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 primordia. 

 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 /^«- 104. — Reconstruction of the 



,^r„n ^e 4-u • 4- 4.- c -1 liver of the chick at the end of 



wall 01 the mtestme oi consider- .i r .i , r ■ ^ .- 



, Til ^'^^ lourth day oi incubation, 



able extent; a little later the base (After Hammar ) 



of the thickening is hollowed out, du., Duodenum. L., Substance 



and the free margin sends off solid ?^ li^^r. Other abbreviations as 



beiore. 

 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. 



