DEVELOPMENT OF THE THORACIC DUCT 59 



Inasmuch as the development of this segment of both ducts 

 is intimately connected with that of the principal abdominal 

 lymphatic channels, its consideration will be included in that of 

 the latter system (Part III). The present communication deals 

 only with the first three segments above enumerated. 



Fig. 29 shows in a color schema the genetic composition of the 

 thoracic ducts according to the segments just outlined, and their 

 relation to the embryonic venous system. . 



In proceeding caudo-cephalad the post-azygos portion (IV) 

 develops in the larger right segment (52) by replacement of the 

 caudal part of the ventro-medial tributary plexus of the right 

 azygos vein. 



The smaller channel of the left side replaces the caudal part of 

 the left azygos vein. In the azygos area (III) the left channel 

 follows, as a much reduced left thoracic duct, the prevertebral 

 portion of the left azygos vein. The larger right lymphatic 

 channel (36), gradually inclining cephalo-sinistrad, develops 

 along the ventral aspect of the inter-azygos cross-anastomosis 

 and unites with the left duct eventually in a close plexiform inter- 

 lacement perforated by the intersegmental arteries, which con- 

 tinues cephalad to near the level of the aortic arch. Here the 

 plexus condenses into two more or less well defined channels 

 which turn up to the right and left into the preazygos segment 

 (II). The smaller right channel follows, cephalad of the right 

 azygos terminal, the dorso-medial aspect of the right precaval, 

 innominate and common jugular veins, developing by confluence 

 of extraintimal spaces along the course of these vessels (cf . recon- 

 struction of series 88, figs. 27 and 28), and forms the cephalic 

 end of the reduced right thoracic duct of the adult. It terminates 

 by joining the thoracic duct approach (12) of the right jugular 

 lymph sac. The left and larger trunk replaces the extra-peri- 

 cardial portion of the left precava, and ascends dorsal to the left 

 innominate confluence to its junction with the thoracic duct 

 approach (12) of the left jugular lymph sac. 37 in fig. 29 indi- 

 cates the broncho-mediastinal trunk in one of its commonly 

 encountered types, joining caudally the preazygos portion of the 

 thoracic duct (35), and terminating cephalad in the ventral 



