THORACIC DUCT DEVELOPMENT IN THE PIG 421 



(B) Postcardinal division . Small venous derivatives are formed 

 by a process analogous to fenestration along the mesial border of 

 the postcardinal (6b, figs. 12, 28 and 32) of each side and appar- 

 ently always in the line of fusion of this vein with the middle 

 segment of the originally independent supracardinal channel, 

 thus suggesting their derivation from the supracardinal system 

 also in this division. During their development consecutive 

 ages may be distinguished among them by the amount of individ- 

 uality they manifest. Some are merely little bulging irregulari- 

 ties in the circumference of the parent vein, others describe 

 the first step of separation by the presence of thin strands or 

 partitions, and still others are complete throughout a number of 

 sections but open to the veins at one or both ends. Being in the 

 direct axis of the precardinal veno-lymphatics and homologous 

 with them, these venous spurs or derivatives may be called the 

 postcardinal veno-lymphatics. Ordinarily they are exhibited 

 more distinctly on the right side. Later they constitute longer 

 and shorter venules (6b, figs. 29, 32) parallel to the postcardinal, 

 but they are never quite independent of the latter, remaining 

 joined to it here and there until their reduction when they break 

 up into degenerating segments and disappear in the mesenchyme. 

 Such a final procedure will become clearer in the consideration of 

 the second or transition phase. 



(C) Supracardinal division. In the region of the mesonephroi, 

 plexuses of vessels spring from the ventral aspect of the supra- 

 cardinal veins, and in some embryos they become so extensive 

 as to encompass the aorta almost completely. For this reason 

 and the fact that they are the equivalents of the anterior veno- 

 lymphatics in function, they were named the supracardinal peri- 

 aortic veno-lymphatics. A further description of them would be 

 superfluous considering their clearness in the accompanying micro- 

 photograph and reconstruction (6c, figs. 3 and 32). 



Caudally, at the level of the superior mesenteric artery, sub- 

 sidary channels arise from the supracardinals ventro-medially, 

 become more and more plexiform, and approach one another from 

 both sides to anastomose in the area dorsad of the aorta (6c, 

 fig. 4). Since eventually they will be concerned in the pro- 



