Embryonic history of the aortic arches in mammals. 425 



tainly whether the sinuses present in the first two arches in these 

 embryos are remnants of the former larger blood vessels which 

 eventually disappear, or are associated with the formation of the 

 external carotid. It is evident however, that the external carotid 

 is for the most part not primarily a remnant of the ventral parts 

 of the first two arches. The small vessels passing- forward from 

 the ventral ends of the third arches, in the earlier stages observed, 

 lie near the median plane. In the latter half of the twentieth day 

 they appear considerably nearer the exterior. The anterior prolonga- 

 tion of the truncus arteriosus, not found in the rabbit, disappears 

 in the pig in the latter part of the twentieth day. 



The third arch probably reaches its maximum development early 

 in the twentieth day. Subsequent changes bring this arch to about 

 the diameter of the dorsal aortic root anterior to it, and the two 

 vessels eventually form the internal carotid. The dorsal aorta 

 between the third and fourth arches decreases in size from the 

 latter half of the twentieth day, until, in probably tlie twenty-third 

 day, it has become extremely attenuated, with thickened walls, and 

 has broken away entirely from the dorsal end of the third arch. 



Changes of considerable extent take place in the relations of 

 the arches to each other as the common carotids develop, but since 

 these are also related to earlier changes, they will be considered in 

 speaking of the relations of the ventral ends of the aortic arches. 

 The fourth arches are of about equal size until late in the twentieth 

 day, when the vessel of the left side becomes larger, and from this 

 time onward the difierence increases until in the oldest stage re- 

 constructed the right fourth arch is of about one-half the diameter of 

 that of the left side. The right aortic root is also diminished greatly 

 in calibre, and in the oldest stage considered, appears continuous 

 with the fourth arch of that side, to the root of the vessel passing 

 to the forelimb. Behind the subclavian artery the right aortic root 

 is gradually obliterated. 



The series of embryos observed furnishes a comparatively 

 complete history of a fifth aortic arch for the pig. This arch when 

 complete is connected ventrally with the fourth arch, and dorsally 

 with the aortic root, and the dorsal end of the sixth aortic arch. 

 It is, where complete, a well-defined vessel with continuous walls. 

 In each embryo in which rudimentary structures were present 

 between the fourth and sixth arches, a fifth entodermal pouch was 

 observed. The completed fifth arch was found in the twenty-first 



