DEVELOPMENT OF THE THORACIC DUCT 123 



the same stage. If this is done there can remain no doubt that 

 the two thoracic ducts in the embryos of the cat develop by 

 confluence of extraintimal perivenous lymphatic spaces. These 

 anlages appear at first as isolated spaces, either surrounding the 

 retreating veins or closely applied to part of their circumference 

 and subsequently to the ventral wall of the main azygos trunks, 

 usually lateral to the points where the ventral plexus connected 

 with the main azygos channel. 



A General Analysis of the Development of the Thoracic Duct in the 



Azygos Region 



The clearest preliminary survey of the development of the 

 azygos segment of the cat's thoracic ducts can be obtained be 

 comparing the series of reconstructions shown in figs. 188 to 193, 

 all representing the structures in the ventral view and in a magnifi- 

 cation of 225 diameters. The individual sections of these embryos 

 will presently be described and figured in detail and will be re- 

 ferred to the reconstruction, but a general consideration of the 

 six figures will, at this point, help to make the genesis of the azy- 

 gos segment of the thoracic ducts clear. 



The earliest stage is a 13.5 mm. embryo (series 76), shown in 

 fig. 188. The reconstruction includes the azygos-Cuvierian junc- 

 tion of each side, the right (3) and left (6) azygos veins and their 

 interazygos supraaortic anastomosis, and extends caudad to the 

 lower azygos region. 



The intercostal arteries (red) and the sympathetic nerves (1) 

 are shown. The numerous extraintimal anlages of the thoracic 

 ducts (green) incrust the ventral aspect of the azygos veins and 

 of their anastomotic plexus. In the caudal half of the preparation 

 these lymphatic anlages follow fairly symmetrically the main 

 azygos trunks. On the right side they are still very numerous and 

 separated from each other, while the corresponding spaces on the 

 left side have already become confluent to form larger, although 

 still discontinuous, segments of the future lymphatic channel. 



At the level marked C in the figure the line of the lymphatic 

 anlages following the right azygos trunk divides. A part continues 



