AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 67 



The central feature in the post-branchial arterial development is the evolution 

 of the aortic arch. It comes into being from various sources. Its beginning is indi- 

 cated by the replacement of the left fourth arch and the dorsal aorta between it and 

 the left pulmonary by a tube of continuous curvature at the time the aorta cranial 

 to it is narrowing its lumen preparatory to obliteration. In the arterial system of a 

 14-mm. embryo such as has just been described (plate 3, figs. 37 to 39), angles and 

 inequalities of diameter block out roughly the arterial regions which are losing their 

 individuality in the formation of the arch. These are the aortic trunk, the tube 

 derived from the left half of the aortic sac, the left fourth arch, the left dorsal aorta 

 between the fourth and pulmonary arches, and, finally, that portion of the left 

 paired aorta lying distal to the pulmonary arch. The irregularities of the arch have 

 disappeared by the time the embryo reaches a length of 17 mm. 



The radius of curvature of the early aortic arch changes in connection with 

 alterations in the direction of the long axis of the heart as it shifts downward into the 

 thorax. While the arch is in the lower neck region and the ribs of the two sides have 

 not become united in front by the rudiments of the presternum and sternal bands, 

 the curve of the arch is rather open, though it will be seen that its radius is already 

 less than when first forming (figs. 20, 24). As the heart passes into the dorsal con- 

 cavity of the thorax and is encircled by the ribs, its apex points less ventrally 

 and more caudally. In consequence the pars ascendens of the arch assumes a more 

 longitudinal direction. Since the more distal part of the arch is held by a number of 

 branches, a sharp bend develops between the two at the origin of the innominate and 

 left common carotid. Fty the time the summit of the arch has reached the level of 

 the first thoracic vertebra and the rudiments of the sternum have fused to complete 

 the superior thoracic aperture, the pars ascendens is nearly aligned with the long axis 

 of the body, and the arch for the time has more the form of a letter V than of a seg- 

 ment of a circle (figs. 21, 25). 



The arch is also peculiar at this time in that it lies almost completely in the mid- 

 sagittal plane. This is because the dorsal aorta has not yet moved to its position 

 at the side of the vertebral bodies, which are at this time so immature as not to have 

 assumed the strong convexity which later characterizes them in this region, and the 

 heart has not yet taken on its obliquity relative to the long axis of the body. 



The tracing of the regions of the arch system into the later arteries, as well as an 

 understanding of the changes in the latter, is largely a matter of inference based 

 upon changes in dimensions. Accordingly, the length and circumference of various 

 parts of the arch system were obtained, as also the length and circumference of the 

 parts of later vessels with which they were to be compared. For the study of most 

 regions a series of 11 embryos of graded development were used. Of these, 6 rep- 

 resented the branchial stage and 5 the post-branchial. The measurements were 

 made on models and then reduced to their true value by dividing by the magnifica- 

 tion. The reliability of the data was considerably increased by correcting approx- 

 imately for shrinkage of the vessels by a comparison of the length of the embryo at 

 the time of fixation and after embedding and sectioning. It will suffice here to state 

 the chief conclusions derived from the tables which were prepared from the measure- 



