*3 2 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



proximal end of the inferior vena cava. The smaller right umbilical vein after 

 entering the liver breaks up into sinusoids. It soon atrophies, while the left vein 

 persists until after birth. 



The Vitelline Veins. — Of these a distal portion of the left and a proximal 

 portion of the right are persistent. The left vitelline vein, fused with the right, 

 courses from the yolk-sac cephalad to the intestinal loop. Near its dorsal anas- 

 tomosis with the right vein just caudal to the duct of the dorsal pancreas, it 

 receives the superior mesenteric vein, a new vessel arising in the mesentery of the 



Ventral 



pancreas 



Caecum- 



Lvitelline 

 Vein 



5ma!l inlesT 



Sup mesenteric 

 vein 



L-UmbilicaJ Vein 



Allantois 



R. umbilical Vein • 



ft. umbilical artery 



Fir,. 122. — Reconstruction of a 10 mm. pig embryo to show the umbilical and vitelline veins from the 

 ventral side. X indicates sinusoidal connection between left umbilical vein and portal vein. 



intestinal loop. Cranial to its junction with the sup. mesenteric vein, the left 

 vitelline with its dorsal anastomosis and the proximal portion of the right vitel- 

 line vein form the portal vein, which gives off branches to the hepatic sinusoids 

 and connects with the left umbilical vein. For the development of the portal 

 vein, see Chapter IX. 



Transverse Sections of a 10 mm. Pig Embryo 

 Figures arc shown of sections passing through the more important regions 

 and should be used for the identification of the organs. The level and plane of 



