Charles F. W. McClure - 187 
The separation of the azygos system into two subdivisions (thoracic 
and lumbar) is a marked feature at this stage of its development. The 
separation may, however, be apparent rather than actual since a capillary 
anastomosis may exist between the two which cannot be determined in 
section. In later stages the two subdivisions do become connected so that 
the blood from the lumbar azygos tributaries is returned to the heart, for 
the most part, by the left thoracic azygos vein. In the 10.5 mm. pouch 
young, however, the large size of the azygos veins in the lumbar region 
precludes the possibility of any such route for all of the blood collected 
by them; and I am, therefore, inclined to believe that it is returned, for 
the most part, through capillaries directly to the postcava, which is the 
course pursued at a subsequent stage of development in which large and 
frequent anastomoses are formed between this vessel and the lumbar 
azygos veins. 
In the 8 mm. embryo the azygos veins of the lumbar region have not 
as yet been formed and this region is drained by the dorsal somatic trib- 
utaries of the postcardinal veins. This circumstance leads one to infer 
that the lumbar azygos veins as met with in the 10.5 mm. pouch young 
may also be formed from branches of the postcardinals and in the same 
manner as a portion of the azygos veins in the thoracic region, although, 
on account of the lack of intermediate stages, it is impossible to deter- 
mine this question. 
The azygos veins in the 11.5 mm. pouch young appear to present the 
same arrangement as in the preceding stage, although on account of the 
circumstance that the specimen was cut along the frontal plane it is dif- 
ficult to determine the exact extent of the thoracic azygos veins, as well 
as whether a direct anastomosis exists between them and the azygos veins 
of the lumbar region. There can be little doubt, however, if such an an- 
astomosis exists that it is still of minor importance as compared with 
that at a later stage, and that the thoracic and the lumbar azygos veins 
are, as in the 10.5 mm. pouch young, practically independent of each 
other. 
At the caudal end of the body the sections are cut almost at right 
angles to the long axis of the body so that in this region the lumbar 
azygos veins are not difficult to follow. 
In the region of the permanent kidneys and craniad of the point where 
the left anterior revehent vein joins the posteava (Fig. 49, Plate IV) 
the lumbar azygos veins are extremely prominent and lie, for the most 
part, dorsal to the segmental branches of the aorta. Between its junc- 
tion with the left anterior revehent vein and that with the two post- 
cardinals, the posteava gradually approaches the aorta and in the region 
