DEVELOPMENT OF THE THORACIC DUCT 121 



3. THE AZYGOS SEGMENT OF THE THORACIC DUCT 



This main part of the thoracic duct develops by the confluence 

 of extraintimal lymphatic anlages which begin to appear in the 12 

 mm. embryo, are clearly marked in the 13 and 13.5 mm. embryo, 

 increase in the 14 mm. stage, become confluent to form larger seg- 

 ments in the 15 and 15.5 mm. embryos, and finally unite into the 

 plexiform and nearly continuous channels of the thoracic ducts in 

 the 16 mm. stage. These extraintimal lymphatic anlages develop 

 at first along and around portions of the detached and degenerat- 

 ing veins of the ventro-medial tributary plexus of the azygos veins 

 at some distance from the latter. Subsequently they extend 

 dorsad along these decadent venules and then come to lie in close 

 apposition to the ventral aspect of the main azygos trunks and the 

 terminals of the ventral branches. They are from the beginning 

 genetically distinct and independent of the same. 



In the earlier and purely venous stages the azygos veins receive, 

 in addition to the terminals of the extensive supracardinal plexus, 

 which unites the veins of the two sides dorsal to the aorta, larger 

 dorsal somatic tributaries from the body walls and from the inte- 

 rior of the vertebral canal, and smaller ventro-medial branches 

 which drain the periaortic space close to the wall of the main arte- 

 rial vessel. When these ventral azygos tributaries appear they 

 occupy in general the position described as characteristic for 

 the cardinal collateral plexus of the Marsupialia (McClure. 30 ). 



The ventro-medial azygos tributary plexus, which furnishes 

 the venous line along and around which the first extra-intimal 

 anlages of the azygos segment of the thoracic ducts develop, begins 

 to appear with the first differentiation of the supracardinal plexus. 

 The recognition of its course and extent, and of the subsequent 

 changes in which it is involved, is of the greatest importance to 

 the correct interpretation of the placental thoracic duct develop- 

 ment. The plexus, and the subsequently more defined ventro- 

 medial tributary vein, parallel to the main azygos trunks in the 

 caudal part of the area involved, will be frequently referred to in 

 the following pages. 



McClure 15 was the first investigator who definitely recognized 



