132 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



Figs. 214 to 219 inclusive show transverse sections in the central 

 thoracic region in this embryo, (series 214, slide xiv, sections 

 15, 16, 17, 19, 20, 22, X 225), in which the extraintimal thoracic 

 duct development and its relation to the atrophying central venule 

 of the ventro-medial azygos plexus is shown at the height of its 

 development. In the interval between aorta, right azygos vein 

 and oesophagus all six sections show the empty and partially 

 collapsed central endothelial tube of the decadent ventro-medial 

 azygos venule (4), surrounded by the extraintimal lymphatic 

 anlage of the thoracic duct (5), about to replace it topographically. 

 The central atrophying venule (4) is here still of large size, 

 although empty and completely separated from the functional 

 azygos venous channels. It, together with the enveloping peri- 

 venous extraintimal lymphatic anlage of the thoracic duct (5), 

 forms a conspicuous histological object in the field above defined, 

 between aorta, oesophagus, right azygos vein and interazygos 

 plexus. Figs. 220 to 224 inclusive show transverse sections of the 

 same embryo further caudad on the same slide (series 214, slide 

 XIV, sections 26, 27, 28, 32 and 34, x 225). The central atro- 

 phying venule (4) in the interior of the replacing extraintimal peri- 

 venous lymphatic space (5} is undergoing further reduction and 

 diminution in size and extent, until, in fig. 224, it has receded to 

 an insignificant tab of tissue (4) still attached at one point to the 

 interior of the endothelial lining of the lymphatic space (5). The 

 reduced central venous remnant (4) and the surrounding extrain- 

 timal lymphatic anlage (5} have shifted in these five sections dorsad 

 until they come into apposition with the ventral aspect of the 

 broad interazygos anastomosis in the interval between the same, 

 the aorta and the oesophagus. 



In figs. 225 to 229 inclusive five successive sections from the 

 beginning of the same slide are given (series 214, slide xiv, sec- 

 tions 1, 2, 3, 4, and 5, X 225). This series corresponds to the one 

 just discussed, from the caudal part of the same slide. The cen- 

 tral decadent venule (4) in sections 1 and 2 (figs. 225 and 226) 

 can still be made out in the interior of the peri venous extraintimal 

 lymphatic space (5), but in the succeeding sections 3 and 4 (figs. 

 227 and 228) the latter alone occupies the field, showing the elimi- 



