196 FREDERICK TILNEY 



as well as in placental circulation. This is shown by the rich 

 umbilico-cardinal plexus which permeates the lateral somatic 

 wall. Each vein opens into the sinus venosus of its own side 

 in common with the vitelline entrance and forward of the Cuvier- 

 ian approach (fig. 2, 10). 



2. The omphalomesenteric veins (fig. 2, 9). These veins, also 

 paired and symmetrical, are smaller than the umbilicals. Al- 

 though their point of entrance into the sinus venosus is the 

 most mesial and ventral of all vessels entering the heart, the 

 plane of the main trunk passes about midway between the um- 

 bilical and post-cardinal veins, so that the course of the vessel 

 as it approaches the sinus is deflected sharply forward and inward. 

 Two divisions of this vessel may be recognized, an intra-hepatic 

 portion which has entered into the formation of the sinusoids 

 and is situated at the dorso-mesial angle of the liver. It thus 

 comes into close proximity to the coelomic angle. The second 

 or omphalic portion passes out upon the 3^olk sac, leaving the 

 body through the external umbilicus. In this way, each om- 

 phalomesenteric vessel, in its intrahepatic portion, has its long 

 axis parallel both with the umbilical and post-cardinal veins, 

 but interposed between it and each of these channels are the 

 mesial branches of the perimesonephroic plexus. No evidence 

 of any distinctly mesenteric branch was found. 



3. The post-cardinal veins (fig. 2, 5). These are present as 

 a pair of vessels of medium size which have become definite 

 channels throughout their entire length. They hold their typi- 

 cal position dorso-lateral to the mesonephros and still retain 

 part of the plexiform connection with the umbilical veins (fig. 2, 

 7) . This connection becomes greatly reduced as the promontory 

 is reached. The size of the vessel increases as it ascends to 

 become confluent with the pre-cardinal, but before doing so it 

 turns forward almost at right angles to its axial course and by 

 this horizontal arm enters the duct of Cuvier. Immediately 

 below this, sharp forward bend, the post-cardinal receives a 

 large tributary from the cephalic extremity of the perimeson- 

 nephroic plexus. In the greater part of its course many branches 

 from the plexus join this vessel. At some points the vein has 



