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, shde 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-inedial 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 j^artially 

 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 topographicalh*. 

 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 jxM-i- 

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

 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 interioi' 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 renmant (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 intei-wd ))etween the same. 

 the aorta and the oesoi)hagus. 



In figs. 225 to 229 inclusi\'e fiNc successive sections from the 

 beginning of the same slide ai'e gi\(Mi (series 214, slide xiv, sec- 

 tions 1, 2, 3, 4, and 5, X 225). This series corresi)()iids to the (me 

 just discussed, fiom the caudal i)art of the same slide. The cen- 

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

 can still be made out in the interior of the ])ej'i\-eiiousextraintimal 

 lynii)hatic si)ace (5), but in the succeeding sections 3 and 4 (figs. 

 227 and 228) the latter alone oc('ui)ies the field, showing the elimi- 



