THE DIGESTIVE SYSTEM 



325 



Development of the Liver. The anlage of the liver appears in a 

 2.5 mm. human embryo as a ventral outpocketing of the foregut near the 

 anterior intestinal portal, between the two vitelline veins. The liver 

 diverticulum projects into the ventral mesentery and the mesoderm of 

 the septum transversvmi which separates the pericardial cavity from the 

 abdominal cavity. The outgrowth soon becomes differentiated into an 

 anterior mass of branching tubules surrounded by branches of the vitel- 

 line veins, and a posterior hollow sac which later becomes the gall bladder. 

 The multiplication of the tubules correlated with that of the blood capil- 

 laries associated with them produces the lobules. Mesenchyme cells 



fev.w4 ft:-''' 



^- —PORTAL VEIN" 



LOBULE. 



HEPATIC VEIN 



B C 



and C — diagrams of successive stages in the development of the 

 The subdivision of the Hver anlage into lobules is correlated with 

 branching of the portal and hepatic veins. The branches of the hepatic vein are 

 intralobular, and those of the hepatic portal vein — shown in black — are interlobular. 

 (Redrawn after Mall.) 



Fig. 272. — A, B 

 lobules of the liver. 



form the interlobular connective tissue which binds the lobules together. 

 Bile capillaries appear within the cell cords, and the blood capillaries 

 acquire endothelial walls. As a result of this, the lumen of each bile 

 capillary is separated from that of each blood capillary by a layer of 

 gland cells and a layer of endotheHal cells. (Figs. 261, 267, 272) 



The multiplication of tubular cords and of blood spaces results in a 

 rapid enlargement of the liver, which begins to bulge out from the septum 

 transversum and the ventral mesentery and to push into the abdominal 

 cavity between the septum and the stomach. In this way, the liver 

 becomes covered by the peritoneum and acquires its two chief lobes. 

 The ventral mesentery into which it originally grew forms the falciform 

 ligament. (Fig. 267) 



