262 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



At that time, the anterior wall of the yolk-sac and that part of 

 the fore-gut which becomes the stomach, lie in the septum 

 transversum (Figs. 211, 212). When the liver bud grows out, 

 it springs from the junction of the fore-gut and yolk-sac 

 (Fig. 221); and spreads into the ventral mesentery and septum 

 transversum. The part of the gut from which it springs after- 

 wards becomes the second stage of the duodenum It is at first 

 a hollow diverticulum of the gut hypoblast (Fig. 218). The 

 diverticulum is surrounded in the mesogastrium by a mass of 

 mesoblastic cells which form the vessels, capsule and connective 

 tissue of the liver. It divides almost at once into right and 

 left solid processes of hypoblastic cells, to form the right and 

 left lobes of the liver (Fig. 218.) 



The hepatic buds are developed just behind the sinus venosus and 

 between the vitelline veins which are also situated in the ventral 

 mesentery (Figs. 202 and 212). The veins are broken up by 

 the ingrowth ; from them starts an invasion of venous capillaries, 

 which, with the mesoblast of the septum transversum, penetrates 

 the liver buds and breaks the solid processes of hypoblast into 

 reticulating cylinders. Secondary processes start from the primary 

 hepatic reticulating cylinders and form smaller and smaller meshes 

 of hepatic cells. By the third month the hepatic lobules, with 

 the intra- and sub-lobular arrangement of portal and hepatic 

 veins, have appeared. The bile ducts probably represent the 

 lumina of the original tubular hepatic buds. The umbilical veins 

 also are cut by the hepatic invasion (Figs. 188 and 190). 



The liver develops rapidly; in the 2nd and 3rd months it 

 occupies more than half the abdominal space. At the com- 

 mencement of the 2nd month, the yolk-sac becomes gradually 

 smaller and, owing to the development of the duodenum and 

 jejunum, it gradually retreats from the abdomen and region 

 of the diaphragm and liver. The second and third stages of 

 the duodenum and the jejunum are developed behind the ventral 

 mesentery. As the liver grows it gradually frees itself from the 

 diaphragmatic part of the ventral mesentery. The right lobe 

 develops more rapidly than the left and comes in contact with the 

 inferior vena cava. The stomach also frees itself from the basis 

 of the diaphragm and comes to lie with the liver in the mesentery. 

 The common bile duct is developed out of the hollow stalk of the 



