1 80 EMBRYOLOGY 



Development and regression of the 

 renal portal circulation 



Changes in the blood system are correlated with changes in the structure 

 of other organs, such as the kidney (Figs. 102 and 106) . During the develop- 

 ment of the pronephros the blood from the body of the embryo is drained 

 by the anterior and posterior cardinal veins and a jugular vein. The posterior 

 cardinal empties directly into the heart by means of the common cardinal. 

 During the development of the mesonephros, however, some very striking 

 changes occur in this simple system. 



First of all, a subcardinal vein develops parallel to the posterior cardinal 

 vein and enters it (Fig. 107). Second, a new vessel develops between the 

 ductus venosus in the liver and the anterior end of the subcardinal. This new 

 vessel, when completed, opens up a new channel to the heart via the liver. 

 With this new channel open, the former circulation through the posterior 

 cardinal vein is shut off and blood is forced through vessels in the meso- 

 nephros. This means that blood from the posterior parts of the embryo flows 

 through the posterior cardinal vein, through the mesonephros into the 

 subcardinal and through the new vein, the inferior vena cava, through the 

 liver and into the heart. This type of circulation is a renal portal circulation, 

 in which the mesonephros carries the blood. This renal portal circulation is 

 found in the adult frog but is not the system of the adult chick. 



The renal portal circulation is gradually replaced by the renal veins, 

 which drain the metanephros. The metanephros develops as the mesonephros 

 breaks down. With this breakdown a new connection is made between the old 

 posterior cardinal vein and the inferior vena cava (Fig. 108) with the result 

 that blood coming from posterior parts flows directly to the heart and is not 

 carried by the kidney. Branches of the inferior vena cava supply the legs, 

 tail, and internal organs, and the system takes on the aspect of the adult 

 circulatory system. 



Hepatic portal system 



Changes in the circulatory system accompany the development of the 

 liver (Fig. 109). During early development the blood coming into the 



