542 



by Sabin for the pig embryo, these ducts are filled by an injection 

 mass at different stages of development. The communication in the 

 adult which, at times, exists between the thoracic duct and the azygos 

 system may also be explained on the basis that certain of its primary 

 venous connections have been retained. Another inference which may 

 be drawn is that the varicose structure, so characteristic of the tho- 

 racic duct and other lymph channels, may be due to the presence and 

 persistence of the narrow connecting channels which are secondarily 

 formed between the isolated, spindle-shaped spaces. 



The writer's conception of the development of the thoracic and 

 right lymphatic ducts, as illustrated by Figs. 10, 11, 12 and 13, may 

 finally be summed up as follows: 



The anlages of the thoracic and right lymphatic ducts consist of 

 a series of independent outgrowths which first appear along the com- 



Fiff. 10. 



Fig. 11. 



Fig. 12. 



Fig. 13. 



Figs. 10, 11, 12 and 13. Diagrams illustrating the development of the thoracic 

 duct in the domestic cat. 1 external jugular vein ; 2 subclavian vein ; 8 innominate 

 vein; 4. duct of CüVlEK; 5 jugular lymph sac; 6 process of lymph sac which joins the 

 independently formed thoracic duct; 7 veno-lymphatic anlages of thoracic duct ; 8 isolated 

 lymph spaces; 9 thoracic duct; 10 internal jugular vein; 11 azygos vein. 



mou jugular and innominate and then along the azygos veins exactly 

 in the line subsequently followed by these ducts; these outgrowths 

 are subsequently spht 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, except through the mediation of the jugular lymph sac^). 



1) In his paper on the development of the lymphatic system in 

 rabbits (American Journal of Anatomy, Vol. 5), Lewis states that 



