148 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



ment of the thoracic ducts is merely a question of continuous 

 enlargement of the lymphatic channels at the expense of the azy- 

 gos veins. The latter recede still further, while the former cor- 

 respondingly increase until the adult relative condition of both - 

 systems is attained. In this readjustment the thoracic ducts 

 of the adult cat acquire for themselves in large part the topo- 

 graphical position occupied in earlier embryonic stages by por- 

 tions of the azygos complex. The method of this substitution has 

 been sufficiently and repeatedly detailed in the preceding pages. 



Figs. 265 to 270 inclusive show the results in transverse sections 

 of the middle thoracic region in a 19 mm. cat embryo (series 253, 

 slide xxiv, sections 9, 11, 15, 16, 17 and 18, X 225). 



In fig. 265 (section 9) both azygos veins are reduced, but the 

 right trunk (3) has maintained itself as the principal vessel, still 

 connected by the interazygos anastomosis (15) with the very much 

 smaller left azygos (6). Ventral to the veins, on each side of the 

 aorta (7), are the right and left thoracic ducts (36). The right 

 duct, as is invariably the case in this region, is the larger, and shows 

 a pointed dorso-medial extension which passes behind the aorta 

 towards the ventral aspect of the interazygos anastomosis. This 

 is a further development of the same conditions seen in the 15.5 

 mm. and 17 mm. embryos, in figs. 254, 255 and 259, and is responsi- 

 ble for the formation of the intermediate dorsal lymphatic ele- 

 ment seen in the 16 mm. embryo, (figs. 261 and 262) behind the 

 aorta, between the azygos veins and the intercostal arteries, 

 (cf. especially left side of fig. 262). 



On the left side of fig. 265, ventral to the left thoracic duct, is 

 seen a venous radicle (4) in relation to an extraintimal anlage (5). 

 In section 11 (fig. 266) this space has enlarged. In the succeeding 

 sections 15, 16, and 17 (figs. 267, 268 and 269) the same structure 

 appears as a well developed lymphatic channel (51), in the root 

 of the dorsal mesentery, and in section 18, (fig. 270) it opens into 

 the left thoracic duct. It marks one of the points of connection 

 of the mesenteric lymphatic plexus with the thoracic duct, which 

 leads to the ultimate differentiation of the postazygos segment of 

 this channel. The anlage is the same previously noted in the 16 

 mm. embryo, (4 and 5, in figs. 261 and 262, series 96), and in the 

 15.5. mm. embryo, series 143 (51 in figs. 257 and 258). 



