136 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



dinal receives blood not only from the anterior end of the mesonephros 

 but from the region between the two mesonephroi. This is accomplished, 

 however, not by the postcardinal proper, but by a new outgi'owth that 

 takes its origin from the postcardinal near the anterior end of the me- 

 sonephros. The walls of these new vessels, which may be called the 

 accessory veins (yn. ace), are extremely delicate ; the exact places of 

 union between them and the postcardiuals is indicated not only by the 

 general topography, but also by the rather abrupt change in the thick- 

 ness of the walls where the accessory vessel meets the postcardinal. 

 Moreover, the cavities of the accessory vessels are not freely open 

 throughout their whole extent, as those of the postcardiuals are, but 

 are here and there partly interrupted by flattened trabeculse. In fact, 

 posteriorly these trabeculse become so numerous that the cavities of the 

 vessels are finally merged in the interspaces thus formed. The acces- 

 sory vessels at this stage may be traced posteriorly to a point about 

 midway the length of the mesonephros. 



In slightly smaller embryos the accessory veins are much shorter, but 

 even in these they always open freely into the postcardiuals, and we 

 therefore believe them to be outgrowths of the postcar- 

 dinal vessels. The place of union between the accessory 

 vessels and the postcardinals in the specimens studied 

 was at the level of the tenth rib, and the accessory ves- 

 sels could usually be traced posteriorly some distance 

 beyond the last or fourteenth rib. 



In an embryo whose greatest length was about forty- 

 eight millimeters, the postcardinal and the accessory con- 

 stituents of each vein could no longer be distinguished, 

 for they had fallen so well into line with each other that 

 they were represented by a perfectly continuous vein 

 (Figure 3). That on the left side, which may now be 

 called the hemiazygos vein (v7i. hni'az.), retained its 

 earlier connections and extended from the left Cuvierian 

 duct along the left side of the vertebral column to a point 

 iGUKE . ^Q^Q distance posterior to the last rib. That on the 

 right side, the azygos vein {vn. az.), had lost its anterior connection with 

 the right Cuvierian duct, but otherwise extended over a tract corre- 

 sponding in the main to that of the hemiazygos. The blood collected 



Fig. 3. Reconstruction of the azygos and hemiazygos veins from an embryonic 

 pig about forty-eight millimeters long. Ventral view. X 12. vn. az., azygos vein ; 

 vn. hm'az., hemiazygos vein. 



-■- VNMM'*2 



