Development of the Fourth Day 245 



branch retaining its connection with the third 

 and fourth arches, as the systemic trunk. 



The dorsal communication between the third 

 and forth aortic arches (Fig. 76) soon becomes 

 obHterated ; and the ventral ends of the third 

 arches become converted into the subclavian 

 arteries, which carry blood to the anterior 

 appendages. The blood from the left side of 

 the heart now passes through the third aortic 

 arch to the anterior appendages, and through 

 the fourth arch to the dorsal aorta. The 

 fourth pair of aortic arches are from the fifth 

 day much larger than either of the other pairs 

 that are still present. For a time the two 

 arches of the fourth pair are of the same size, 

 but the arch of the left side soon begins to 

 diminish, and eventually almost completely dis- 

 appears ; the right arch increases in size as 

 the left diminishes, and forms the systemic 

 arch of the adult chick. It is to be noticed 

 that in man it is the left arch that persists as 

 the systemic arch. 



As early as the middle of the third day, the 

 pulmonary arteries are formed in the walls of 

 the lungs, and when the fifth pair of aortic 

 arches makes its appearance the pulmonary 

 arteries become attached to and open into the 



