THE HYPOMERE (LATERAL PLATE MESODERM) 241 



later to become the hepatic vein emptying into the sinus venosus. The 

 right posterior vitelline vein shortly disappears and the left one re- 

 mains as the hepatic portal vein, bringing blood from the viscera to 

 the liver. Shortly after hatching, a hepatic vein arises from the liver 

 substance to pass directly to the sinus venosus via the path of the 

 original anterior vitelline vein. After the development of the posterior 

 vena cava the two main liver lobes direct their separate hepatic veins 

 into the sinus venosus. 



The paired common cardinals grow obliquely in a dorso-lateral 

 direction from the sinus venosus toward the body wall and then divide 

 to send branches anteriorly and posteriorly. The paired outgrowths 

 of the sinus venosus are known as the ducts of Cuvieri or Cuvierian 

 sinuses. The anteriorly growing extension of the common cardinal 

 vein is the anterior cardinal vein. This receives blood from the tongue, 

 hyoid, thyroid, parathyroid, and the floor of the mouth by a branch 

 known as the external (inferior) jugular vein. It also receives blood 

 from the brain, shoulder, and forelimb by way of another branch, the 

 internal (superior) jugular vein. A third contributing vessel is the 

 subclavian vein which brings blood from the brachial vessel of the 

 forelimb and the cutaneous vein of the skin. All of these vessels con- 

 tribute to the original anterior cardinal vein and at their point of 

 junction are known as the anterior or superior vena cava. Since the 

 posterior cardinal veins fuse mesially and become separated from the 

 common cardinal, the anterior vena cava becomes the only remnant 

 of the embryonic ductus Cuvieri. 



The posterior cardinal veins are directed posteriorly from the 

 paired common cardinals (ductus Cuvieri). Each of these forms a 

 sinus around the temporary pronephric tubules and then grows pos- 

 teriorly as the single median cardinal vein between the mesonephroi 

 and into the tail. During its course it receives vessels from the dorsal 

 body wall. 



The posterior cardinals go through considerable change during 

 the pre-metamorphic development of the frog. There is degeneration 

 of the entire left cardinal and most of the right cardinal veins. The 

 sections posterior to the pronephric sinuses tend to fuse mesially to 

 form the single posterior vena cava (post-caval vein or inferior vena 

 cava) which empties directly into the sinus venosus, having moved 

 posteriorly from the ductus Cuvieri. The paired hepatic veins, from 

 the liver lobes, are now incorporated into the enlarged posterior vena 



