DEVELOPMENT OF THE THORACIC DUCT 149 



Fig. 266 (section 11) is taken at the level of a pair of intercostal 

 arteries (#), which perforate the interazygos anastomotic plexus 

 (15) and separate the same from the azygos trunks. The dorso- 

 medial extension of the right thoracic duct towards the interval 

 between the interazygos channel (15) and the dorsal circumference 

 of the aorta (7) has increased. In section 15 (fig. 267) it has 

 crossed to the mesal side of the right intercostal artery and occu- 

 pies part of the area filled in the preceding sections by the inter- 

 azygos anastomosis. This latter is reduced to insignificant 

 remnants. The left azygos vein (6) appears as a minute channel 

 between the left intercostal artery (2) and the left thoracic duct 

 (36). It can be traced in the same condition in the succeeding 

 sections. 



In sections 16, 17 and 18, (figs. 268, 269 and 270) the inter- 

 mediate lymphatic channel appears along the dorsal circumfer- 

 ence of the aorta, between the intercostal arteries, which now 

 pass through meshes separating the three chief channels of the 

 lymphatic plexus, viz. the left and right thoracic ducts and the 

 intermediate supra-aortic lymphatic. 



The three last figures of this series show very clearly, by com- 

 parison with figs. 261 and 262, the advances in lymphatic devel- 

 opment, and the recession of the azygos system, especially of the 

 left azygos vein, in passing from the 16 mm. (series 96) to the 19 

 mm. stage (series 253). The latter presents practically adult 

 conditions in both the venous and the lymphatic systems. 



It only remains to be noted that, in the adult cat, the right azy- 

 gos vein shifts secondarily dorsad to a submuscular prevertebral 

 anastomotic venous chain at the 9th or 10th costo-vertebral level, 

 and thus affords the opportunity for the greater development of 

 the caudal part of the right thoracic duct which replaces the origi- 

 nal azygos channel and establishes connection with the abdominal 

 lymphatics, forming the above defined "Post-azygos segment of 

 the thoracic duct." 



It is not the purpose of the present paper to enter into the 

 development of the abdominal connections of the thoracic ducts. 

 This subject requires a minute consideration of the conditions 

 encountered in the adult mammal, and is hence properly reserved 

 for future detailed consideration. 



