DEVELOPMENT OF THE VEIN'S IN T1IK EMHKVO I'I<;. 29 



of the pig. Its interest is increased by the fact (hat Kampmeier (1912) thought that it 

 disappeared and was replaced by the thoracic duct. The thoracic duct dues develop along 

 the course of this vein and Kampmeier stated that beyond the stage of 'l'\ mm. (see lig. I > 

 this vein disappeared. A glance at figure 17 (from a pig 28 mm. kmg) and again al figure IS 

 (from a pig 40 mm. long) will show at once that it is on the contrary a developing vein. 



The two figures of the stage measuring 2S mm. (namely figures 10 and 17) show a 

 number of important points: first, the relative and marked development of the right 

 oblique vein and the left descending azygos (accessory hemiazygosi ; secondly, the drag- 

 ging of both right and left posterior cardinal veins along the azygos veins as the \Yolffian 

 body comes to lie farther and farther caudalward. It is now much plainer that the pos- 

 terior cardinals are becoming tributaries of the azygos system than it was in figure .">. 

 Figure 10 is intended to show that the left descending azygos (accessory hemiazygos) is 

 present alongside of the anterior cardinal to the duct of Cuvier and that it lies dorsal 

 to the anterior cardinal. 



The essential characteristics of the azygos veins are also shown in figure IS from a 

 specimen measuring 40 mm. This particular injection shows a variation of the usual 

 pattern in the pig, which is that the right and left veins are about equal, as in figure 17. 

 In figure 18, however, the marked anastomosis of the two veins, which is pretty constant 

 opposite the eighth to the tenth ribs (see fig. 17), has become very large, and below that 

 point a single trunk represents both veins. 



The development of the azygos system makes its nomenclature very simple: On either 

 side the azygos opens into the duct of Cuvier. According to the position of the duct of 

 Cuvier there is a short descending branch, the so-called superior intercostal and accessory 

 hemiazygos, and a long ascending branch, the so-called azygos and hemiazygos. For 

 the pig, where both ducts of Cuvier are permanent, the only nomenclature necessary is 

 ascending and descending right and left azygos veins, together with the special branch, the 

 right oblique vein. 



CONCLUSIONS. 



This study has demonstrated that the posterior cardinal veins disappear with the 

 Wolffian bodies. They are not transformed either into a part of the vena cava or into a 

 part of the azygos veins. They develop along the edge of the Wolffian body and disappear 

 with that organ. The vena cava is primarily a vein to drain the Wolffian bodies, and, being 

 formed from the omphalo-mesenteric vein within the liver, and from the mesial cardinal 

 vein below the liver, may be said to be foreshadowed in the primitive venous system of the 

 embryo. Subsequently, as the Wolffian bodies disappear the lower segment of the vena 

 cava develops from a prevertebral caval plexus by which the caudal veins and veins of tin- 

 legs become shifted from the Wolffian bodies to the vena cava. 



The azygos veins are new prevertebral veins, representing a vascular arch between the 

 spinal arteries and the cardinal system of veins. They drain into the anterior and the 

 posterior cardinal veins by very numerous connections. Of the many connections the 

 ones which remain as the permanent ones are those opposite the ducts of Cuvier. and hence 

 no part of the posterior cardinal vein is incorporated in the azygos system. The position 

 of the duct of Cuvier in the adult determines the length of the ascending and descending 

 branches. The more dorsal position of the azygos system throughout its development 

 enables one to distinguish it from the cardinal system at every stage. The development of 

 the azygos system enables one to simplify its nomenclature into right and left ascending 

 and descending azygos veins. 



