FROM TWELVE TO THIRTY-SIX SOMITES 



181 



moses where branches meet, so that a complete ring of anas- 

 tomosing columns of hepatic cylinders is rapidly formed around 

 the center of the ductus venosus 

 (Figs. 103 B and 104, cf. also Figs. 

 119 and 120). But the anterior 

 and posterior ends of the ductus 

 venosus are not yet completely 

 surrounded by the basket-work of 

 liver substance, owing to the ab- 

 sence of any part of the posterior 

 diverticulum in its anterior por- 

 tion, and of the anterior divertic- 

 ulum in its posterior portion. 



The floor of the intestine be- 

 tween the anterior and posterior 

 liver diverticula is depressed; later 

 it becomes separated from the 

 intestinal cavity to form a tem- 

 porary common bile-duct; which 

 then receives the two primary di- 

 verticula (Figs. 103 B, 104 and 

 187). 



The pancreas arises from a dor- 

 sal and a pair of ventral primorclia. 

 The former is an outgrowth of 

 the dorsal wall of the intestine 

 immediately above the posterior 

 liver diverticulum (Figs. 103 B 

 and 104). At the 35 s stage it is 

 a solid thickening of the dorsal 

 wall of the intestine of consider- 

 able extent; a little later the base 

 of the thickening is hollowed out, 

 and the free margin sends ofT solid 

 buds into the dorsal mesentery 

 just behind the stomach. The ventral primordia arise from the 

 posterior liver diverticulum in a manner to be described later 

 (Chap. X). 



Mid-gut. At the 35 s stage the mid-gut is still open to the 

 yolk-sac. Its subsequent history is given in Chapter X. 



Fig. 104. — Reconstruction of the 



liver of the chick at the end of 



the fourth day of incubation. 



(After Hammar.) 



du., Duodenum. L., Substance 

 of liver. Other ahbreviations as 

 before. 



