18 DEVELOPMENT OF THE VEINS IN THE EMBRYO PIG. 



From the stage of about 13 mm. onward, the blood of the anterior third of the organ is 

 collected by three short trunks (a posterior cardinal vein, a ventral vein, and a short anterior 

 mesial vein) and drained into the duct of Cuvier. All of the rest of the blood of the organ 

 passes through transverse veins into the vena cava. The anastomosis of these two areas 

 through the capillary bed is very complete. This stage of the circulation is shown in 

 figures 4 and 5. The complete disappearance of the posterior cardinal veins, as the Wolftian 

 bodies disappear, will be followed in connection with the development of the azygos system. 

 The short trunk of the posterior cardinal vein in the cephalic part of the Wolffian body 

 eventually lies obliquely across the dorsal surface of the organ and is only in its primitive 

 position, just at the anterior pole (fig. 4). Its length is subject to great variation, even 

 on the two sides of the same embryo (as seen in fig. 4). The ventro-lateral vein and the 

 short mesial trunk are likewise limited to the anterior pole of the organ. The system of the 

 vena cava is represented in this figure only by the transverse vein of the left Wolffian body. 

 The relative position of the Wolffian body (in the two stages of 10 and 20 mm.) with refer- 

 ence to the duct of Cuvier is to be seen at a glance by comparing figures 3 and 4. The 

 Wolffian body is gradually dragged caudalward, and hence the part of the posterior cardinal 

 vein anterior to the organ itself is gradually lengthened. Subsequently the anterior pole 

 of the Wolffian body is pushed far to the side by the developing kidney. 



From the preceding description it will be seen that the circulation of the Wolffian body 

 belongs to the primitive type that is, it is a circulation in which the veins never accom- 

 pany the artery, but rather run as far as possible from it. The arteries are in the center of 

 the organ and the veins on the periphery. In the early stages three longitudinal veins make 

 up the cardinal system, draining into the duct of Cuvier. These three veins are very abun- 

 dantly connected by transverse veins. As soon as the vena cava is established by the con- 

 nection of the right mesial cardinal with the liver, a marked development of the transverse 

 system takes place by which the circulation of about two-thirds of the organ is shifted from 

 the duct of Cuvier to the vena cava. In this process the posterior cardinal vein eventually 

 disappears. 



AZYGOS AND HEMIAZYGOS VEINS. 



GENERAL SURVEY. 



The azygos and hemiazygos veins are new longitudinal veins which develop in relation 

 to these changes in the circulation of the Wolffian bodies and in connection with changes in 

 the circulation of the body-walls. They are not transformed posterior cardinal veins. 

 The facts following have been emphasized already: that the posterior cardinal vein drains 

 primitively the spinal veins and the caudal veins; that there are two primitive veins of the 

 Wolffian bodies, the posterior and the mesial cardinal ; and that the posterior cardinal system 

 breaks up within the Wolffian body into a number of veins after the mesial cardinal estab- 

 lishes its connection with the liver. The vena cava, which then drains most of the Wolffian 

 bodies and thus subsequently the renal veins and the iliacs, may be considered the substitute 

 for the cardinal system of the Wolffian bodies. The anterior portion of the posterior cardinal 

 system draining the anterior pole of the Wolffian bodies disappears entirely with the 

 Wolffian bodies, and the corresponding spinal veins drain into the new azygos system. 

 The remnants of the posterior cardinal system in the caudal pole of the organ likewise 

 disappear, and the corresponding spinal veins drain into the vena cava and ascending 

 lumbar veins. Confusion has arisen from the conception that the azygos veins and a part 

 of the inferior vena cava were transformed posterior cardinal veins. 



The prevailing view in regard to the origin of the azygos veins has developed through 

 the early work of Rathke and especially through that of Hochstetter. Rathke's first idea 



