546 THE VEINS OF MAMMALIA. 



common iliac veins there is no renal portal system like that ()f the Reptilia 

 and Amphibia. 



Posterior vertebral veins, similar to those of Reptilia and Birds, are 

 established in connection with the intercostal and lumbar veins, and unite 

 anteriorly with the front i)art of the posterior cardinal veins (fig. 373 A)'. 



On the formation of the posterior vertebral veins, and as the inferior 

 vena cava becomes more important, the middle part of the posterior car- 

 dinals 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 (as), 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 s\ sti-m 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 con- 

 siderably elongated. This trunk is soon enveloped by the liver. 



Tlie succeeding changes have been somewhat differently described by 

 Kolliker and Rathke. According to the former the common trunk of the 

 allantoic and vitelline veins in its passage through the liver gives off 

 Ijranches 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 v/hich originally brought back blood into the ductus 

 venosus. The ductus venosus becomes moreover merely a tmall branch of 

 the vena cava. 



At the close of foetal 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 poi'tal vein''. 



1 Eathke, as mentioned above, holds that in the Snake the front part of the 

 posterior cardinals completely aborts. Further investigations are required to shew 

 whether there really is a difference between Mammalia and Reptilia in this matter. 



2 According to Eathke the original trunk connecting the allantoic vein directly with 



