Mesenteric Sac and Thoracic Duct in Embryo Pig 317 



1. The first lymphatics appear in the cervical region near the internal 

 jugular vein, in the pig, between 14 and 16 mm. (Dr. Sabin). 



2. The thoracic duct appears first at 23 mm., and is located in the 

 exact position of the branches of the azygos veins in the earlier stages. 

 In all probability, it is derived from branches of these veins. 



3. The mesenteric sac begins as a mass of blood capillaries, lying 

 ventral to the renal anastomosis of the sub-cardinal veins. They become 

 fused into a definite sac at first having extensive venous openings, which 

 later become gradually obliterated, thus making the sac independent at 

 one time of either venous or lymphatic connections. 



4. By a growth and fusion of capillaries from both the sac and the 

 lower portion of the thoracic duct proper, the former becomes a definite 

 part of the lymphatic system. 



