64 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



and to the left of this origin after the segment is broken. The intermediate part of 

 the interrupted segmeit lies closely applied to the cranial surface of the loop formed 

 by the vagus and its recurrent branch. The arch, in the character of its curve, 

 shows molding by the nerve, and frequently the aorta just caudal to it is flattened. 

 The molding is still more clearly seen on the left pulmonary arch, which does not 

 become interrupted. 



In spite of indications of pressure from the vagus on the degenerating arch, 

 models of two embryos in which the arch is not reduced do not show any consider- 

 able flattening of the vessel walls against one. another due to pressure. The lumen is 

 rounded, and in one specimen, in which the mesenchymal layer of the wall can be 

 made out satisfactorily, this is much thickened. The first distinguishable step in the 

 reduction, then, is a contraction. 



The disappearing segment of the arch seems to have been exposed to unfavor- 

 able conditions in regard to both longitudinal tension and pressure by the vagus 

 nerve. Yet a comparison of the history of the right and left arches at this time 

 brings out clearly that these factors are not the exclusive cause. The left arch 

 shows a well-marked molding by the vagus and its recurrent branch, but it does not 

 retrogress; on the contrary, at this time it is increasing in diameter. The reason for 

 its persistence in spite of unfavorable surroundings is probably to be found in its 

 more advantageous position relative to the pulmonary current. The bifurcation 

 between the pulmonary trunk and the arches is well to the left of the mid-sagittal 

 plane, due to the presence of the aortic trunk on the right. In consequence, the left 

 arch has a much shorter and more direct route to the dorsal aorta than the right, 

 thus receiving more blood and being better able to maintain itself. 



One embryo, in which the arch as a functioning element had gone, still had a 

 cellular cord extending from the junction of the right pulmonary artery and the per- 

 sisting ventral segment of the arch to the ventral edge of the caudal pharyngeal 

 complex. Though its cross-section was made up of a number of cells, the 

 endothelial and mesenchymal elements could not be distinguished from each 

 other. The post-mortem changes in the surrounding tissue made it impossible to 

 determine whether or not its cells were degenerating before the death of the embryo. 



We are fortunate in having models of three stages in the breaking of the dorsal 

 aorta between the third and fourth arches. In the first, a continuous curvature of 

 the third arch and the aorta cranial to it had developed, while the fourth arch had 

 similarly formed a common arch with the aorta on its caudal side (figs. 9, 11 ; plate 2, 

 figs. 34, 36). This indicates that, as the current in the fourth arch passes caudally, 

 that of the third arch moves in a cranial direction. With the perfection of these 

 curves, the intermediate aortic segment becomes more slender (fig. 12) and its 

 ends are pulled slightly downward and away from each other to give it an arched 

 form. It shows contraction by a thickening of its wall and decrease of its lumen. 

 Lehmann describes a condition in the pig (missing in our series) in which the further 

 moving apart of the distal portions of the two arches results in the pulling out of the 

 intermediate segment to a mere thread. In our next stage this filament is probably 

 broken, as we find a rounded mass at the upper end of the fourth arch, evidently due 



