THE CIRCULATORY SYSTEM 601 



the postcaval veins degenerate as the pronephroi degenerate, and ulti- 

 mately disappear entirely, even before metamorphosis is complete These 

 leave the ductus Cuvieri as the proximal portions of the anterior cardi- 

 nal veins, and it is these remaining proximal portions which are called 

 the anterior, superior, or precaval veins. All blood from the posterior 

 parts of the body-wall and from the tail now passes directly to the heart 

 through the median cardinal and postcaval veins. 



Iliac Veins. The pronephroi are followed by the mesonephroi as 

 in the chick, and an alteration in the relation of the median cardinal vein 

 follows. On each side of the body the developing mesonephroi push 

 into the median cardinal vein, so that this vein is divided into one me- 

 dian and two lateral parallel channels. The caudal vein empties into 

 the median channel and finally disappears, and the iliac veins which 

 :ome from the hind-legs open into the lateral channels. It is the iliac 

 ve'ms which become the chief vessels leading to the mesonephric region 

 after the caudal vein disappears. 



Adult Venous System. After an understanding of the formation 

 and change which takes place in the venous system during the embryonic 

 period, the adult system can be understood. 



Afferent or Advehent Mesonephric Veins, or Renal Portal Veins. 

 These are merely the iliac veins, together with the lateral channels of 

 the median cardinal vein, with which they are continuous. 



Posterior Vertebral Veins. These are the small veins from the pos- 

 terior body w r all which open into the renal portal vein. 



Renal Veins, or Revehent Mesonephric Veins. These are the short 

 connecting vessels which connect the vascular space in the mesonephroi 

 with the median channel of the median cardinal vein, so that only this 

 median channel remains as a posterior continuation of the postcaval 

 vein. 



Lateral Veins. A pair of these develop late in the ventral abdominal 

 walls, and open into the sinus venosus. These connect with the iliac 

 veins posteriorly, then fuse medially. 



Anterior Abdominal Vein. The anterior ends of the lateral veins 

 lose their connection with the sinus venosus, while the anterior portion 

 of the right lateral disappears entirely. The left lateral vein forms a 

 new connection with the hepatic portal vein, and is then called the an- 

 terior abdominal vein. 



Pulmonary Veins. These can be seen when the tadpole is about 

 six millimeters in length as projections of the endothelium on the dorsal 

 side of the sinus venosus. These projections form a tube,* opening proxi- 

 mally into the left side of the auricle, which distally leaves the wall of 

 the sinus venosus, and passes dorsally to the lung rudiments. This tube 

 bifurcates at the base of the lungs, where each branch then passes along 

 the medio-ventral side of the lung rudiment. After the lungs begin to 

 function, the pulmonary veins empty into the left auricle. 



