Embryonic history of the aortic arches in mammals. 429 



the vessel, reducing- it to the condition of a cord (aorta between 

 the third and fourth arches), and one in which the middle portion 

 breaks down into disconnected sinuses. The dorsal remnant is more 

 persistent than the ventral. 



5. The subclavians begin from the dorsal aorta and shift for- 

 ward in front of the union of the aortic roots. 



6. The pulmonary arteries arise as symmetrical vessels from 

 each sixth arch, and in the pig become asymmetrical by union of the 

 stems from both sides, reaching the condition figured by Rathke. 

 This is, however, a secondary condition, their symmetrical origin 

 having been pointed out by Beemer. 



