AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 101 



arches and the primitive pulmonary arteries. These vessels are truly indicative of 

 a general structural plan which in some lower vertebrates is elaborated to a degree 

 that permits the development of paired ventral aorta?. There is no phase of human 

 development, however, in which such vessels exist. 



Involution of First Two Aortic Arches. 



After the disappearance of the first and second arches, their corresponding 

 visceral arches are each occupied by two successive channels; these may be termed, 

 respectively, the earlier and later mandibular and hyoid arteries. The earlier 

 lie near the axis of the arch ; the later run close to its caudal border. These arteries 

 all break up in a plexus in the subpharyngeal region. Their current is from the 

 dorsal aorta, and they supply the visceral arches. Only the later hyoid artery 

 could be found in the post-branchial period, when it constituted the stem of the 

 stapedial artery. There was no evidence of cell degeneration during the involution 

 of the arches. Small endothelial saccular enlargements or lacunae were found in 

 the subpharyngeal region after the arches had disappeared, but these were always 

 parts of vessels. They may be due to proliferation of endothelium later to be used in 

 the rapid differentiation of vessels. 



The precursors of the external carotid arteries are seen, soon after the first 

 and second arches have gone, as a pair of irregular and inconstant sprouts from the 

 aortic sac. They sometimes send branches into the bases of the mandibular and 

 hyoid arches. They first he near the mid-line, but gradually they either move 

 lateral ward or are replaced by more lateral vessels. In the ear her part of the post- 

 branchial period, when the identity of the third arch is becoming lost, these arteries 

 are found coming off from the middle of the third arch. Lingual, thyroid, and 

 other branches are distinguishable at this time. 



Principal Changes during Post-branchial Phase. 



The early post-branchial phase is the time when rapid disintegration of the 

 arch system takes place. Since the identity of its parts is largely topographic, 

 their walls differing little in structure, one can not expect to trace the parts, as hard 

 and fast units, into the later vessels. It is of interest, however, to learn in what 

 manner the earlier vessels give up their identity in the mature arteries which evolve 

 from them, since this subject in any part of the vascular system seems to have 

 received little attention. 



The rapid breaking up of the arch system is effected principally by its inter- 

 ruption in four places. The loss of the segment between the dorsal aorta on either 

 side, between the third and fourth arches, helps especially in the formation of the 

 carotids. The cutting off of the right paired aorta at its caudal end makes possible 

 the completion of the right subclavian artery. The degeneration of a part of the 

 right pulmonary arch permits the development in the fetus of the main pulmonary 

 channel, the two parts of which are termed the definitive pulmonary artery and the 

 ductus arteriosus. 



In each instance one immediate cause of the interruption seems to be a reduc- 

 tion of current-flow. In the two aortic segments between the third and fourth arch 



