104 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



Fusion does not progress farther cranially because the developing cervical 

 vertebra? and the digestive tube, pressing upon the aorta? from above and below, 

 tend to crowd the latter apart. If we take the paired aortse in connection with their 

 cranial extensions, the longitudinal neural arteries, we find that during growth these 

 two vessels, at first nearly parallel, are carried widely apart in three places and to 

 only a slight extent in two intermediate regions. The two vessels together produce 

 a trilobed figure. The most caudal separation, which is also the least in extent, 

 is due to the pressure of the cervical vertebra? and digestive tube, as already de- 

 scribed. The next is over the anterior part of the pharynx and is the result of the 

 rapid widening of this region. The third is on the forebrain and is produced by the 

 rapid lateral expansion of the latter. At the caudal end of the pharynx is a region 

 in which the arteries are in close approximation. The pharynx here is in a condition 

 of regression and increases in width very slowly. It may be that the pressure of 

 the vagus nerve on the lateral side of the vessels here, as it passes downward across 

 them, has some effect in preventing their being carried apart, since they sometimes 

 show its impress. Just cranial to the pharynx another approximation is due to a 

 less rapid increase in width here than is shown by the forebrain and pharynx lying, 

 respectively, cranial and caudal to it. 



The paired aortse and a part of the unpaired vessel shift backward relative to 

 their immediate environment, the pharynx and digestive tube. The shifting first 

 occurs at the cranial end of the aorta?, since here they first fall behind the surround- 

 ing structures in longitudinal growth. The withdrawal then takes place progres- 

 sively in more and more caudal parts of the paired aorta? and then involves the 

 unpaired aorta to an increasing extent. In the earlier part of the branchial period, 

 when fusion of the aorta? has just been completed, the withdrawal has not pro- 

 gressed to the fusion point, but is shown only by the bending backward of the dorsal 

 ends of the more caudal arches soon after each appears. The next indication is a 

 sharp forward bend of the proximal end of the cervical segmental arteries as far 

 back as the sixth. The aortic bifurcation remains at rest for a while, but the region 

 of withdrawal has extended back to it at about the end of the branchial period. 

 Beginning in embryos of 14 mm., there is a rapid caudal shifting of the point of 

 bifurcation, which ends at about the 17-mm. stage. The caudal movement con- 

 tinues and is not complete in the 24-mm. embryo, in which the superior thoracic 

 aperture is closed and the heart is in the thorax. 



The fusion of the paired aorta? also progresses caudally as well as cranially. 

 The caudal paired vessels are always very short, for while they are elongating dis- 

 tally they are fusing proximally. The paired condition, unless perhaps in the form 

 of very slender terminals, does not remain at this end of the body. In 4 to 6 mm. 

 embryos only very short double vessels are present and later no definite aorta? 

 could be recognized. The distance which the territory of the aorta derived from 

 the bifurcation shifts can not be told precisely. There is a type of anomalous right 

 subclavian artery, however, which evidently taps the aortic system by retaining 

 the caudal end of the right paired aorta, since in the adult it comes off as the distal 

 branch of the aortic arch. Inasmuch as it has been found coming off as low as the 



