126 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



phatic channel, extending cephalad of the line C in fig. 190 along 

 the ventral aspect of the right intersegmental arteries and of the 

 interazygos plexus. It covers the cephalic part of the right 

 ventro-medial para-azygos venous ridge (34), the remnants of 

 which, detached from the main vein, are to be seen on section 

 occupying the interior of the perivenous extraintimal lymphatic 

 channel which is replacing them (Compare figs. 214 to 240, show- 

 ing sections from slides xiv and xv of series 214). A part of the 

 ridge (34) extends caudad of line C in fig. 190, but the process of its 

 replacement by lymphatic spaces has proceeded much further 

 than in series 34 (fig. 189). 



Fig. 191 shows the ventral view of the reconstruction of the 

 same region in a 15 mm. embryo (series 219). In comparison 

 with the 13.5 and 14 mm. stages this embryo shows more uniform 

 and in general more advanced development in all regions of the 

 thoracic duct anlage. The individual and separate lymphatic 

 anlages seen in series 76 and 34 (figs. 188 and 189) have become 

 confluent to form longer and better defined lymphatic channel 

 segments. These have arranged themselves in two groups. Cau- 

 dad of the point marked C in fig. 191 they follow pretty symmetri- 

 cally the ventro-medial margin of the left and right azygos trunks, 

 occupying here the site of the earlier ventro-medial azygos trib- 

 utary plexus which they have in part secondarily replaced. The 

 segments are relatively long, especially on the left side. Cepha- 

 lad of the level marked C the lymphatic spaces of the left side 

 follow the left azygos trunk, becoming gradually smaller and more 

 disconnected. On the right side they divide at C into two lines. 

 One of these continues upward along the right azygos trunk and 

 arch. The other and larger turns c'ephalo-mesad, and is prolonged 

 upward and to the left, in front of the right intercostal arteries 

 and the interazygos anastomosis. Caudad of the level C we have, 

 therefore, two lines of lymphatic anlages, folio wing the right and left 

 azygos trunks, while cephalad of this level we encounter three, the 

 right, left and intermediate. The former, diverging from the other 

 two in ascending to right, marks the path of the future right lym- 

 phatic duct. The two caudal parallel lymphatic lines below C and 

 the central and left lines cephalad of this point become subse- 



