THORACIC DUCT DEVELOPMENT IN THE PIG 



423 



Fig. 4 Transverse section taken near the level of the superior mesenteric artery 

 in a 20 mm. embryo (series 194, slide 40, section 16), X 200. 6c, posterior supra- 

 cardinal veno-lymphatics, the precursors of the subsequent cisterna chyli; 12d, 12s, 

 right and left supracardinals; 13, aorta; 13ds, dorsal segmentalartery; 14, sympa- 

 thetic nerve trunks. 



ated by the writer. According to the descriptions, their character 

 and derivation in the rabbit and cat embryos accord accurately 

 with their tendencies in the pig embryos and allow of no other 

 conclusion than that they are homologous structures ; for not only 

 do they occupy identical positions but they also have their sources 

 in the same venous trunks. This similarity is even more strik- 

 ingly emphasized in somewhat older pig embryos where they are 

 on the verge of evanescence and break up into isolated segments. 

 Until they have reached their culmination these veno-lympha- 

 tics can be followed and reconstructed with remarkable ease and 

 accuracy. Their caliber or lumen is as uniform as a plexiform 

 condition permits, and constrictions, irregular shrinkage, collapse, 



