HISTOGENESIS OF THE LIVER 297 



3. The cells of the bile duct epithelium are structurally farther 

 removed from the primitive hepatic cell type than are the cells 

 of the hepatic cylinders. 



C. The hepatic jnesenchyma 



1. The hepatic mesenchyma is derived from the splanchnic 

 mesothelium from two zones and at two periods. In both cases 

 the mesenchymal proliferation is associated with marked irregu- 

 larities of the mesothelium. 



2. The first mesenchymal proliferation is ventral and is as- 

 sociated with the formation of mesothelial villi on the right side 

 and irregular mesothelial growths about the omphalo-mesenteric 

 vein on the left side. This proliferation forms the mesenchymal 

 tissue about the gall bladder, cystic duct, main trunk of the 

 omphalo-mesenteric vein, and to an indeterminable degree the 

 interstitial mesenchymal tissue of the ventral part of the liver. 



3. The second and dorsal proliferation is associated with the 

 formation of mesothelial funnels and anastomosing mesothelial 

 tubules which at one time occupy the dorsal fifth of the liver. 

 These mesothelial invaginations break down into mesenchymal 

 cords which in turn form the interstitial mesenchymal tissue of 

 the dorsal part of the liver. 



4. These two zones of mesenchymal proliferation correspond 

 with the two zones of splanchnic mesenchymal proliferation of 

 the trunk region of selachian embryos as described by H. E. 

 Ziegler. 



5. The mesothelial covering of the liver is at first made of 

 columnar cells which later become squamous and thereafter 

 apparently give rise to little or no mesenchymal tissue. The 

 last areas from which the primitive form of mesothelium dis- 

 appears are the tips of the lateral lobes and the dorsal margins 

 of the lateral and median lobes. 



6. Generally the splanchnic mesothelium becomes reduced 

 at once from a columnar to a squamous cell layer upon contact 

 with a hepatic tubule or cylinder. 



