600 THE EMBRYOLOGY OF THE FR<><; 



gion of the blood-islands. They are paired but not alike on both sides. 

 They pass along the lateral surface of the yolk and liver, and enter the 

 sinus venosus. In fact, the sinus venosus is really formed by a fusion 

 of these vessels from each side. 



Ductus Cuvieri or Cuvierian Sinuses. These are a pair of large veins 

 which enter into the sinus venosus also and may even form part of that 

 organ. They come from the body-wall opposite the sinus venosus. 



Hepatic Vein. As the liver develops, the omphalomesenteric veins 

 which pass through that organ break up into capillaries within the sub- 

 stance of the liver. Then the parts of both omphalomesenteric veins, 

 which lie between the liver and the heart, fuse into a single hepatic vein. 



Hepatic Portal Vein. The right omphalomesenteric vein disappears 

 caudad to the liver, while the left partly remains as the root of the future 

 hepatic, portal vein. This vein will later receive branches from the 

 digestive tract as well as from those organs which have arisen from the 

 digestive tract. 



Anterior Cardinal Veins. As the ducts of Cuvier pass dorsad to the 

 dorsal body wall, they divide. One branch passes headward as the an- 

 terior cardinal vein. 



Superior Jugular Veins. This is the name given the anterior cardi- 

 nal veins as they pass forward into the head, where they drain the brain 

 and the dorsal portions of the head. 



Inferior Jugular Veins. These drain the mouth, sucker, and ventral 

 surface of the head, and open into the roots of the duct of Cuvier just 

 before these in turn enter the sinus venosus. 



Posterior Cardinal Veins. These are the posterior or caudal por- 

 tions of the divided ducts of Cuvier, and are primarily the drainage sys- 

 tem of the body-wall and excretory system. They pass caudad along the 

 medial side of the pronephric ducts and receive the veins from the body- 

 wall known as the segmental veins. The posterior cardinal veins form 

 large sinusoids in the region of the pronephros, but as the metanephros 

 develops, all this is modified, so that at fifteen millimeters the caudal 

 ends of the veins fuse to form the single median cardinal vein. 



Caudal Vein, This begins at the tip of the tail and drains that 

 region. It is unpaired, but upon reaching the body cavity, divides above 

 the cloacal region, and then empties into the posterior cardinal veins. 



Posterior or Inferior Vena Cava, or Postcaval Vein. This begins 

 as a branching of the left omphalomesenteric vein lying dorsal to the 

 liver. From here, the postcaval vein passes through the suspensory fold 

 of the liver to the right posterior cardinal vein and connects with it just 

 anterior to the point where the median cardinal vein begins. This vessel 

 enlarges rapidly and becomes the largest blood vessel in the body. It 

 passes through the liver to the sinus venosus. The hepatic vein then 

 opens into it instead of into the sinus venosus as formerly. 



Anterior, Superior, or Precaval Veins. The pronephric portions of 



