156 A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



middle and distal thirds. Each receives from the body-wall tributaries coming 

 from both dorsal and ventral directions. The tributaries which reach the um- 

 bilical vein on its ventral wall arise for the most part within villus-like processes 

 of the body-wall. On the left side, one of these (found at the level of the tenth 

 and eleventh body-segments) is even larger in cross-section than the main stem 

 of the vein itself and forms a venous sinus in the villus (plate 5, fig. 1.) It is 

 drained by a relatively small vessel. Above this are a number of tributaries 

 which drain a longitudinal vessel situated in line with the venous sinus below. 

 Concerning the significance of these vessels I am in doubt, but believe them to 

 be either the remnants of the plexus from which the umbilical vein has developed 

 (Evans 6 ) or the beginning of the anterior body-wall plexus (Smith 41 ). 



Opposite the seventh somite the left umbilical vein receives a branch from 

 the vitelline vein, which lies ventral and caudal to the main junction of these 

 vessels. Approximately at the level of the interspace between the sixth and 

 seventh body-segments, the left umbilical vein receives from above the vena cardi- 

 nalis communis, entering by three distinct tributaries (plate 5, fig. 1), as described 

 above. Turning sharply medially into the septum transversum, it unites with 

 the vitelline vein (plate 6, fig. 1), and with it forms the left vitello-umbilical trunk. 

 This trunk, which is represented by one main vessel and two smaller ones, passes 

 medial to join the sinus venosus. 



The right umbilical vein likewise receives tributaries from the body-wall all 

 along its course and at its most cephalic point receives the right common cardinal 

 vein. It is united to only a portion of the vitelline vein and at but one point. 

 The vitello-umbilical trunk is represented on this side by a network of smaller 

 veins which connect it with the sinus venosus. 



VEN/E VITELLINE. 



The vitelline veins arise on the surface of the yolk-sac from the yolk-sac 

 plexus. Two principal veins, the right and left, are formed on the yolk-stalk by 

 the convergence of numerous tributaries. These course cephalad, one on either 

 side, and enter the septum transversum (plate 6, fig. 1). They pass dorsally along 

 the sides of the hepatic diverticulum, lying quite close to it and the wall of the 

 fore-gut. Several small tributaries proceeding from the mesenchyma surrounding 

 the fore-gut are received by them from above. Each breaks up into several 

 branches, which form a plexus within the transverse septum. A minute com- 

 missural branch connecting the veins of both sides is found in the notch between 

 the hepatic diverticulum and the fore-gut wall. The two connections of the left 

 vitelline vein with -the left umbilical vein have already been described. All the 

 blood carried by the left vein, except that which may cross over to the opposite 

 side in the small commissural branch, joins that of the umbilical vein before reach- 

 ing the sinus venosus. Near its termination the right vitelline vein breaks up to 

 form a plexus, the branches of which diverge. Some of these pass directly into 

 the sinus venosus, while others join the umbilical vein to form a plexus represent- 

 ing the right vitello-umbilical trunk. Only a part of the blood which is carried by 

 the right vitelline vein, therefore, passes through the right vitello-umbilical trunk. 



