412 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



lumbar veins, and unite anteriorly with the front part 

 of the posterior cardinal veins (Fig. 138 A). 



Upon the formation of the posterior vertebral veins, 

 and upon the iaferior vena cava becoming more im- 

 portant, the middle part of the posterior cardinals be- 

 comes completely aborted (Fig. 139 c), the anterior and 

 posterior parts still persisting, the former as the con- 

 tinuations of the posterior vertebrals into the anterior 

 vena cava (aa), the latter as the hypogastric veins (%). 



Though in a few Mammalia both the posterior verte- 

 brals persist, a transverse connection is usually established 

 between them, and the one (the right), becoming the 

 more important, constitutes the azygos vein (Fig. 139 

 az), the persisting part of the left forming the hemi- 

 azygos vein Qia). 



The remainder of the venous system is formed in the 

 embryo by the vitelline and allantoic veins, the former 

 being eventually joined by the mesenteric vein so as to 

 constitute the portal vein. 



The vitelline vein is the first part of this system 

 established, and divides near the heart into two veins 

 bringing back the blood from the yolk-sac (umbilical 

 vesicle). The right vein soon however aborts. 



The allantoic (anterior abdominal) veins are origin- 

 ally paired. They are developed very early, and at first 

 course along the still widely open somatic walls of the 

 body, and fall into the single vitelline trunk in front. 

 The right allantoic vein disappears before long, and the 

 common trunk formed by the junction of the vitelline 

 and allantoic veins becomes considerably elongated. 

 This trunk is soon enveloped by the liver, and later in 

 its passage through, gives off branches to, and also 



