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, follow r s 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. They are, however, always present and their eventual 

 connection with the larger peri venous lymphatic spaces can be 

 ascertained definitely by following their development through 

 the proper stages. 



