THE DEVELOPMENT OF THE VASCULAR SYSTEM 



255 



vessel which comes to lie parallel to the internal jugular and opens into it 

 near the subclavian. The opening, however, shifts to the subclavian, 

 where it is usually found in the adult (Figs. 323 and 234). 



The changes which occur in the posterior cardinal veins are very extensive 

 and result in conditions which bear but little resemblance to those in the 

 earlier stages. In connection with these changes the development of the 

 inferior vena cava must be considered. The posterior cardinal veins appear 

 very early as paired, bilaterally symmetrical vessels which extend from the 

 duct of Cuvier to the tail region and are situated ventro-lateral to the aorta 



Ant. cardinal 

 (int. jugular) 



Ext. jugular - 



Innominate (right) 



Sup. vena cava - 



Post, cardinal 

 (azygos) - 



Inf. vena cava ... 



Subcardinal 



Subcardinal 



Hiac 



FlG. 233. Diagram representing a stage (later than Fig. 232) in the development of the superior 

 vena cava and the inferior vena cava, also of the azygos vein. Hochstetter. 



(Fig. 231). From the first they receive blood from the body wall through 

 segmental branches, and as the primitive kidneys (mesonephroi) develop 

 they receive blood from them also, as well as from the mesentery. They 

 return practically all the blood from the region of the body situated caudal 

 to the heart, just as the anterior cardinals return the blood from the region 

 of the body situated cranial to the heart. In other words, the two sets of 

 cardinal veins are the body veins par excellence during the earlier stages of 

 development. While the anterior set persists for the most part as permanent 

 vessels and increases with the development of the body, the posterior set 



