16 DEVELOPMENT OF THE VEINS IN THE EMBRYO PIG. 



transverse veins receive many vessels from the depth of the organ (fig. 6) and have tiny 

 anastomoses with each other. 



The transverse veins of the mesial surface (fig. 2) connect the mesial vein with the 

 other two. They are always larger than the lateral and are few in number. The import- 

 ance of the transverse veins is that they develop into the largest tributaries of the vena 

 cava as long as the Wolffian bodies persist. This transition will be readily followed by 

 comparing figures 2, 3, 4, and 5, from embryos 8,11, 22, and 23 mm. long. By means of the 

 marked development of the transverse veins the circulation of the caudal two-thirds of the 

 organ is shifted over to the mesial vein (fig. 5, plate 2). Indeed, the entire relation of the 

 vena cava is foreshadowed in figure 2, from an embryo pig 8 mm. long, in the large anasto- 

 mosis of the mesial and posterior cardinal veins about the middle of the organ. Within the 

 Wolffian body the lateral veins opposite the median mesonephritic vein become the chief roots 

 of the vena cava, so that from the dorsal view the vena cava appears to bifurcate into these 

 great transverse veins (fig. 14, plate 6). The median mesonephritic vein corresponds in the 

 adult to the vena cava opposite the renal veins. Thus the posterior cardinal vein becomes 

 more and more limited to the anterior pole of the organ (fig. 4), while in the caudal pole only 

 remnants remain connecting the transverse veins (figs. 5 and 14). This change (by 

 which the posterior and ventral veins become limited to the anterior pole of the organ) takes 

 place while the embryo is developing from 12 to 13 mm. in length. The posterior cardinal 

 vein, then, does not become a permanent part of the vena cava, but, disappears as the Wolf- 

 fian body disappears. This point will not be wholly proved until the subject of the rela- 

 tion of the primitive fibular and caudal veins is taken up in detail. In general, in the stage 

 represented by figure 3, the caudal vein is a continuation of the posterior cardinal vein 

 and the blood from the tail and from the hind limb bud passes through the Wolffian body 

 to the heart, either by way of the posterior cardinal vein or by way of the vena cava. At 

 the stage of 12 to 13 mm. the changes of the circulation within the Wolffian body bring 

 about the condition that almost all of the blood from the tail and hind limb bud passes 

 through the veins of the Wolffian body, chiefly through remnants of the posterior cardinal 

 veins in the caudal half of the organ, to the vena cava; while at the stage of 22 mm. (shown 

 in fig. 5) most of the blood from the tail and hind limb bud passes around the Wolffian 

 body to the vena cava and only a small part of it takes the earlier route through the veins 

 of the Wolffian body. The process by which this shifting of the circulation around the 

 Wolffian body takes place is bound up with the study of the prevertebral plexus of veins, 

 which is taken up later, first in connection with the azygos veins and secondly in connection 

 with the vena cava. 



The importance of figure 5 is that it represents all of the permanent fundamental rela- 

 tions of the adult vena cava. The inferior vena cava thus consists primarily of a part within 

 the liver derived from the omphalo-mesenteric vein and certain hepatic sinusoids, and 

 of a part within the Wolffian body derived from the mesial cardinal veins and their tribu- 

 taries, the transverse veins connected by remnants of the posterior cardinal veins. At first 

 the vena cava drains the tail and hind legs only through the Wolffian body. Subsequently 

 the blood from the caudal end of the embryo is shifted around the Wolffian body and 

 that organ disappears. From this history the vena cava, inasmuch as it arises mainly 

 from the omphalo-mesenteric and mesial cardinal veins, is to be regarded as one of the 

 earliest veins of the embryo rather than as a late development. The final vena cava is 

 made from three main sources: above and within the liver mainly from the omphalo- 

 mesenteric veins; in the interrenal segment from the mesial cardinal veins; and below the 

 kidneys from the prevertebral plexus. In other words, its ventral segment comes from the 



