150 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



Junction of azygos and preazygos segments of the thoracic duct 



These segments of the main adult lymphatic channel usually 

 unite by end-to-end fusion in embryos between 15 mm. and 16 

 mm. crownrump measure, although, as previously stated, the 

 establishment of a continuous lymphatic channel may be delayed 

 in individual instances beyond this period. 



Figs. 271 to 275 inclusive show five successive transverse sec- 

 tions of this junctional area of the two segments in the cephalic 

 portion of the dorsal mediastinal region in a 15.5 mm. cat embryo 

 (series 143, slide xii, sections 22, 23, 24, 25 and 26, X 200). 



Fig. 271 (section 22) contains the right precava (3) and the left 

 azygos-Cuvierian arch (6), with the cephalic termination of the 

 interazygos plexus (15,) dorsal to the oesophagus (8). The caudal 

 extremity of the preazygos segment of the thoracic duct (35) is 

 seen on the left side, and continues in the next section (fig. 272) as 

 the link between the preazygos and azygos segments of the duct 

 (35, 36 in fig. 272). In the following section 24 of this slide (fig. 

 273) the darker area dorsal to the lymphatic channel marks the 

 blind cephalic end of the left azygos arch, and hence characterizes 

 the lymphatic channel (36) as the beginning of the azygos por- 

 tion of the duct. In fig. 274 (section 25) the left azygos arch 

 (dorsal 3) is opened by the section and receives the terminal of 

 the cephalic end of the interazygos plexus (15) seen in the three 

 immediately preceding sections (sections 24, 23, and 22, figs. 

 273, 272 and 271) . We are therefore now at the cephalic end of the 

 azygos segment of the thoracic duct (36), ventral to the azygos 

 vein (3) and the interazygos anastomosis, and can trace the same 

 uninterruptedly from this point cephalad into continuity with the 

 beginning of the preazygos channel (35), as seen in sections 23 and 

 22, figs. 272 and 273. 



In the following section (26, fig. 275) the left azygos arch (3) 

 is fully opened, in its turn ventrad to join the left precaval vein, 

 and the azygos segment of the thoracic duct (36) occupies the 

 typical position of this period, between the ventro-medial aspect of 

 the right azygos trunk (3) and the dorso-lateral circumference of 



