140 THE STRUCTURE OF THE FOWL 



By this time important changes have taken 

 place in the vessels of the embryo. The two 

 primitive aortas have united to form a single 

 dorsal aorta arising from the heart as the 

 right and left aortic arches. Other arches 

 (making a total of six) appear in the region 

 of the neck, but only three or four are present 

 at one time. The first, second, and fifth 

 arches do not remain long. The third arch 

 ultimately forms the carotid artery, and from 

 it springs the subclavian. The fourth arch 

 on the left side disappears ; while the cor- 

 responding arch on the right becomes the 

 single aortic arch of the adult. Part of the 

 sixth arch becomes the pulmonary artery. 

 The truncus arteriosus or common origin of 

 the aortic arches is divided longitudinally 

 into the aorta and pulmonary artery. Com- 

 plicated changes of the venous system within 

 the embryo also occur. 



The formation of an extra-embryonic system 

 of vessels in the wall of the yolk-sac takes 

 place, as has been said, at an early period. 

 The formation of the allantois results in a 

 second extra - embryonic circulation. The 

 allantois carries arteries from the dorsal aorta 

 to the chorion, which is thus rendered capable 



