RELATION OF LYMPHATIC TO BLOOD-VASCULAR SYSTEM 33 



stage (fig. 18) is marked both in the venous and the lymphatic 

 channels. The former are relatively much reduced, while the 

 latter have correspondingly increased in extent. The azygos is 

 mainly represented by the right channel (3}. The left channel (6) 

 has become small, but is still connected by the transverse inter- 

 azygos anastomosis (15} with the larger and permanent right 

 trunk. The right thoracic duct (36} is likewise large. A dorso- 

 medial extension of the same, which carries into the interval 

 between aorta and interazygos plexus, will, in later stages, replace 

 the latter secondarily, after the complete recession of the left 

 azygos vein. The left thoracic duct (86} is also of large size and 

 fills a considerable part of the area formerly (fig. 18) occupied 

 by the left azygos trunk. Some decadent remnants of the ventro- 

 medial azygos^ plexus (4) are still seen associated with the left- 

 duct, and are in process of replacement by a lymphatic space (5} 

 destined to make connection between the left thoracic duct anlage 

 and the mesenteric lymphatic plexuses (cf. p. 148, figs. 266 to 

 270, 51}. 



Figs. 20 to 24 show transverse sections in the region of the devel- 

 oping mesenteric lymphatics and of the ascending lumbar lym- 

 phatic trunks in a 17 mm. embryo (series 258, slide xxiii, sections 

 34, 33, 32 and 31.) The embryonic veins (74) occupying the root 

 of the mesentery caudal to the subcardinal cross-anastomosis are 

 in the process of being replaced by extraintimal lymphatic spaces 

 (51} which are destined to become confluent and form the anlage 

 of the future mesenteric lymphatic sac. 



The region in question is not only interesting in reference to 

 the ontogenesis of the abdominal lymphatic sacs and channels, 

 but the arrangement of the periaortic axial venous trunks and 

 their relation to the developing lymphatics is, in combination with 

 the next following stage (20 mm., figs. 25 and 26), of the highest 

 importance in interpreting the phylogenetic relations of the main 

 abdominal veins in mammalia. For these reasons a somewhat 

 more detailed consideration of the sections may properly be in- 

 troduced here. 



Fig. 20 shows section 34 of slide xxiii of series 258 in a magnifica- 

 tion of 75 diameters and affords a comprehensive picture of the entire 



