54 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



SUCCESSIVE DEVELOPMENT OF ARCHES AND SHIFTING OF CURRENT. 



The existence of aortic arches is the result of the interposition of the pharynx, 

 with its pouches, in the path of the blood-stream from heart to dorsal aorta. Since 

 the arterial end of the heart at first lies below the cranial end of the pharynx and 

 later shifts backward relative to it, the aortic arches develop in regular order from 

 before backward. As the more caudal ones are completed, the first and then the 

 second undergo involution. Later, the third arches cease to carry part of the 

 aortic stream. The current from heart to aorta is in this way shunted caudally. 

 Successive stages in the process are represented in figures 1 to 16. 



The earliest channel is the first arch, which for a time carries the entire aortic 

 current. It curves dorsally in a groove behind the head process in the mandibular 

 arch. At first it faces forward, but with the increasing curvature of the head region 

 it becomes more and more exposed to ventral view. 



In an embryo of 3 mm. (No. 2053) a second arch is forming (fig. 1, and plate 

 1, figs. 29, 30.) In the next individual of the series (No. 12016) the second arches 

 are well developed and the first have already decreased greatly in caliber (fig. 2). 

 The next available stage has a large third arch and a dwindling second (fig. 3). 

 Models were made from 8 embrj^os in which the fourth but not the sixth arch has 

 developed. In all but the youngest of these the first arch has gone and only a 

 slender channel passes through the mandibular arch. In the more mature speci- 

 mens the second arch also has disappeared (figs. 4, 5). The hyoid arch is now occu- 

 pied by a channel too slender and tortuous to be regarded even as a remnant of 

 an aortic arch. The phase in which the fourth arch is the most caudal feeder to 

 the aorta begins with embryos averaging about 4 mm. in length and ends with 

 embryos averaging 6 mm. The succeeding portion of the branchial period, which 

 is characterized by the presence of a pair of pulmonary arches and is terminated 

 by the interruption of the right arch, is represented by embryos from about 6 to 

 12 mm. in length. The approximate length in days of the various divisions of the 

 developmental period can be obtained from table 1. 



During the branchial period the changing bed of the stream from heart to 

 aorta follows these successive paths: first arch, first and second arches, second and 

 third arches, third and fourth arches, third, fourth, and sixth arches, and, not rarely, 

 the latter three in company with the so-called fifth arch. It is possible that the 

 first, second, and third arches also for a time share the current, though this con- 

 dition was not observed in our series. For most of the interval before the comple- 

 tion of the fourth arch, a single pair of vessels carries the greater part of the blood- 

 stream, so rapidly do the first and second arches dwindle. In the later part of the 

 branchial period, covering 9 of approximately 22 days which constitute the total 

 branchial span, there is comparative stability in the arch system, while the current 

 is divided between the third, fourth, and pulmonary arches. 



The length of the arches is surprisingly fixed during their entire existence, 

 although the body more than doubles in length during the same interval. The 

 length of the third and of the fourth arch was measured on models of 4 embryos in 



