VEINS AND LYMPHATICS IN TRAGULTJS 201 



from the placenta to the heart, pending the establishment of 

 the well defined ductus venosus. 



3-. The post-cardinal veins {fig. 5, 5). These vessels occupy 

 the typical post-cardinal position. They may be traced caudad 

 as far as the cloaca and there lose their identity in an indefinite 

 plexus. Followed cephalad, each vessel is seen to attain its 

 maximum diameter at the level of the middle of the mesonephros. 

 They still give evidence of the early influence of the perimeso- 

 nephroic plexus; in several regions a channel is observed lying 

 parallel to the main vessel and continuous with it above and 

 below. The redundant element may be ental or ectal in position. 

 The genesis of this intermediate cardinal element is not yet 

 entirely clear. Like the sub-cardinal vein, it seems to represent 

 the general process by which definite channels are evolved from 

 more primitive plexus formations along the lines of axial growth 

 and drainage. The cephalic portion of the post-cardinal vein 

 is still intimately related to the supra-sinal segment of the umbili- 

 cal vein by branches of the umbilico-cardinal plexus. In the 

 region of the promontory, each vessel increases in size and re- 

 ceives a number of dorsal tributaries. 



4- The sub-cardinal veins and the perimesonephroic plexus (fig. 

 5, 12). One of the characteristic changes in this stage is the 

 development of a definite sub-cardinal vein out of the perimeso- 

 nephroic plexus. This vein is a slender vessel with here and 

 there a remnant of the former plexus draining into it. This 

 is particularly the case about the lower pole of the mesonephros 

 where the plexus is still rich and shows a tendency to give rise 

 to other axial lines besides the sub-cardinal channel. Here it 

 is possible to recognize a longitudinal drainage line which is being- 

 selected along the inner aspect of the plexus, mesial to the sub- 

 cardinal itself and ventro-lateral to the aorta. This element 

 is in the position of the cardinal collateral channel of McClure. 

 Because of its course and relations, three portions of the sub- 

 cardinal vein may be recognized. 1. The caudal vertical por- 

 tions 2, the middle or horizontal portion and 3, the cephalic 

 vertical portion. The caudal vertical portion begins as a slender 

 vessel from the caudal extremity of the peri-mesonephroic plexus ; 



