626 



THE DIGESTIVE SYSTEM 



)^\ LUNGS 

 TOMACH\ I -f^^"/ 



RIGHT ^fc-:ifi '■ LEFT 



AT METAMORPHOSIS 



Fig. 296. Development of coils in the digestive tracts in the dog shark, Squalus 

 acanthias, and in the frog, Rana pipiens. (A) Squalus acanthias embryo of 110 mm. 

 (B-F) Rana pipiens, digestive tube development, shown from ventral aspect. Arrows in 

 B and C denote primary movements of the primitive gut tube resulting in condition shown 

 in D. 



e) Human Embryo. In the human embryo, the liver arises in a similar 

 manner to that of the pig embryo from the ventral wall of the foregut, just 

 posterior to the forming stomach (fig. 294F). The hepatic outpushing invades 

 the area of the ventral mesentery and becomes intimately associated with the 

 substance of the septum transversum (fig. 362H). Secondary evaginations or 

 liver cords ramify extensively within the mesenchyme of the mesentery, and the 

 vitelline or omphalomesenteric veins, as in other vertebrates, become broken 

 up into sinusoids, surrounding the outgrowing hepatic cords. The gallbladder 

 arises as a secondary outgrowth from the posterior wall of the original hepatic 

 outgrowth (fig. 294F). The gallbladder rudiment enlarges distally and gives 

 origin to the cystic duct which joins the common bile duct. 



2) Histogenesis of the Liver. As the liver pushes out into the ventral mesen- 



