SUMMARY AND CONCLUSIONS 163 



mediastinal region, which form the basis for the extraintimal lym- 

 phatic channel development in the correlated segments of the 

 thoracic ducts, are sufficiently described above (p. 85). 



There is no question that the internal and common jugular veins 

 and the innominate vein receive, along their mesal and dorso- 

 mesal aspects during the 13 and 14 mm. stage, numerous tributaries 

 of these two plexuses, as indicated above schematically in fig. 101. 

 These terminals cannot, however, be interpreted as ' outgrowths' 

 of the main venous line, constituting the venous anlages of the 

 thoracic duct in this region and subsequently uniting to form the 

 continuous channel of this duct in its preaortic segment. They 

 are a direct caudal continuation of the extensive inter- and peri- 

 neural venous plexus connected, cephalad of the thoracic duct 

 level, with the mesal aspect of the internal jugular vein through- 

 out nearly its entire extent. 



The elements of these early venous plexuses serve as lines of 

 extraintimal perivenous lymphatic development. 



Both in the prevertebral region cephalad of the aortic arch and 

 in the ventral mediastinal area, as above described (pp. 84 to 110) 

 the extraintimal anlages of both channels, the preazygos segment 

 of the thoracic duct dor sally, and the mediastinal trunk ventrally, 

 can be followed in the proper stages from their earliest inception 

 to their full completion and union with each other. 



The lymphatic channel system in both areas, as in all other 

 regions of the body, is at first very extensive and plexiform, com- 

 posed of a larger number of intercommunicating spaces. Later 

 the definite channel, smaller in cross-cut area and more direct 

 in course, crystallizes along definite static lines out of the pre- 

 existing lymphatic network, just as we now know that haemal 

 channels do. In other words, the systemic lymphatic channels, 

 as would be expected, repeat in their development the ontogenetic 

 stages of the ha3mal channels. Both sets of vessels, lymphatic 

 as well as haemal, result from the condensation of an antecedent 

 plexus along definite and determined lines and thus give rise to 

 the characteristic vessels of the adult. 



In this entire process there is never a question of the direct 

 implication of a vein in the formation of a 'ymphatic space. The 



