74 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



do not approach each other. If the slight difference in the average distance between 

 the earlier and later periods has any significance, it shows that the vessels are being 

 carried slightly apart by the general growth of their surroundings. The models 

 show that at this time the rudiments of vertebrae and nerve-tube are expanding, so 

 that the aortae are gradually taking on a medial position relative to their lateral 

 borders. Though the vessels as a whole do not approach, their increase in 

 caliber prepares for their fusion by approximating their adjacent walls. 



The fusion in the aorta? occupies about a week. In its first stage the two aortas 

 are connected by transverse anastomoses and He almost in contact. No. 2053, 

 a 3-mm. embryo, is apparently the only recorded example of this condition in 

 man. It has 4 cross connections, the largest being of nearly aortic caliber. 

 Sabin (1917) figures a slightly more advanced condition in a 20-mm. pig embryo, 

 in which about 15 are present, some of the more caudal being of large dimensions. 

 Embryos No. 2053 to No. 2841, inclusive (table 1), all show the process of fusion 

 still under way, though a long, more caudal region of continuous fusion is already 

 present in each. Tortuous swollen capillaries or straight transverse channels of 

 larger dimensions connect the two vessels just cranial to the fused region. Enlar- 

 gement of capillaries connecting the arteries and the development of larger trans- 

 verse communications from them are clearly in progress. The process is com- 

 parable with the development elsewhere of vessels from a capillary plexus. At the 

 cranial end of the region of continuous fusion the unpaired aorta has the cross- 

 section of a figure 8, often for a considerable distance. This evidently is the 

 result of the recent blending of a series of transverse communications. Tracing 

 caudally, a remodeling can be followed into a vessel of the usual form. 



It has sometimes been assumed that the fusion of the aorta progresses cranially, 

 and the spinal ganglia or vertebral rudiments have been used as points of compari- 

 son. This method leads to entirely erroneous conclusions. During the time in 

 which fusion is taking place, the nerve-tube and cervical vertebral column are 

 growing cranially relative to the pharynx, to which the aorta is moored by its 

 arches. The relative position of the pharynx and these more dorsal structures 

 also shows much individual variability in the fixed embryo. Whether this occurs 

 in life was not determined. It is by a comparison with the immediate environment 

 of the aorta, especially the pharynx and digestive tube, upon which, for the time 

 being, it does not shift, that changes in the region of fusion may be recognized. 

 The most cranial communication, or, in the absence of a communication, the end 

 of the region of continuous fusion, is found in all but the youngest embryo, showing 

 fusion to vary in the branchial period from a position of 3^ to one of 5^ body 

 segments caudal to the pulmonary arch. For these measurements in embryos too 

 young to show the pulmonary arch, the position later to be occupied by the arch 

 was used in place of it and was recognized by the caudal pharyngeal-pouch complex. 

 In the youngest fusion stage the most cranial communication is 9 body segments 

 behind the sixth-arch region. The fusion is thus shown to begin more caudally 

 and progress forward. The presence of a region of continuous fusion caudal to 

 the territory where it is in progress in the next older embryos points to the same 



