AORTIC- ARCH SYSTEM IN THE HUMAN EMBRYO. 69 



or both of them moving in a certain sense through its substance. As no good reason 

 for assuming that the arteries undergo a decrease of substance while maintaining 

 their diameter and function was found elsewhere in this study or in the literature, 

 this alternative may be dismissed from consideration, and it may be safely con- 

 cluded that the substance of the wall has shifted about to permit a movement of the 

 origin of one or both branches. The two arch divisions lying proximal and distal to 

 the interval between the innominate and carotid arteries differ greatly in their 

 changes in length. The segment proximal to the innominate, taken with the truncus 

 aorticus, to which it is equivalent at the beginning of the post-branchial period, 

 shows an increase in length which becomes very rapid when the innominate and 

 carotid are approaching at the time of rapid descent. The part distal to the left 

 common carotid, extending to the upper end of the sixth arch, shortens in the late 

 branchial and early post-branchial periods and later remains constant during the 

 time of rapid descent. 



It is clear that in the history a sharp distinction must be drawn between the 

 period before and the period occupied by the rapid descent. Before the descent the 

 truncus arteriosus and the succeeding division of the arch which has developed from 

 the left half of the sac increase in length. As has been seen, no marked increase in 

 diameter is required, since in the branchial period these vessels are relatively capa- 

 cious parts of the arch system. The distal portion of the definitive aortic arch com- 

 ing from the left fourth arch and from the aorta distal to it is in contrast with the 

 more proximal part of the forming arch. They remain unchanged in length up to the 

 time of rapid descent. In circumference the part derived from the fourth arch 

 undergoes an especially rapid enlargement, since in the branchial period it is only 

 one of six conveyers of the blood from heart to aorta, while at this stage it transmits 

 more than half of the entire current. At the time of rapid descent the innominate 

 and the left common carotid approach, while the distance between the innominate 

 and the aortic valves increases with especial rapidity. It is natural to conclude that 

 the innominate has moved toward the left common carotid. Since the distance 

 between the left common carotid and the ductus arteriosus remains constant, the 

 former probably does not change its position on the arch. These inferences, drawn 

 from the changes in length of the various parts, agree with expectations based upon 

 the relation of the two vessels to the forming arch at the beginning of the post- 

 branchial period. As the left carotid is at its summit and the innominate comes off 

 from its ascending limb, only the innominate could respond to the tension upon it 

 by moving along the wall of the descending arch. 



A result of the retardation in the elongation of the distal part of the arch relative 

 to the proximal is a change in the region which forms its summit. In the 14-mm. 

 embryo, in which the arch is just taking form, the entire left fourth arch is the 

 summit (plate 3, fig. 37). The relative shortening of the distal part of the definitive 

 arch results in a drawing down of the fourth-arch zone into the descending limb, 

 thus leaving the left common carotid at the summit (fig. 24). The distal migration 

 of the innominate on the ascending limb also serves to bring it to a position on the 

 highest part of the arch. 



