126 EARLY EMBRYOLOGY OF THE CHICK 



ahead of them in their caudo-lateral growth and comes to 

 constitute the outer investment of the lung-buds. The en to- 

 dermal buds give rise only to the epithelial lining of the bronchi, 

 and the air passages and air chambers of the lungs. The 

 connective tissue stroma of the lungs is derived from mesen- 

 chyme surrounding the lung-buds, and their pleural covering 

 from the investment of splanchnic mesoderm. 



The Oesophagus and Stomach. — Immediately caudal- to the 

 glottis is a narrowed region of the fore-gut which becomes the 

 oesophagus, and farther caudally a sHghtly dilated region which 

 becomes the stomach (Fig. 43). The concentration of mesen- 

 chyme cells about the entoderm of the oesophageal and stomach 

 regions foreshadows the formation of their muscular and con- 

 nective tissue coats (Fig. 46, C). 



The Liver. — In all vertebrates the liver arises as a diverticu- 

 lum from the ventral wall of the gut immediately caudal to the 

 stomach region. In chick embryos the Uver diverticulum 

 appears just as the part of the gut from which it arises is 

 acquiring a floor by the concrescence of the margins of the 

 anterior intestinal portal. As a result the liver evagination 

 appears for a short time on the lip of the intestinal portal, and 

 grows cephalad toward the fork where the omphalomesenteric 

 veins enter the sinus venosus. As closure of the gut floor is 

 completed, the liver diverticulum comes to lie in its character- 

 istic position in the ventral wall of the gut. In embryos of four 

 days the original evagination has grown out in the form of 

 branching cords of cells and become quite extensive in mass 

 (Fig. 43). In its growth the Uver pushes ahead of it the 

 splanchnic mesoderm which surrounds the gut, with the result 

 that the liver from its first appearance is invested by mesoderm. 

 (Fig. 46, E). 



The proximal portion of the original evagination remains open 

 to the intestine, and serves as the duct of the liver. This 

 primitive duct later undergoes regional differentiation and gives 

 rise in the adult to the common bile duct, to the hepatic and cys- 

 tic ducts, and to the gall bladder. The cellular cords which bud 

 off from the diverticulum become the secretory units of the 

 liver (hepatic tubules). 



The same process of concrescence which closes the floor of 

 the fore-gut involves the proximal portion of the omphalo- 



