112 THE LIVER. 



with its fellow to form a median organ. Its lower end then gives off solid, bud-like 

 excrescences, and lateral buds come off again from these, so that this part of the 

 organ acquires a ramified, lobulated appearance like an acinous gland. The acini are, 

 however, solid, and remain so, although the upper end of the tube still has a narrow 

 lumen. 



The lymph oid cells next invade the epithelium, growing into every part of the 

 tubular gland, and converting it into a mass of adenoid tissue. In this process the 

 epithelium becomes broken up into small isolated portions, some of which remain in 

 the medullary portion of the lobules as the epithelial nests which are seen in sections 

 of the fully developed organ, and are known as the concentric corpuscles of Hassall. 



The liver. This organ arises in the form of two diverticula of hypoblast, which 

 grow from the ventral wall of the duodenum immediately beyond the stomach (figs. 

 117, 118, Z, 123, Ll\ They extend into a mass of mesoblastic tissue which connects 

 the stomach and duodenum with the anterior wall of the abdomen, and which (with 

 the mesentery, with which it is continuous round the gut) separates the body-cavity 

 here into a right and left half. In this tissue is the omphalomeseraic or vitelline 

 vein (and later the umbilical vein) proceeding on either side to the sinus venosus, 

 and the liver diverticula grow into the mesoblast above and in front of these veins. 

 Here they ramify, giving off solid buds of cells which grow into columns or cylinders, 

 and these again give off lateral diverticula of the same kind. So far the development 

 of the liver resembles that of a compound tubular or acino-tubular gland, except that 

 the ramifications of the original gland diverticula are from the first solid instead of 

 hollow. But soon an important difference appears in the fact that the cylinders 

 unite and anastomose with one another everywhere to form a close network, and 

 from the cords of this network solid sprouts are again constantly being given off to 

 form fresh cylinders, thus producing a yet closer and more intricate network. In 

 the meantime, capillary blood-vessels are formed in the mesoblastic tissue in which 

 this formation of cell-cylinders of hypoblast is going on, and these vessels, which 

 form a network interlocking with that of the anastomosing cell- cylinders, become 

 connected with branches of the vitelline vein on the one hand (vena advehentes), 

 and on the other with veins (vena revehentes) which pass towards the sinus venosus, 

 and eventually are found opening as the hepatic veins into the inferior vena cava. 



The two original hollow diverticula are the rudiments of the right and left hepatic ducts. 

 The common bile duct is formed later by a protrusion of that part of the duodenal wall with 

 which the original diverticula are connected. This protrusion also eventually receives the 

 duct of the pancreas, which becomes shifted towards it. As the common bile duct lengthens, 

 the liver becomes separated from the duodenum, with which it was at first in close connection. 

 The portal and interlobular bile ducts are formed by the hollowing out of some of the 

 anastomosing cell-cylinders, so that a lumen is produced within them surrounded by hepatic 

 cells, which lose their original polyhedral character, and become changed into the columnar 

 epithelium of the ducts, the anastomoses between the cell-cylinders here disappearing. 

 The remaining cylinders form the secreting substance of the liver. The biliary canaliculi 

 appear as minute passages between the cells, and come into continuity with the bile ducts. 

 With a further development of the connective tissue of the organ, the glandular substance of 

 the liver, which was at first continuous throughout, becomes separated into lobules, and the 

 network of cell-cylinders tends with multiplication of their cells to become fused into a con- 

 tinuous mass within each lobule, the bile canaliculi forming by numerous lateral junctions and 

 anastomoses a close network of intercellular passages within the lobule. 



The g-all "bladder and cystic duct are formed by a diverticulum from the common bile 

 duct, which appears in the second month. 



In the elasmobranch fishes, and in amphibia, there is only a single hepatic diverticulum. 

 The anastomosing cell-cylinders which sprout from this are not solid, but hollow, with a 

 narrow lumen, and the liver has from the first and retains permanently the character of a 

 compound gland formed of anastomosing tubules. In reptiles the cylinders also have from 

 the first a narrow lumen. In birds and mammals the cylinders are solid, as in man. 



As the liver grows, it projects on either side into the pleuro peritoneal cavity. The 

 mesoblast which unites it to the anterior wall of the abdomen, becomes thinned out to form 



