1724 



HCMAN ANATOMY. 



which connects the stomach and the duodenum with the anterior body-wall. The 

 hepatic diverticulum grows forward and upward into the anterior mesentery until it 

 comes into relation with the imperfect partition which partially separates the thoracic 

 and abdominal divisions of the body-cavity. This partition, the septum transrcrsum, 

 primarily consists of lateral folds, projecting at right angles from the anterior mesen- 

 tery, caused by the large vitelline veins traversing the anterior mesentery on their 

 way to the sinus venosus of the early heart. The relation of these structures is 

 more fully considered in connection with the development of the diaphragm (page 

 1701); for the present purpose it is sufficient to note that the liver-anlage early 

 comes into relation with the septum transversum. The ventral portion of the pri- 

 mary liver-evagination, clothed with the entoblastic lining of the gut-tube, very soon 

 differentiates into two diverticula : the one nearer the head, or hepatic division, pro- 

 duces the liver proper ; the other, or cystic division, later becomes the gall-bladder 

 and its duct. These divisions are gradually removed from the primitive duodenum 

 by the growth of the primary diverticulum, which at one end becomes converted 

 into a tube connected with the digestive canal and at the other bifurcates into the 

 hepatic and cystic channels. This tube, evidently later the common bile-duct, is at 

 first short and wide, but later rapidly lengthens. 



FIG. 1459. 



: ^*foztfrj&/~;* Septum transversum 



*\:/.-. iJOk* 



&*+'.%*!* '' 



ft m^s. 



Liver-anla 

 Cystic diverticulum 



^T,-.", '.' 



'"J"/4 ' ;.', 'i t ' t 



* " - ,-ft; '/ t '-'w'~f ^jff:-' 1 Hepatic diverticulum 



.^> : **'VJ$y 



^S^L_ Dorsal pancreatic 

 vJ> diverticulum 



. 

 Portion of sagittal section of early rabbit embryo, showing liver-anlage and ducts. X 95- 



The cells lining the longer hepatic diverticulum undergo marked proliferation 

 and produce the liver-mass which invades the septum transversum almost as far as 

 the sinus venosus and surrounds the vitelline veins. The formation of the liver-mass 

 follows at first the type of development seen in tubular glands, outgrowths of the 

 hepatic tube branching and subdividing to form solid sprouts and buds composed of 

 epithelial cells. In some of the lower animals, as the amphibians, the tubular type 

 is retained in the adult organ; but in the higher forms, including man, the tubular 

 character of the young liver is soon lost and replaced by the reticular arrangement 

 produced in consequence of the growing together and union of the terminal divis- 

 ions of the gland. 



Coincidently with the formation of the net-work of glandular tissue by the 

 junction of the cylinders of hepatic cells, the meshes of the rcticulum become occu- 

 pied by blood-vessels derived from vitelline veins. These are now represented at 

 the hepatic anlage by venous stumps from which numerous afferent branches (vcnce 

 hi-pnticn- mfi'fhcnffs) penetrate the liver-mass to become the portal system. The 

 division, subdivision, and union of these blood-vessels keep pace with the increasing 

 complexity of the net-work of hepatic cords, the intergrowth of these constituents 

 eventually leading t<> the intimate relations between the hepatic secreting tissue and 

 the intralolmlar eapillaries seen in the fully developed organ. The cell-trabeculae 

 composing the primary hepatic net-work are partly solid and partly hollow ; the 



