THORACIC DUCT IN THE CHICK 133 



In the same year (1908) McClure (21) abandoned the view 

 previously held jointly by him and Huntington as regards the 

 development of the thoracic and right lymphatic ducts in cat 

 embryos, and states: 



The anlages of the thoracic and right lymphatic ducts consist of a 

 series of independent outgrowths which first appear along the common 

 jugular and innominate and then along the azygos veins exactly in the 

 line subsequently followed by these ducts; these outgrowths are sub- 

 sequently split off from the veins, by a process of fenestration, in the 

 form of a series of isolated, more or less spindle-shaped spaces which 

 later become confluent with one another and with a process of the 

 jugular lymph sac to form a continuous system disconnected from the 

 veins, .... (p. 542). 



In 1909 Sabin (7) reiterates her original view, based on the study 

 of injected pig embryos, stating that the "presumption seems to 

 lie on the side that the thoracic duct develops in the same manner 

 as all other ducts," namely, "from endothelial sprouts from the 

 sacs" (p. 58). 



McClure in 1910 (23), after further studies of critical stages 

 in lymphatic development, retracted his former view and stated : 



The venous line .... along which the cat's thoracic duct develops 

 is topographically replaced by the lymphatic channel, not directly, as 

 assumed by me (in 1908), but secondarily by extra-intimal lymphatic 

 space development, .... the 'extra-intimal theory,' as originally out- 

 lined by Huntington and myself (in 1906), establishes a fundamental 

 principle of development for the main systemic lymph channels in 

 mammals (p. 105). 



Sabin, in a later article ('11) (22), states that "the thoracic 

 duct develops in part as a down growth of the jugular sac and in 

 part, especially its dilated portion or cisterna chyli, as a direct 

 transformation of the branches of the azygos veins" (p. 424) . This 

 expression of opinion seems to be a correction of her earlier state- 

 ments and a partial adoption of Lewis's view. In place of her 

 former concept of an uninterrupted centrifugal lymphatic growth 

 from the sacs, she now appears to hold that in addition a portion 

 of the thoracic duct develops as the result of direct transformation 

 of azygos venous tributaries into lymphatics. 



