THORACIC DUCT DEVELOPMENT IN THE PIG 417 



distinction is clearly grasped all possible confusion will be avoided. 

 There is a firm suspicion, however, that all veno-lymphatics vessels, 

 whether apparently direct or indirect antecedents of some lymphatic, 

 are fundamentally identical or homologous structures. 



A brief account of the early history of the supracardinal or 

 azygos system of veins will simplify the explanation of the source 

 and character of the veno-lymphatics in the thoracic duct area 

 and their grouping into three divisions. In 14 and 15 mm. em- 

 bryos, the precardinal and jugular veins give rise, in the region of 

 the anterior lymph sac, to a number of dorsal tributaries which 

 are continued back to the posterior part of the body as two slender 

 channels immediately above and parallel to the pre- and post- 

 cardinal veins. On account of their position and subsequent 

 history, these longitudinal channels may be called the supra- 

 cardinal lines. Throughout their course they are joined to the 

 pre- and postcardinals by numerous cross-anastomoses. They 

 also possess branches which may be described as dorsal segmental 

 veins, because they drain the regions of the back on each side of 

 the vertebral column and spinal cord and appear to be arranged 

 metamerically. The disposition and fate of the supracardinal 

 lines in a later stage (19 mm. embryo, fig. 28) can be indicated as 

 follows: Their precardinal division (A), that segment extending 

 between the levels of the jugular lymph sacs and the Cuvierian 

 ducts, is complicated into a plexus, some channels of which become 

 veno-lymphatics (6a), to be considered presently, and others are 

 absorbed by the dorsal branches (8) of the precardinal veins. 

 Their middle or postcardinal division (B, 12ld, 12ls), between the 

 Cuvierian ducts and the anterior extent of the mesonephroi, fuses 

 longitudinally with the postcardinal veins. The posterior divi- 

 sion (C) furnishes the true supracardinal veins 20 (12d, 12s), 



20 George S. Huntington and Charles F. W. McClure: The development of the 

 postcava and tributaries in the domestic cat. Am. Jour. Anat., vol. G, 1907, 

 Abstr. Anat.Rec, vol. 1: "A bilateral and originally symmetrical venous channel 

 develops dorso-medial to the primitive postcardinal vein by longitudinal anas- 

 tomoses between somatic postcardinal tributaries. This secondary vein channel 

 forms what we have termed the supracardinal system of veins. It extends from 

 the level at which the posterior limb veins open into the postcardinals to a point 

 cephalad where it joins that portion of the postcardinal which alone persists to 

 form the anterior end of the adult azygos." 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 13. NO. 4 



