SUMMARY AND CONCLUSIONS 159 



and involved problems in the entire genetic history of mammalian 

 vascular organization. 



I am convinced that the theory of direct derivation of the mam- 

 malian systemic lymphatic vessels from the embryonal veins is 

 utterly wrong and that it is based primarily on insufficient obser- 

 vation. It lends itself to an easy interpretation of a relatively small 

 number of mammalian embryos of each appropriate stage, but 

 fails utterly in reckoning with the conditions presented by a large 

 consecutive series of the critical periods. 



I will analyze this view on the basis of the material discussed 

 in the preceding pages, first in the region of the thoracic duct ap- 

 proach of the jugular lymph sac and in the preazygos segment 

 of the thoracic duct, and subsequently in the azygos district of the 

 latter. 



I. THORACIC DUCT APPROACH AND PREAZYGOS SEGMENT OF THE 



THORACIC DUCTS 



The terminal portion of the thoracic duct has been described 

 as developed by confluence of venous outgrowths, arising along 

 the dorsal surface of the left common jugular and innominate veins 

 in the earlier (14 mm.) stages, which occupy the same relative 

 position with respect to the main venous channel, the thyro- 

 cervical artery and the sympathetic nerve as does the thoracic 

 duct in the later (16 mm.) stages. 



In my opinion the so-called ' outgrowths' arising from the main 

 venous trunks are the terminals of dorso-medial tributary branches 

 entering the same, and have nothing to do with the thoracic 

 duct. They are part of the pre vertebral capillary plexus in rela- 

 tion in this stage to the dorso-medial surface of the main venous 

 channels, as described in detail in the second part of this paper 

 dealing with the development of the preazygos segment of the thor- 

 acic duct (supra, p. 84). This opinion is based on the fact that in 

 many 14 mm. embryos sections through this region show the so- 

 called ' venous outgrowth' and the thoracic duct approach of the 

 jugular lymph sac coexisting side by side. 



As previously described in detail (supra, pp. 56 to 77) the thor- 



