On the formation of the posterior vertebral veins, and as the inferior vena cava becomes more important, the middle part of the posterior cardinals becomes completely aborted ([fig. 374], c), the anterior and posterior parts still persisting, the former as the continuations of the posterior vertebrals into the anterior vena cava (az), the latter as the hypogastric veins (hy).
Though in a few Mammalia both the posterior vertebrals persist, a transverse connection is usually established between them, and the one (the right) becoming the more important constitutes the azygos vein ([fig. 374], az), the persisting part of the left forming the hemiazygos vein (ha).
The remainder of the venous system is formed in the embryo of 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-sack (umbilical vesicle). The right vein soon however aborts.
The allantoic (anterior abdominal) veins are originally 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.
The succeeding changes have been somewhat differently described by Kölliker and Rathke. According to the former the common trunk of the allantoic and vitelline veins in its passage through the liver gives off branches to the liver, and also receives branches from this organ near its anterior exit. The main trunk is however never completely aborted, as in the embryos of other types, but remains as the ductus venosus Arantii.
With the development of the placenta the allantoic vein becomes the main source of the ductus venosus, and the vitelline or portal vein, as it may perhaps be now conveniently called, ceases to join it directly, but falls into one of its branches in the liver.
The vena cava inferior joins the continuation of the ductus venosus in front of the liver, and, as it becomes more important, it receives directly the hepatic veins which originally brought back blood into the ductus venosus. The ductus venosus becomes moreover merely a small branch of the vena cava.
At the close of fœtal life the allantoic vein becomes obliterated up to its place of entrance into the liver; the ductus venosus becomes a solid cord—the so-called round ligament—and the whole of the venous blood is brought to the liver by the portal vein[239].
Owing to the allantoic (anterior abdominal) vein having merely a fœtal existence an anastomosis between the iliac veins and the portal system by means of the anterior abdominal vein is not established.