92 F. W. THYNG 



teric arteries. The cephalic part of this plexus lies between the 

 subcardinal segment of the inferior vena cava and the left supra- 

 renal vein (left subcardinal) , the caudal portion lies dorsal to the 

 renal anastomosis. In places the diameter of the plexus is equal 

 to, or even greater than, that of the adjacent aorta. Although 

 the lumina of the vessels are closely packed with corpuscles, it is 

 difficult to connect the vessels with any of the definitely formed 

 veins. Nevertheless, it would be impossible at least without 

 reconstructions to say that such connections do not exist. From 

 the relations of the plexus, given above, there seems little doubt 

 that it corresponds to the anlage of the mesenteric lymph sac, 

 discovered in rabbit embryos by Lewis ('02 and '05). 



Baetjer ('08) who has investigated the development of this sac 

 in a series of pig embryos, concludes that it originates in a series 

 of small veins which separate from the renal anastomosis. 



Cisterna chyli. A series of anastomosing venous channels is 

 found on the right and left side, dorso-lateral to the aorta. The 

 cephalic end of this plexus apparently connects with the azygos 

 and hemiazygos veins respectively. Many of the channels open 

 into the supra-ureteral venous channels, previously described. 

 The intersegmental arteries in this locality extend dorsal to these 

 channels, although offshoots of the latter frequently anastomose 

 between the successive pairs of arteries, dorsal to the aorta. 

 These vessels may represent the anlage of the cisterna chyli, for 

 they correspond in position to the anlage as described by Sabin 

 ('09). 



Sacci lymphatici posterior es. Definite posterior lymph sacs 

 in relation to the sciatic veins have not been found, but numerous 

 small tributaries, entering the proximal part of the veins may 

 foreshadow them. Sabin ('09) states that they first appear in an 

 embryo of 20 mm., as a plexus of small veins. 



