126 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



cardinal veins anastomose just caudal to the origin of the superior mesen- 

 teric artery, and the posterior cardinals are interrupted at this level. The 

 proximal portions of the posterior cardinals open into the common cardinal 

 veins as in the 6 mm. embryo. Of the two subcardinal veins, the right 

 has become very large through its connection with the right posterior 

 cardinal vein and the common hepatic vein, and now forms the middle 

 portion of the inferior vena cava. For the development of this vein, see 

 Chapter IX. 



Spinal cord 



Nolochord 

 Pharynx 



R. ant. cardinal vein 

 Pericardial cavity 

 Sinus venosiis 



Inf. vena cava 



Portal vein 



Ventral pancreas 



Cacum 



L. vitelline vein 

 Small intestine 



Sup. mesenleric vein 

 R. umbilical vein 

 R. umbilical artery 



Ant. cardinal vein 

 Esophagus 



Trachea 



Upper limb 



Common car- 

 dinal vein 



Ductus venosus 



Liver 



Py/oric stomach 

 Hepatic diverliculum 

 Dorsal pancreas 

 Duodenum 

 .. umbilical vein 



Allantois 



PIG. 127. Reconstruction of a JO 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. X 15. In the small orientation figure (cf. Fig. 123) the various planes are indi- 

 cated by broken lines * *. 



The umbilical veins (Figs. 126 and 127) anastomose in the umbilical 

 cord, separate on entering the embryo, and course cephalad in the ventro- 

 lateral body wall of each side to the ventral lobe of the liver. The left 

 vein is much the larger, and, after entering the liver, its course is to the 

 right and dorsad. After connecting with the portal vein, it continues 

 as the ductus venosus and joins the 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, 



