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 outgrowth 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 (vn. ace.), are extremely delicate; the exact places of 
union between them and the postcardinals 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 postcardinals are, but 
are here and there partly interrupted by flattened trabecule. In fact, 
posteriorly these trabecule 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 sthaller embryos the accessory veins are much shorter, but 
even in these they always open freely into the postcardinals, 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- 
q vum eight millimeters, the postcardinal and the accessory con- 
“VNAZ 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 (vn. hm’az.), retained its 
earlier connections and extended from the left Cuvierian 
duct along the left side of the vertebral column to a point 
some distance posterior to the last rib. That on the 
right side, the azygos vein (wn. 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 
Zo 
cam 
FIGURE 3. 
Fio. 3. Reconstruction of the azygos and hemiazygos veins from an embryonic 
pig about forty-eight millimeters long. Ventral view. 12. vn. az., azygos vein; 
vn. hm’az., hemiazygos vein. 
