bile commences at about the end of the third fetal month. 



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

 culiar 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 that 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 

 secondary 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 liver 

 lobules are developed. 



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

 from the coeliac axis, conies into relation first with the hepatic duct and gall 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 multiple 

 lobes. Absence or duplication of the gall 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 outpocket- 

 ing of the dorsal duodenal wall, just cranial to the hepatic diverticulum 

 (Fig. 177). At 7.5 mm. it is 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 between the hepatic diver- 

 ticulum and the gut (Lewis), and its wall is at first continuous with both. 

 With the elongation of the ductus choledochus its origin is transferred 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 

 ii regular nodules upon its surface (Fig. 185 B}. The ventral pancreas 

 is smaller and develops a short, slender duct that opens into the ductus 

 choledochus. When the stomach and duodenum rotate, the pancreatic 

 ducts shift their positions as well. At the same time, growth and bending 

 of the bile duct to the right bring the ventral pancreas into close proximity 

 with the dorsal pancreas (Figs. 185 and 187). 



In embryos of 20 mm., the tubules of the dorsal and ventral pancreatic 

 anlages interlock (Fig. 187 B}. Eventually, anastomosis takes place be- 



