180 THE THIRD DAY. [CHAP. 



Early on the fourth day each mass sends out ventral 

 to the meatus venosus a solid projection of hypoblas- 

 tic cylinders towards its fellow, that from the left side 

 being much the longest. The two projections unite 

 and form a long solid wedge, which passes obliquely 

 down from the right (or from the embryo lying on its 

 left side, the upper) mass to the left (or lower) one. In 

 this new wedge may be seen the same arrangement of a 

 network of hypoblastic cylinders filled in with vascular 

 mesoblast as in the rest of the liver. The two original 

 diverticula with their investing masses represent respec- 

 tively the right and left lobes of the liver, and the wedge- 

 like bridge connecting them is the middle lobe. 



During the fourth and fifth days the growth of the 

 solid, lobed liver thus formed is very considerable; the 

 hypoblastic cylinders multiply rapidly, and the network 

 formed by them becomes very close, the meshes contain- 

 ing little more than blood-vessels. The hollow processes 

 of the diverticula also ramify widely, each branch being 

 composed of a lining of hypoblast enveloped in a coating 

 of spindle-shaped mesoblastic cells. The blood-vessels 

 are in direct connection with the meatus venosus have 

 become, in fact, branches of it. It may soon be observed, 

 that in those vessels which are connected with the pos- 

 terior part of the liver (Fig. 74), the stream of blood is 

 directed from the meatus venosus into the network of 

 the liver. In those connected with the anterior part the 

 reverse is the case ; here the blood flows from the liver 

 into the meatus venosus. The thick network of solid 

 cylinders represents the hepatic parenchyma of the adult 

 liver, while the hollow processes of the diverticula are 

 the rudiments of the biliary ducts; and we may suppose 



