DEVELOPMENT LYMPHATIC SYSTEM IN AMNIOTES 269 



links or bond-pieces between the independently formed system 

 of the Ijrmphatic channels proper and the veins, and I have thus 

 interpreted them in several previous publications (8, 9, 21, 22, 

 23). 



At the time when McClure and I published our results in 

 detail ('10) the question as to the 'venous' or 'non-venous' origin 

 of the vertebrate lymphatic system was still an active one. The 

 only criterion applied at that period of the investigation to the 

 diagnosis of an embryonal lymphatic as against a venous vessel 

 was the presence within the lumen of the latter of free red blood- 

 cells. As above stated the early anlages of the jugular lymph 

 sacs become filled with blood-cells and communicate freely with 

 the channels of the systemic veins- We consequently at that 

 time regarded these early anlages as direct derivatives of the veins. 



Two facts, however, impressed themselves upon us even then 

 during the progress of our investigation: 



1. We were always able to differentiate sharply between the 

 regular series of the metameric dor so-medial pre- and postcard i- 

 nal somatic tributaries, forming definite components of the per- 

 manent venous system, and the more irregular and plexiform 

 dorso-lateral mass of vascular spaces, filled with blood for the most 

 part, which we recognized as the anlages of the future jugular 

 lymph sacs, and which we consequently defined as the 'veno- 

 lymyhaticsJ 



2. Among these latter we observed, described and figured in 

 numerous embryos of the earlier stages closed endothelial-lined 

 sacs, some containing red blood-cells, others entirely empty. 

 We interpreted the former as either accidental separations from 

 the general veno-lymphatic plexus, or as elements which had 

 not yet joined the same. The latter we regarded as possibly 

 representing veno-lymphatic elements which had, on a small 

 scale, anticipated the evacuation of the blood contents and the 

 separation from the veins which becomes typical for the entire 

 dorso-lateral veno-lymphatic plexus in the succeeding stages. 

 Up to this point the direct derivation of the mammalian jugular 

 lymph sacs from the early pre- and postcardinal veins, adjacent 

 to and including their Cuvierian junction, has not been questioned. 



