DEVELOPMENT OF THE THORACIC DUCT 145 



relative change in position of the earlier lymphatic spaces is the 

 appearance of additional thoracic duct anlages, which develop 

 as extraintimal lymphatic elements along those lines of the em- 

 bryonic azygos system which are destined to undergo, hi the 

 later stages, still further reduction and final elimination in the 

 definite organization of the adult condition. [Compare on this 

 point the sections of series 96 (figs. 261 and 262) with those of 

 series 253 (figs. 265 to 270)]. 



One other fact deserves careful consideration in analyzing the 

 development of the azygos segment of the thoracic ducts in stages 

 between 14 mm. and 15 mm. In a certain proportion of the em- 

 bryos of this period the extraintimal replacement of the earlier 

 ventro-medial azygos plexus in the cephalic part of the future 

 azygos segment of the thoracic duct is delayed: In these cases 

 peculiar histological pictures are obtained in this area. The com- 

 ponents of the ventro-medial azygos plexus have become detached 

 from the main veins, and have fused together into an elongated 

 spindle shaped bag, completely filled with blood, beginning and 

 ending blindly and not in communication with any venous channel. 

 This structure, when found, invariably occupies the site of the 

 long lymphatic segment seen hi the reconstruction of series 214 

 (fig. 190) on the right side, in the areas A and B (slides xiv and 

 xv), ventral to the right intercostal arteries, in the interval 

 between aorta and right azygos vein. The appearance presented 

 by it on section is shown in figs. 263 and 264 in a 14 mm. cat 

 embryo (series 211, slide xii, sections 2 and 8, X 225). It is 

 indicated in these figures by the leader 34, as a detached and con- 

 fluent part of the original ventro-medial azygos plexus. Fig. 263 

 shows this structure hi the interval between the intercostal arter- 

 ies. Fig. 264 gives a typical view of it at the level of one of the 

 right intercostal branches. It lies ventral to the same, between 

 aorta (7) mesad, the right azygos trunk (3) laterad, dorsal to the 

 oesophagus (8). The blood cells, which completely fill its lumen, 

 differentiate from those circulating hi the continuous vein channels 

 by remarkably intense reaction to the Orange-G stain. As 

 stated, this structure is completely detached from all adjacent 

 veins, can be followed cephalad and caudad through many sec- 



