THORACIC DUCT DEVELOPMENT IN THE PIG 



455 



Fig. 25 Transverse section through the postcardinal region in a 26 mm. pig 

 embryo (series 69, slide 44, section 11), X 150. 5d, 5s, right and left thoracic 

 ducts showing the manner of their growth by the addition of spaces from the mesen- 

 chyme; lid, lis, right and left postcardinals; 13, aorta; 17, oesophagus; 19, embry- 

 onic vertebral column. 



recall that in the later transition stages the most anterior portion 

 of the thoracic duct anlage is characterized by an extensive and 

 complicated lymphatic plexus, especially well developed in Sabin's 

 series 23a (3, fig. 30) . During the lymphatic phase such a plexus is 

 completely reduced and converted into a simple channel (22-26 

 mm. embryos) whose fork or division into the right and left limbs 

 of the duct becomes shifted far back of the lymph sac. Series 

 192 (3, fig. 35) presents an intermediate stage in which the orig- 

 inal plexiform condition is still suggested but has been almost 

 entirely obliterated by the transverse fusion and consequent 

 reduction of the number of interanastomosing channels. 



No matter whether the thoracic duct anlagen arise as extra- 

 intimal spaces or entirely apart from the veno-lymphatics, dur- 



