90 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



bifurcation at the time when fusion ceases, it is somewhat variable, and it is not 

 unlikely that an occasional subclavian may arise from a paired aorta even at the 

 time of its first appearance. The subclavians do not share in the descent of the 

 aorta but move cranially upon it as it descends. As a result, they are each soon 

 found to be coming off from the paired aorta of its side. In 4 embryos of the 

 branchial period, in which the pulmonary arch was complete, the subclavians of 

 3 were already cranial to the bifurcation. In all of 8 slightly older specimens the 

 subclavians came from the paired aorta?. Important in the shifting of the sub- 

 clavians on the aorta? is the mooring of the vessel by its large vertebral branch which 

 is passing into the rudimentary vertebral foramen of the sixth cervical ver- 

 tebra. Its comparatively large size is also a factor, for, as will be seen in the dis- 

 cussion of the development of the vertebral arteries, the segmental arteries lying 

 cranial to the subclavians, though having a similar relationship to the paired aorta? 

 at this time, are not able to move upward on these vessels and so are stretched and 

 finally interrupted. 



In the post-branchial period the asymmetrical changes in the paired aorta? 

 involve their branches (the subclavians), and the history of the latter becomes 

 very different on the two sides. On the left the subclavian continues its movement 

 along the aorta; on the right this is rendered unnecessary by the interruption of 

 the right paired vessel close to the bifurcation. This also makes it possible for the 

 right fourth arch and the paired aorta caudal to it to become the proximal end of 

 the right subclavian, since there has already been an interruption of the right 

 paired aorta cranial to the fourth arch. A decrease of the right dorsal aorta 

 to a diameter equivalent to the subclavian has taken place before it was separated 

 from the aorta at its caudal end. Because the more cranial break occurs con- 

 siderably earlier than the caudal, the sharp angle between the arch and aorta has 

 gone and the two vessels have formed a nearly straight channel before the aorta 

 has lost its lumen at its distal end. This distal interruption of the aorta is accom- 

 panied by abrupt changes in the arterial channel where the more primitive sub- 

 clavian enters the aorta. Just before the wall of the involuting segment weakens, 

 and while it is exerting its maximum tension at this point, due to the pull from the 

 shifting aorta, the subclavian and aorta meet at a downwardly directed acute 

 angle. The degeneration of the wall of the disappearing aortic segment releases 

 its tension on the point of union of subclavian and dorsal aorta, thus permitting 

 this point to withdraw upward until the primitive subclavian passes obliquely 

 upward to go over into the aortic segment by a moderate curve (figs. 14, 15, 19). 

 By the time the costal and sternal rudiments have swung toward one another 

 and fused in the mid-line, the curves have disappeared and the fourth arch and 

 aortic territory of the subclavian are no longer distinguishable. In this manner a 

 channel of great tortuosity is reduced to a nearly straight segment at a time the 

 embryo is increasing only about 30 per cent in length. It must be accomplished 

 by a great slowing in the growth of its wall. Later, the forward and medial growth 

 of the ribs produces another marked curve just distal to the origin of the vertebral 

 artery, so that the part of the subclavian proximal to the rib forms a letter U 



