102 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



it is due to a stagnation at this point, the streams in the aortic segments cranial 

 and caudal to it flowing in opposite directions. This in turn may be ascribed to 

 the rapid increase in the mass of the head region and the consequent enlargement 

 of the territory supplied by the cranial end of the aorta?. The right pulmonary 

 artery receives less current than the left, because it is a less direct route to the aorta, 

 due to the pulmonary trunk ending to the left of the mid-sagittal plane. The 

 stream to the right paired aorta caudal to the fourth arch is reduced, because the 

 sixth arch on this side has become interrupted and the aortic trunk is at this time 

 directing its current more into the left than into the right fourth arch. The short 

 interval of the aorta between the subclavian and the bifurcation suffers a still 

 greater reduction of current, as it does not carry the stream to the subclavian. 



The downward pull exerted by the descending heart upon the derivatives of 

 the arch system also probably contributes to these interruptions. That the effect 

 of tension is less important than current-strength is shown by a comparison of the 

 right pulmonary arch and of the right end of the right aorta with the corresponding 

 regions on the left side, since the latter were likewise exposed to the tension yet did 

 not succumb. The excessive tension at least hastens the degeneration, therefore, 

 after it is once begun. The regions affected first contract and then are pulled out 

 into long filaments. These in turn are broken. With the exception of the pul- 

 monary arch, which leaves behind a long cord of degenerating cells, the recoil 

 of the broken filaments brings back most of the substance of the involuting region 

 to a position close to the adjacent vessels, where it is worked over into their walls. 



The development of the definitive aortic arch is complex, since its material 

 comes from many sources. In a 14-mm. embryo the last trace of the division of 

 the dorsal aorta between the third and fourth arches is just about to disappear, 

 but the ends of the arches are still defined. Since the aortic arch is just taking form, 

 one can learn the sources of its respective regions, and it is seen that the aortic 

 trunk, left half of aortic sac, left fourth arch, left dorsal aorta between the fourth 

 and pulmonary arches, and the part of the aorta lying next most caudally lose their 

 identity in it. The right half of the sac elongates to become the innominate artery. 

 The right and left third arches, as far up as the external carotids, develop into the 

 common carotid arteries. 



The history of regions of the forming definitive aortic arch may be inferred 

 by a study of measurements of the divisions of the arch system in the late branchial 

 period and the parts of the arch apparently arising from them. The segment 

 between the innominate and left common carotid, originally equivalent to the left 

 half of the aortic sac, keeps pace with the growth of the body-length until the heart 

 and arch make their l'apid descent into the thorax at the stage of 14 to 18 mm. 

 At this time the innominate and left common carotid approach. This is almost 

 certainly due to the origin of the innominate moving distally on the arch. The prox- 

 imal part of the arch, as far as the innominate, has been elongating steadily, but 

 its sudden extension, as the arch rapidly descends, is probably due to the origin 

 of the innominate moving distally through the more distal region of the definitive 

 arch as far as the ductus arteriosus. As a result, the fourth-arch region, which was 



