DEVELOPMENT OF T1IK VKINS IN' THE EMKUYO IMC. !'."> 



now drain through an excessively complex venous plexus, either directly into the median 

 mesonephritic vein or vena cava, or into its direct tributaries. This is clear in figure 12, 

 from an embryo 20 mm. long. It is this venous plexus which surrounds the cisterna chyli at a 

 slightly later stage and makes it practically impossible to get a pure lymphatic injection In- 

 direct puncture. Returning to figure 11, the fifteenth and sixteenth together and the 

 seventeenth spinal vein still drain through the Wolffian body. Opposite the seventeenth 

 segment is a transverse lumbar vein which drains into the \Yolfiian body. It is analogous 

 to the thoraco-epigastric vein. 



On the other hand, the common stem of the fibular and caudal veins, shown cut off in 

 figure 11, no longer drains mainly into the veins of the Wolffian body as it did in the 

 earlier stages (fig. 3), but has been shifted, by means of the sub vertebral plexus shown 

 in figure 12, to the median mesonephritic vein or the vena cava. The history of this 

 process is best studied in thick serial sections of injected embryos. In embryo pigs up 

 to 19 mm. long the common stem of the primitive fibular and caudal veins still enters 

 remnants of the posterior cardinal vein in the dorso-median angle of the caudal pole of the 

 Wolffian bodies. These remnants of the posterior cardinal vein connect the transverse 

 veins (figs. 5 and 14) and join the vena cava. The caudal vein, however, connects freely 

 with the prevertebral plexus, as shown, for a level farther cerebralvvard in figure 12. 

 When the embryo pig is about 20 mm. long, the permanent kidney begins and lies against 

 the posterior cardinal vein in the Wolffian body and at that time the anastomoses of the 

 caudal vein with the prevertebral plexus develop to such an extent that more of the blood 

 of the caudal veins passes through the prevertebral plexus mesial to the permanent kidneys 

 than through the more lateral primitive pathway through the caudal pole of the Wolffian 

 body. This change in the circulation is shown in the diagram of figure 14, which repre- 

 sents the conditions found in embryo pigs measuring from 20 to 27 mm. In the preverte- 

 bral plexus (as seen in figure 12) symmetrical longitudinal trunks develop lateral to the 

 aorta, which shunt the blood from the caudal pole of the Wolffian body to the median 

 mesonephritic vein. The diagram of figure 14 is given to analyze the large vein which 

 represents the bifurcation of the vena cava at this stage. In the mid-line is the anastomosis 

 of the two mesial cardinal veins and this central vein leads on either side into three main 

 dorsal, transverse veins of the Wolffian body. The number of these great transverse veins 

 is not constant, but they drain all but the cephalic pole of the organ. These transverse 

 veins are connected along the dotted line of the diagram by remnants of the posterior car- 

 dinal veins, which are marked by crosses. A remnant of the posterior cardinal vein 

 is shown passing ventral to the kidney. These remnants of the posterior cardinal 

 veins (as can be seen in the diagram, or better in the actual sections) lie within the Wolffian 

 body, distinctly lateral to the sub vertebral plexus. They have the same position with 

 reference to the Wolffian body as the posterior cardinal vein in figure 6, and when the 

 Wolffian body disappears they disappear with it, as is indicated by the dotted line on figure 

 14. Thus the statement of Kerschner (1888, p. 813), referring to the lower segment of the 

 vena cava, is entirely correct: "Bin medial und ventral von den Urnieren (den Niereiu 

 gelagertes Gefass kann den Cardinal Vene nicht entsprechen." 



In the stages represented by figure 14 it can be said that symmetrical vena? cava? empty 

 into a large median vein which has remnants of the posterior cardinal vein along its sides; 

 but when the Wolffian bodies disappear, only the median part of this great vein persists 

 as a part of the vena cava; the more lateral part, consisting of the posterior cardinal veins 

 and the transverse veins of the Wolffian bodies, disappears with that organ and hence the 

 inferior segment of the inferior vena cava (which has been thought to be a persistent pos- 



