132 THE THIRD DAY. [CHAP. 



hollow processes, lined with hypoblast. Each diverticulum 

 becomes in this way surrounded by a thick mass composed 

 partly of solid cylinders, and to a less extent of hollow 

 processes, continuous with the cylinders on the one hand, and 

 the main diverticulum on the other, all knit together with 

 commencing blood-vessels and unchanged mesoblastic tissue. 

 Between the two masses runs the meatus venosus, with the 

 bulgings on which, referred to above, the blood-vessels in 

 each mass are connected. 



Early on the fourth day each mass sends out underneath 

 the meatus venosus a solid projection of hypoblastic 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 respectively 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 containing little more than 

 blood-vessels. The hollow processes of the diverticula also 

 ramify widely, each branch being compo3ed of a lining of 

 hypoblast enveloped in a coating of spindle-shaped meso- 

 blastic 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 posterior part of the liver (Fig. 53), 

 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. 



The exact morphological significance of these anastomosing cylinders, and 



