. AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 99 



have probably contributed to its survival. The dorsal aorta, as was earlier shown, 

 does not shift equally throughout its entire extent, in relation to its environment, 

 but the movement begins at the cranial end. At the time of the formation of the 

 vertebral artery the process has just reached the caudal end of the cervical region. 

 It may be, then, that there has as yet been no shifting at the origin of the subclavian. 

 If there has already been a slight movement, then we must infer that since the sub- 

 clavian still comes off perpendicularly it must have moved along the aorta, just as 

 we know it does in the succeeding phase of its development. It is not improbable 

 that of the cervical segmentals it alone can do this, since it has a greater current 

 than its companions, due to its supply of the limb-bud. 



Because of its success in maintaining itself, it naturally falls heir to the distal 

 territory of the more cranial segmentals by means of the anastomotic chain which 

 connects them and which earlier seems to have supplied them with blood from the 

 opposite direction. Its ultimate capture of the vertebral as a branch, as soon as 

 time is given it to expand, is a natural sequel of its closer connection with the main 

 arterial stream than is possessed by it's rival, the internal carotid. 



SUMMARY. 



The evolution of the aortic-arch system is one of the most striking and com- 

 plete instances of recapitulation in human development. The arches are not, 

 however, all present at one time, as in many anamniotes, the first disappearing 

 before the last arises. The arches develop as a result of the interposition of the 

 pharynx with its pouches between heart and aorta in the early embryo. As soon 

 as the heart moves away from the pharynx they disappear. The developmental 

 period during which the arches are present may be termed the branchial-phase, 

 and the remaining time, up to the attainment of the adult condition, the post- 

 branchial phase. The interruption of the sixth arch was arbitrarily taken as mark- 

 ing the division between the two. The branchial phase occupies about 22 days 

 and the post-branchial 28 years, yet the organization of the adult arterial system of 

 the head, neck, and thorax is far along toward completion in the first 14 days of 

 the post-branchial period. 



The first arch has been found in mammals to develop from a preexisting angio- 

 blastic and capillary net. In man the arches are formed of sprouts converging 

 from the dorsal aorta above and the aortic sac below. These seem not to be pre- 

 ceded by a very complete net, though a sparse plexus does first grow out from a 

 bulging of the aorta and the sac. The sprouts, because of their rapid formation 

 and large size relative to the net, seem to develop as much from an outgrowth of 

 endothelium as by differentiation of the net already present. In the development 

 of the pulmonary arch the simple dorsally directed sprout from the sac does not 

 appear. Instead, there is an outgrowth of the same nature, directed caudally. 

 By fusion with a sprout from the dorsal aorta it is bisected into a proximal and a 

 distal part, the first forming the ventral end of the arch and the second the primitive 

 pulmonary artery. 



The arches develop in their order from before backward. The first undergoes 

 involution about the time the fourth is complete, and the second disappears before 



