THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



257 



the more distal part of the cardinal becomes the internal jugular vein; the por- 

 tion between the subclavian and the anastomosis (left innominate vein) becomes 

 the right innominate vein ; and the common stem formed by the latter and the 

 left innominate constitutes the superior vena cava which opens into the right 

 atrium (see p. 231). The external jugular vein on each side appears later than 

 the superior cardinal as an independent 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. 233 and 234). 



Ant. cardinal 



Duct of Cuvier 

 Subclavian - 



Inf. vena cava 



Ant. cardinal 

 (int. jugular) 



Post, cardinal 

 Subcardinal 



Iliac 



Iliac 



FIG. 232. Diagrams of two stages in the development of the anterior and posterior cardinal veins, 

 the subcardinal veins (revehent veins of the primitive kidney), and the inferior vena cava. 

 The small branches of the cardinals and subcardinals ramify in the primitive kidneys 

 (mesonephroi). Slightly modified from Hochstetter. 



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 (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 



