CH. LIX.] THE PRIMITIVE ARTERIAL SYSTEM 851 



fourth arch becomes the permanent aorta, curving over the left 

 bronchus. 



Behind the dorsal roots of the fifth arches the dorsal longitudinal 

 vessels fuse together, as far back as the lumbar region, to form the 

 descending aorta, and the lower or posterior end of this vessel is con- 

 tinued at first through the feaudal arches into the posterior ventral 

 portions of the longitudinal vessels which end on the yolk-sac (fig. 

 646). As soon as the allantois forms, each of the posterior ventral 

 vessels gives off a large branch to it, and in front of the origin of this 

 vessel it atrophies so that now the dorsal vessels are continued 

 through the caudal arches into the allantoic or umbilical arteries, 



15 



14 



FIG. 646. Diagram representing arrangement of primitive blood-vessels of left side of embryo. 1, Left 

 primitive jugular vein ; 2, left duct of Cuvier ; 3, left cardinal vein ; 4, protovertebra ; ft, primitive 

 intestine ; 6, caudal aortic arch ; 7, allantoic or umbilical artery ; 8, placenta ; 9, atrophied 

 posterior ventral part of primitive vessel ; 10, yolk-sac artery ; 11, yolk-sac ; 12, vascular area on 

 yolk-sac ; 13, pericardium ; 14, heart ; 15, cephalic aortic arch ; 16, brain. 



which carry blood to the placenta, and new vessels of small size are 

 given off from the descending aorta to the yolk-sac. A little later 

 the primary caudal arches, which lie inside the posterior ends of the 

 Wolffian ducts, are replaced by new arches, which pass outside 

 the ducts, and connect the posterior ends of the dorsal longi- 

 tudinal vessels with the allantoic arteries. At the same time the 

 hind limbs appear, and each receives a branch from the corresponding 

 dorsal vessel ; this is the external iliac artery. After its appearance 

 the part of the dorsal vessel between it and the aorta is the common 

 iliac artery, and the portion of the dorsal vessel behind it, together 



