THE EMBRYOLOGY OF THE BIRD's LUNG 31 



We shall now give attention to the transformations of the sixth 

 aortic arch bj^ means of which the pulmonary artery becomes 

 separated from the aorta so as to form an independent blood 

 vessel coming from the right ventricle of the heart. 



In the embryonic bird, after the sixth day, each pulmonary 

 artery arises from one-half of the sixth arch. In the adult bud 

 this has changed to the condition of right and left pulmonary ar- 

 teries connected with a single tmnk issuing from the right ven- 

 tricle. Not only has there been a complete separation of sys- 

 temic and pulmonic circulation, but, also, the distal extremities 

 of the sixth arch have atrophied. The way in which this is 

 brought about may now be followed. 



About the middle of the fifth day (4| day stage) the fourth, 

 or systemic arch, the rudimentary fifth, and the sixth, or pul- 

 monic arch are present. The fifth arch frequently fails to appear, 

 and when it is present, it is extremely transitory. The disap- 

 pearance of the rudimentary fifth leaves the fourth and sixth as 

 the two posterior arches. 



The left half of the fourth, or systemic, arch atrophies, the 

 right half alone remaining, which becomes much enlarged. 



In the meantime a septum develops that separates the right 

 systemic from the base of the pulmonic, the systemic arch becom- 

 ing connected with the left ventricle and the stem of the pul- 

 monic with the right ventricle. When this has been accomplished 

 the right and left halves of the sixth arch are still present and 

 undiminished in size. 



Figure 64 represents the condition at the close of the fourteenth 

 day of incubation. The lung of the chick does not become func- 

 tional as an organ of respiration until shortly before hatching. 

 Accordingly, most of the blood from the right ventricle passes 

 through both the left and the right divisions of the sixth arch to 

 join the aorta with which these are connected. The pulmonary 

 arteries springing from these divisions remain small (fig. 64). 

 Near the heart the fourth and sixth arches are separated but the 

 persistent right half of the fourth, and both right and left halves 

 of the sixth arch, join the aorta. The fourth arch issues from 

 the left ventricle, as the aortic arch, and the two halves of the 



