DEVELOPMENT OF THE BLOOD-VASCULAR SYSTEM 759 



relation of the layers of the pericardial area is reversed. Each primitive aorta now consists of 

 a ventral and a dorsal part connected cephalad by an arched vessel traversing the first or man- 

 dibular visceral arch. In each succeeding visceral arch a similar vessel develops, so that in all 

 six pairs of aortic arches are formed, of which the r ifth atrophies early. 



Dorsal aorta. 

 Primitive jugular 



Amnior 



Cardinal veir 



Dorsal aorta. 



Body-stall 



Chorionic vill 



FIG. 532. Human embryo of about fourteen days old with yolk sac. (After His, from Kollmann's 



Entwickelungsgeschichte.) 



In the pericardial region the two primitive aorta? grow together and fuse to form the single- 

 chambered primitive heart tube (Fig. 533), the caudal end of which receives the two vitelline 

 veins, while from its cephalic end the two ventral aorta? emerge. By the rhythmic contraction 

 of the tubular heart the blood is forced through the aortre and bloodvessels of the vascular area, 



Aortic bulb 

 Ventricle 



Auricle 

 Sinus venosus 



Vitelline veins 



FIG. 533. Diagram to illustrate the simple tubular 

 condition of the heart. (Drawn from Ecker-Zieeler 

 model.) 



Aortic bulb 



Vitelline veins 



FIG. 534. Heart further advanced than in Fig 

 180. (Drawn from Ecker-Ziegler model.) 



from which it is returned to the heart by the vitelline veins; by this vitelline circulation the 

 nutriment is absorbed from the vitellus. 



Umbilical or Placental Circulation. With the atrophy of the yolk sac the vitelline circu- 

 ation diminishes and ultimately ceases, while an increasing amount of blood is carried through 



