VEINS AND LYMPHATICS IN TRAGULUS 211 



plete. The cephalic division of the pre-azygos segment has met 

 and fused with the thoracic duct approach of the sac. The 

 caudal division is still independent. 



Post-azygos segment {Ug. 10, 29). This segment has also at- 

 tained more definite outline. Its caudal dilatation is larger and 

 its ilio-lumbar appendages more extensive, so that upon the left 

 side there is an uninterrupted thoracic duct line, which has 

 resulted from the fusion of the pre-azygos, azygos and post- 

 azygos segments. The right duct line is still incomplete. The 

 ventral descending process of the sac on both sides has already 

 established connection with the truncus broncho-mediastinalis. 

 In the main, this trunk is still a dense plexus situated ventral 

 to the thymus; in several places, however, it loses its plexiform 

 character to become a distinct channel. Retaining these general 

 relations to the thymus; it passes to the caudal extremity of 

 that organ, where it undergoes considerable reduction, but may 

 be traced across the aortic arch to the root of the lung. 



THE VENOUS ORGANIZATION IN THE LATE EMBRYONIC STAGES 



The advance in the venous organization in the 20 mm. embryo 

 depends on a modification in the relations between the umbilical 

 and omphalomesenteric veins. Coalition of the right sub-cardi- 

 nal with the right emphalomosenteric has produced a definite 

 post-caval system as far caudad as the renal anastomosis. Below 

 this level the cava arises in a manner somewhat different from 

 that observed in the majority of mammals already studied. 



Renal anastomosis and cardinal collateral veins 



Upon reaching the level of the kidneys, the post-caval drainage 

 line becomes greatly expanded to form a large, irregularly quad- 

 rilateral channel situated in front of the aorta. This large chan- 

 nel establishes the renal anastomosis and, from its position, 

 may be termed the inter-renal segment of the post-cava. It 

 presents two cephalic and two caudal angles. From its right 

 cephalic angle the post-caval drainage line is continued toward 

 the heart by means of the right sub-cardinal vein. Its left 

 cephalic angle receives the left suprarenal vein. The renal vein 



