496 



EMBRYOLOGY OF THE CHICK 



lungs, however, is derived from the mesenchyme surrounding the lung- 

 buds. 



In the chick and all birds, there is a characteristic thin-walled, sac- 

 like outgrowth from the hinder edges of the lungs forming the air-sacs 

 (Fig. 292). These do not appear until about the eighth day. 



THE LIVER 



The liver arises as a ventral diverticulum from the duodenum. It 

 can be seen for a short time on the lip of the anterior intestinal portal 



growing cephalad toward 

 the fork where the omphalo- 

 mesentric veins enter the 

 sinus venosus. The liver 

 grows out as a series of 

 cords pushing the splanchnic 

 mesoderm ahead of it as its 

 investing layer. 



The liver evagination as 

 it forms, retains its opening 

 into the duodenum (Fig. 

 293), which later differen- 

 tiates somewhat to become 

 the common bile duct, the 

 hepatic and cystic ducts, as 

 well as the gall bladder. 

 Cellular cords bud off from 

 the diverticulum and be- 

 come the hepatic tubnles 

 which have secretory func- 

 tions. 



As the intestinal portal 

 moves caudad when the 

 fore-gut lengthens, the 

 proximal portions of the 

 omphalomesenteric veins 

 come together and fuse in 

 midline. The fusion extends 

 caudad nearly to the level of 

 the yolk-stalk, beyond which 

 they still remain separate. 

 The liver now surrounds 

 the fused portion of the 

 omphalomesenteric veins. 



It will be noticed, there- 

 fore, that the yolk materials 



Fig. 293. 



Two upper cuts are diagrams to show the develop- 

 ment of the liver, pancreas, and hepatic ligaments. 

 d, intestine; ect, ectoderm; leb, liver anlage ; lig.hep.ent., 

 ligamentum hepato-entericum ; lig.susp.hep ; ventral mes- 

 entery or ligamentum suspensorum of the liver ; mesent. 

 dors., dorsal mesentery ; pancr. dors, and pancr. ventr., 

 dorsal and ventral pancreas. (After Schmikewitsch. ) 



Lower cut is a diagram to show the development of 

 the liver. Lobule 1 shows the principal parts of the 

 gall capillaries ; Lobule 2, shows the anastomoses of these 

 gall capillaries ; in Lobule 3, only the efferent bile capil- 

 laries are shown, together with the arterial and venous 

 capillaries, a, arteries; b, veins. (After Stohr.) 



