DELATION OF LYMPHATIC TO BLOOD-VASCULAR SYSTEM 39 



trunk following its dorso-lateral aspect. As will be shown later 

 (Part v.), departures from the normal type of venous develop- 

 ment in this region, and the substitution of other embryonic 

 pathw^ays for the right supracardinal in building up this section 

 of the adult postcava, produce corresponding and correlated 

 changes in the arrangement of the main retro-aortic lymphatic 

 channels. 



Fig. 26 shows the periaortic region of the same embryo, further 

 caudad, in a magnification of 150 diameters (series 241, slide 

 XXX, section 14.) The section is taken at the level of a pair of 

 dorsal intersegmental arteries which pierce the supracardinal 

 venous {59, 60) and lymphatic {76) complex, and divide the vein 

 of the right side into two components {60, 60). Further ventrad 

 the two postcardinal veins {67, 68) are seen, already consider- 

 ably reduced, ventrolaterad to the ureters {58) and the accom- 

 panying ascending lumbar lymphatic trunks {75). The connec- 

 tion of the latter with the supracardinal lymphatic channel is 

 especially clear on the right side of the embryo. 



Taken together, the 17 mm. and 20 mm. embryos just figured 

 and described afford a very clear and comprehensive picture of 

 venous and lymphatic development in their mutual relationship 

 in this region. 



The schematic text figures A, B and C may help to explain 

 this relationship. 



Fig. A is based on the joint studies which McClure and I made 

 on the development of the postcava in embryos of the domestic 

 cat. The figure was demonstrated to the 21st Session of the 

 Association of American Anatomists in 1906 at the time of pre- 

 sentation of the communications, although not reproduced in the 

 brief abstracts of the papers subsequently published. 2''' ^^ 



The figure represents a composite schema of the main peri- 

 aortic venous axial pathways of the abdominal region. These 

 pathways developing along definite and constant axial hydro- 

 static lines out of the periaortic venous reticulum, have all been 

 determined by us in embryos of the cat. They do not, of course, 

 all coexist at the same time in any embryonic stage, but normally 

 succeed each other in definite sequence. The entire range of 



