258 



TEXT-BOOK OF EMBRYOLOGY. 



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 an- 

 terior set persists for the most part as permanent vessels and increases with the 

 development of the body, the posterior set undergoes regressive changes, its 

 function being taken by a new vessel — the inferior vena cava. 



Not long after the appearance of the posterior cardinals, another pair of longi- 

 tudinal veins appears in the medial part of the mesonephroi. They increase 



Ext. jugular 



Innominate (right) 



Iliac" 



Fig. 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. 



in size as the mesonephroi increase and receive blood from the latter. They 

 also communicate with the cardinals by means of transverse channels which, 

 however, are later broken up as the mesonephroi become more complicated in 

 structure. These vessels are known as the subcardinal veins, or revehent veins 

 of the primitive kidneys (Fig. 232, A). After they have attained a considerable 

 size, a large anastomosis is formed between them ventral to the aorta and just 

 caudal to the omphalomesenteric (superior mesenteric) artery (Fig. 232, B). In 

 the meantime, a branch of the ductus venosus (see p. 263) grows caudally 

 through the dorsal part of the liver and the mesentery, and joins the right 



