66 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



arch. Here, again, direct proof of its action, which in this case would be a marked 

 lateral compression of the degenerating vessel, was lacking. There was no available 

 material in which to study the degenerating first and second arches for evidences of 

 unfavorable effects of tension. 



Before considering in detail the manner in which the large vessels derived from 

 the arch system take form, it might be well to become familiar with a stage midway 

 between the late branchial and the approximately adult condition found in a24-mm. 

 embryo. In plate 3, figure 38, showing a 14-mm. embryo, it can be seen that the 

 right half of the aortic sac is represented approximately by a transverse tube, con- 

 cave cranially, and making, with the modified left limb of the sac and the derivatives 

 of its third arches on either side, the arm of a candelabrum-like figure the upright 

 stem of which is the aortic trunk. From the tube on the right and the sac on the left 

 arises a vessel, which still bears some resemblance to the third arch, and also a deriv- 

 ative of the fourth arch. These vessels, however, take origin more laterally and 

 dorsally, relative to their surroundings, and run more directly dorsal than do the 

 arches in the branchial period. The upper end of the zone arising from the fourth 

 arch is still marked on both the right and the left side by the tapering remnant of the 

 interrupted dorsal aorta as earlier described. The tube of the right side and its 

 fourth arch derivative are much longer than their equivalents on the left side, 

 whereas the latter are of much greater diameter. Those on the left also lie almost a 

 vertebra length more caudally. 



The definitive aortic arch is already roughly outlined at this stage, and the left 

 half of the sac and the widened left fourth arch are parts of it. The tubular deriva- 

 tive of the right half of the sac may be termed the primitive innominate artery, and 

 the regions corresponding to the lower parts of the third arches, up to the origin of 

 the primitive external carotid arteries, are the primitive common carotids. Distal to 

 this point are the primitive internal carotids. 



Individual variation must be reckoned with always in describing a single 

 embryo as a type. In this instance the model of an embryo slightly older than our 

 14-mm. specimen, while also normal in appearance, shows a marked difference in the 

 proportions of the innominate and right common carotid. The innominate has still 

 the form of a slightly elongated half of the aortic sac. To compensate for this the 

 common carotid is longer than in the other embryo. 



The pulmonary vessels no longer show any element suggesting the proximal 

 segment of the right pulmonary arch. The main pulmonary channel is a single large 

 straight vessel leading to the distal end of the definitive aortic arch and giving off a 

 pair of pulmonary arteries near its origin. The right paired aorta, though not inter- 

 rupted at its distal end, is much smaller than its counterpart on the left side. The 

 subclavian arteries are given off from the paired aorta? just before their confluence 

 to form the unpaired aorta. The vertebral arteries are present as branches of the 

 subclavians, and the basilar is completed through most of its later course by the 

 fusion of the longitudinal neural arteries. In position the arch sj^stem is now about 

 midway between its earliest location in the occipital region and its ultimate position 

 in the thorax. 



