210 Vertebrate Embryology 



as a tubular diverticulum from the posterior end of the 

 fore-gut, in the angle between the two vitelline veins, 

 and immediately behind their point of union. A second 

 diverticulum arises from the same spot almost directly 

 afterwards; it is similar to the first, but of rather smaller 

 size. Both these diverticula have hypoblastic walls, 

 with thin mesoblastic investments (Fig. 67, /). 



" Towards the latter part of the third day, as the fold- 

 ing off of the embryo from the yolk-sac proceeds, the 

 liver diverticula are found to arise definitely from the 

 part of the mesenteron which will later become the duo- 

 denum. At the same time they come into very close 

 relation with a very large median vein, the meatus veno- 

 sus, which is formed by the union of the right and left 

 vitelline veins behind the heart (Fig. 76, VE]. 



" The two liver diverticula lie one on each side of the 

 meatus venosus, and in very close contact with this. 

 The hypoblastic cells forming the walls of the diver- 

 ticula now begin to proliferate freely, growing out as 

 solid strands of cells, which form an irregular reticulum 

 closely surrounding the meatus venosus; the meshes of 

 the reticulum being occupied by capillary blood ves- 

 sels, which develop in the mesoblast, and early acquire 

 connection with the meatus venosus itself. These pro- 

 cesses proceed rapidly during the fourth and fifth days, 

 and by the end of the fifth day (Figs. 76 and 83) the 

 liver is an organ of considerable size, consisting of a 

 network of solid rods of hypoblast cells, which branch 

 and anastomose freely in all directions ; the meshes of 

 the network being occupied by blood-vessels, which 

 penetrate all parts of the liver, and are in free com- 

 munication with the meatus venosus, round which the 

 liver is formed. 



