RELATION OF LYMPHATIC TO BLOOD-VASCULAR SYSTEM 49 



Again, in the same way, in adult animals with normally placed 

 right postcava, the main supracardinal lymphatic trunk, draining 

 the abdomen and the posterior extremities, follows the left side of 

 the large artery. 



In the not infrequent instances, however, of left sided postcava 

 or postcardinal vein in the adult the reverse obtains, and the peri- 

 aortic lymphatic channels predominate on the right side and oc- 

 cupy the place usually filled by the large vein in normal venous 

 development. 



I have encountered in the adult series so far examined no in- 

 stance of persistent left precava replacing functionally the normal 

 right superior cava, but have no doubt that this venous variation 

 would involve a transposition of the proximal end of the thoracic 

 duct to the right side, or at least a marked increase in the size and 

 functional importance of the usually insignificant preazygos seg- 

 ment of the adult right lymphatic duct. 



The developmental outline just given describes the mutual 

 ontogenetic relations of the venous and lymphatic systems through- 

 out the greater part of the body. 



Systemic lymphatic development in these regions is, however, 

 by no menas confined to the immediate environment of degener- 

 ating embryonic veins. The same field, which shows the above 

 described histogenetic processes in the development of extrain- 

 timal lymphatic spaces surrounding and replacing a decadent 

 venule, will at the same time contain numerous equivalent lym- 

 phatic mesenchymal clefts and spaces which continue to develop 

 independently of any association with retrograding veins. Nat- 

 urally, these independently developed early lymphatic anlages 

 are less striking than those above described as developing in as- 

 sociation with a receding vein. They are smaller, because they 

 lack the bulk of the contained venous core, and they are more 

 difficult to clearly differentiate against the surrounding mesen- 

 chyme. The}^ are, however, always present and their eventual 

 connection with the larger perivenous lymphatic spaces can be 

 ascertained definitely by following their development through 

 the proper stages. 



