AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 105 



fifth thoracic vertebra, there can be little doubt that the region of the aorta derived 

 from the bifurcation also moves down this far. In fact, it is not unlikely that the 

 subclavian has moved up the aorta somewhat from the region corresponding to the 

 bifurcation, since the left subclavian regularly does so. It is known that the 

 caudal end of the aorta withdraws cranially. There must, therefore, be a point 

 intermediate between the ends which remains more or less fixed in relation to its 

 immediate environment, and this point must be in the proximity of the thoracico- 

 lumbar boundary. 



The interruption of the right paired aorta at its caudal end is quickly followed 

 by a shifting to the left of that part of the definitive aortic channel just back of the 

 point of the former bifurcation, as the angle between it and the right paired aorta 

 is straightened. This is a step in the movement of the thoracic aorta to its final 

 position in the left side of the vertebral column. It is probable that a pressure 

 analogous to that which crowded apart the paired aortse acts later to push the 

 definitive unpaired aorta to the left. 



The shifting of the aorta is paralleled by a caudal displacement of the arches, 

 their ventral connections, and the heart itself. The arches not only shift at their 

 upper ends as far caudally as possible, but the entire fourth arch is curved around 

 the caudal pharyngeal complex at the end of the branchial period, as though this 

 mass were resisting its caudal progress. In a similar way the loop formed by the 

 vagus and recurrens nerves presses against and molds the pulmonary arches. The 

 aortic sac also shifts correspondingly. In the post-branchial period the interrup- 

 tion of the various parts permits a rapid descent of heart, arch, and other vessels. 

 The fourth arch moves, relative to the pharynx, about 4M body segments. Since 

 the nerve-tube grows forward relative to the pharynx, the arch moves on the nerve- 

 tube about 13 body segments. During the period of rapid descent (embryos 14 

 to 18 mm.), the arch moves at a rate of about one-fourth of a segment a day. This 

 displacement is the continuation, in another guise, of the shifting of the blood-stream 

 from heart to aorta, which, in the branchial period, was effected by the loss of the 

 cranial arches and the development of new caudal ones. 



The heart changes its relation to the sac during growth. At first the arterial 

 trunk approaches the sac from a cranial direction, indicating that the apex of the 

 heart is pointing forward. At about the end of the branchial period we find the 

 apex of the heart pointing in the opposite direction, so that the arterial trunk 

 reaches it from its caudal side. The long axis of the heart is at right angles with 

 the perpendicular axis of the body at about the time (near the end of the branchial 

 period) when the proximal ends of the fourth and pulmonary arches are well apart. 

 Therefore, it may be that the heart crowds against these arches at this time and 

 pushes them apart. 



The downward movement of the heart, sac, and arches, like the retreat of the 

 aorta, is due to the failure of the heart and certain territories caudal to it to keep 

 pace with the longitudinal growth of other adjacent parts of the body. The descent 

 of the heart causes a movement of other structures to fill in the space vacated by 

 it, such as the pharyngeal derivatives and probably mesenchyme. The arteries, 



