AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 73 



TOPOGRAPHY OF AORTIC-ARCH SYSTEM AND ITS DERIVATIVES. 

 FUSION OF PRIMITIVE AORT.E. 



The components of the branchial-arch system undergo shifting in the direction 

 of all three axes of the body. Of these, the longitudinal are of greatest extent, 

 while the dorsoventral are inconsiderable and are confined chiefly to movements 

 of the arches which have already been discussed. In their movements the aortae 

 are in several respects in contrast with the rest of the system and require separate 

 consideration. The question of the lateral movements of the primitive paired 

 aortae is bound up with their fusion and the two subjects will be discussed together. 



It is not to be expected that the paired primitive aortae and their continuations, 

 the primitive internal carotid arteries, should maintain equal intervals between 

 each other in all their parts throughout development, since they extend almost 

 the full length of the body and must be exposed to many growth displacements 

 by surrounding structures. There is the possibility that they may come into 

 contact or that they may withdraw from each another, and in fact both conditions 

 are realized in different regions. The more striking changes in the position of the 

 arch system were appreciated by the early investigators. Von Baer (1828) pictures 

 the caudal movement of the heart accompanied by a development of a ventral 

 segment of the first arch and a caudal deflection of the ventral ends of the others. 

 He also shows how the blood-stream is shifted by the loss of cranial and the appear- 

 ance of caudal arches. His (1880) noted that the caudal ends of the third and 

 fourth arches took on a more cranial direction at their proximal ends and described 

 the changing direction of their arterial trunk. Tandler (1902) distinguished three 

 of the various types of wandering: (1) of the "conus," causing a relative lengthening 

 of the aortic arch; (2) upward displacement of the ventral portion of the arches; 

 and (3) caudal shifting of the fourth and pulmonary arches. Kingsbury's analysis 

 of the migration of the pouch derivatives and the related blood-vessels will be 

 referred to later. 



The primitive dorsal aortae during their earlier existence are separated from 

 each other by a contact of nerve-tube, digestive tract, and notochord, which inter- 

 pose between them a barrier of considerable width. This condition exists during 

 the appearance of the earlier somites, but the nerve-tube and notochord gradually 

 separate from the digestive tract, and mesenchyme moves in to fill the gap. 

 Before long the two aortae fuse in their intermediate portions which lie opposite 

 the throacic segments. It was of interest to ascertain whether this is preceded by 

 any actual approach of the vessels as a whole or whether only their adjacent walls draw 

 near due to the increase in diameter of the vessels. A comparison on models was 

 accordingly made between the interval separating the centers of the two vessels 

 soon after their establishment and the corresponding distance in others in which 

 the beginning of fusion was already indicated by the establishment of transverse 

 communications. The distances divided by the magnification are for the earlier 

 aortae 0.18, 0.16, 0.12, 0.15, and 0.12 millimeters; at the time of fusion they are 

 0.24, 0.15, 0.22, 0.14, 0.13, 0.22, and 0.24 millimeters. At the earlier time the 

 average is 0.146 and at the later it is 0.177+ . Clearly, then, the aortae as a whole 



