Superficial Veins in the Pig. 447 



mesenteric artery, a conspicuous, bulky vessel that increases in size 

 as it passes anteriorly. Just below the omphalomesenteric artery 

 the aorta divides into two vessels which in turn break up into two 

 groups of capillaries, each of which reunites to form an umbilical 

 artery. In the head region there are only two branchial arches 

 formed. From the anterior of these a branch is given off which 

 passes into the capillaries that unite to form the anterior cardinal 

 vein (V. cas.). The anterior cardinal vein runs back to meet the 

 posterior cardinal (V. cps.) and the two form a plexiform union 

 before they enter the heart together. The posterior cardinal cannot 

 be traced below the mesonephros, but it is possible that the injec- 

 tion is incomplete as in embryos a very little older than this it 

 extends dorsally to the posterior limb bud. The umbilical veins are 

 plainly visible, running from the allantois along the edge of the 

 membrana reunions to the sinus reuniens which they enter together 

 with the omphalomesenteric veins. (Vv. om.) These vessels are 

 not clean cut, but show clearly their plexiform origin. Along 

 the left umbilical particularly, we see many loops and open- 

 ings. There are also some spicule-like projections here and there 

 along the course of the umbilical veins that form the anlage of the 

 future plexus of the membrana reuniens. 



The second embryo, 7 mm., pictured here in Fig. 2, corresponds 

 roughly, as I have said, with that described by His. Probably the 

 most striking thing about it is the size of the umbilical veins 

 (Vvu). In the figure the right vein is seen to run from the 

 cord in a long curve for the whole length of the mesonephros to 

 the liver. At a point very little below the liver, a relatively small 

 vein is seen looping up superficially and emptying at the anterior 

 portion of the liver into the sinus reuniens (SR) above. This 

 vessel is a part of the umbilical that does not sink into the depth, 

 and receives numerous tributaries from the arm bud. The arm 

 bud, however, drains also into the posterior cardinal vein (Vcp) 

 above, and below by five good connections, directly into the large 

 umbilical vein (Vud). Below these connections, until the posterior 

 limb bud is reached, there is, as yet, only a very narrow strip of 

 body wall to drain. Such few vessels as there are here run into the 



