102 PROCEEDINGS OF THE ANATOMICAL AND ANTHROPOLOGICAL 



into the sinus venosus mesial to the umbilical veins. Before they 

 open into the sinus venosus, a transverse piece connects the two vitel- 

 line veins. 



Umbilical Veins. The umbilical veins are large vessels, and lie in 

 the body stalk at first dorsal to the umbilical arteries ; here they com- 

 municate, and then run cranialwards in the body stalk to gain the 

 lateral body wall. From the lateral body wall the cranial end of each 

 vein bends abruptly, passing mesially and ventrally each to join its 

 own horn of the sinus venosus ; here each vein lies embedded in the 



Fig. 4. Fig. 5. 



Model of heart, x 75. Fig. 4, viewed from the front and left ; Fig. 5, seen from behind. 



An., aortic stem; B.C., bulbus cordis ; K, ventricle; r.Au., right auricle; I. An., left auricle; s.v., sinus 

 v.enosus; r.U.v., right umbilical vein; l.U.v., left umbilical vein; r.V.U.', right vitelline vein; l.l'.ii., 

 left vitelline vein ; D.M., dorsal mesentery ; T/i., depression in mesentery into which median thyroid 

 extends. 



septum transversum and close to the floor of the parietal recess 

 (Fig. 12). 



Aortce. The aortic stem divides into a pair of aortic arch vessels, 

 which pass on either side of the median thyroid anlage backwards in 

 the first visceral arch, and each of these is continued caudally as a 

 dorsal aorta. There are traces also of a second pair of aortic arch 

 vessels. The dorsal aortte rest on the roof of the gut on either side of 

 the notochord, and do not unite in any part of their course, remaining 

 paired throughout and terminating on the sides of the cloaca. A short 

 distance before the termination of the dorsal aortse the umbilical ar- 



