THE LUNGS AND LIVER. 293 



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 hypo- 

 blastic walls, with thin mesoblastic investments. 



Towards the latter part of the third day, as the folding 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 duodenum. At the same time they 

 come into very close relation with a large median vein, the 

 meatus venosus, which is formed by the union of the right and 

 left vitelline veins behind the heart (cf. Fig. 128, VE). 



The two liver diverticula lie one at each side of the meatus 

 venosus, and in very close contact with this. The hypoblastic 

 cells forming the walls of the diverticula 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-vessels, which develop in the mesoblast, and early acquire 

 connection with the meatus venosus itself. 



These processes proceed rapidly during the fourth and fifth 

 days;, and by the end of the fifth day (Figs. 123 and 128) 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 communication with the meatus venosus, round which the 

 liver is formed. 



The liver continues to grow rapidly, and by the tenth day 

 is the largest organ in the abdominal cavity. The trabecular net- 

 work of hypoblast cells becomes the liver parenchyma ; the 

 tubular diverticula from the duodenum branch out freely in the 

 substance of the liver, and become the two bile ducts of the 

 adult bird ; while the gall bladder arises on the fifth day as a 

 saccular outgrowth from the right or larger of the two primary 

 diverticula. 



The early formation of the liver in the chick, and its large 

 size during the greater part of the developmental history, 

 indicate that it must be of considerable functional importance 

 during embryonic life. Its relation to the blood system, and 



