THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



257 



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

 jetween the subclavian and the anastomosis (left innominate vein) becomes 

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

 nnominate constitutes the superior vena cava which opens into the right 

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

 iperior cardinal as an independent vessel which comes to lie parallel to the 

 lal jugular and opens into it near the subclavian. The opening, however, 

 to the subclavian, where it is usually found in the adult (Figs. 233 and 234). 



Ant. cardinal 



Duct of Cuvier 

 Subclavian - 



Inf. vena cava 



Post, cardinal 



Subcardinal 



Iliac 



.... Ant. cardinal 

 (int. jugular) 



Iliac 



A B 



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

 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. 



I he changes which occur in *he posterior cardinal veins are very extensive 

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

 >. In connection with these changes the development of the inferior 

 cava must be considered. The posterior cardinal veins appear very early 

 ired, bilaterally symmetrical vessels which extend from the duct of Cuvier 

 tail region and are situated ventro-lateral to the aorta (Fig. 231). 

 ie first they receive blood from the body wall through segmental branches, 

 is the primitive kidneys (mesonephroi) develop they receive blood from 

 also, as well as from the mesentery. They return practically all the 

 n the region of che body situated caudal to the heart, just as the 



