202 Vertebrate Embryology 



above the pharynx, on each side of, or just 

 below, the notochord. Through small arteries 

 that are given off from the first aortic arch, 

 or from the anterior ends of the dorsal aortae 

 (Fig. 65, I.CA, and E.CA), a small amount 

 of blood finds its way into the head of the 

 embryo, but the greater part of the blood 

 passes posteriorly through the at first double 

 and then single (Fig. 65) aorta ; the single 

 aorta soon becomes double again, as has been 

 previously described, and through these two 

 posterior aortae the blood passes to the hinder 

 end of the embryo. Only a small part of the 

 blood is distributed to the hinder end of the 

 embryo, at this period, however, the greater 

 part passing to the vascular area through the 

 two large vitelline arteries that are given off, 

 one on each side, from the posterior paired 

 aortae (Fig. 65, Of.A). The blood from the 

 vascular area returns, through the vitelline 

 veins, to the meatus venosus, and thence to the 

 auricular region of the heart. The blood from 

 the anterior end of the embryo returns, by the 

 anterior cardinal veins, to the Cuvierian veins, 

 where it meets the blood from the posterior 

 end of the embryo that has been brought 

 forward to that point by the posterior car- 



