202 FREDERICK TILNEY 



it passes cephalad along the ventro-mesial surface of the meso- 

 nephros, having its long axis parallel with the aorta. Upon 

 reaching the level of the sub-hepatic sinus the vessel swings 

 dorsad and slightly lateral so that its axis is now turned almost 

 at right angles to its caudal portion. The position which it 

 now occupies distinguishes it as the middle or horizontal portion 

 of the vein. In this portion of its course the vessel closely fol- 

 lows the direction of the intra-hepatic segment of the vitelline 

 vein as the latter is sweeping dorso-cephalad away from the 

 sub-hepatic sinus. The two vessels are still completely sepa- 

 rated by the coelom with a measured distance of 1(V between 

 them. The angle determined by the junction of the caudal 

 and middle portions of the vein lies in a direct line with the intra- 

 hepatic portion of the vitelline vein, so that the projection cau- 

 dad of the axis of the latter vessel would coincide with the axis 

 of the caudal portion of the sub-cardinal vein, and would thus 

 foreshadow the axis of the future post-cava (on the right side). 

 It is not possible to state that the region of closest approach 

 is the region of the final confluence of these two vessels, but it 

 seems probable from the relations of the sub-hepatic sinus, that 

 the ductus venosus, intrahepatic vitelline and sub-cardinal veins 

 of the right side unite at this, rather than at some more cephalic 

 point. The cephalic vertical portion continues the vein cephalad 

 and is the smallest as well as the shortest of the three divisions. 

 It taps the post-cardinal vein 200/x above the cephalic pole of 

 the mesonephros. It is parallel to the intra-hepatic portion of 

 the omphalomesensenteric vein, from which it is separated by 

 a mean distance of 20/x. 



5. The pre-cardinal veins (fig. 5, 2) and veno-lymphatics. The 

 general arrangement of the pre-cardinal veins has undergone 

 no marked change. The vessels still present an arched and a 

 a straight limb. There has been, however, considerable modifi- 

 cation in the character of the dorsal tributaries. These have 

 expanded and become confluent in several places. The expan- 

 sion has continued after confluence has taken place and the 

 channels have been converted into irregular spaces. The four 

 dorsal tributaries to the pre-cardinal vein have all united, while 



