AORTIC ARCHES OF BIRDS — GLENNY 539 



there is also present a branch from the dorsal aorta to the anlage of 

 the wmg, and that this latter vessel precedes the appearance of the 

 secondary or definitive subclavian artery. The secondary subclavian 

 makes its appearance on or about the sixth day of incubation, while 

 the primary subclavian appears on about the fifth day according to 

 Hochstetter. 



Evans (1909b) has shown that the segmental subclavians commonly 

 occur in the 16th to 19th intersomitic spaces. Hughes (1934) later 

 pointed out that the segmental subclavian of the first intersegmental 

 space enlarges at the expense of the others, and soon becomes the 

 single dorsal subclavian artery although considerable variability in 

 the primary subclavian development exists. Fleming's (1928) studies 

 are largely confirmatory of Hughes' observations. 



The two independently derived vessels (primary and secondary 

 subclavians) come to form a junction on about the sixth day of 

 incubation with the result that the limb-bud receives its blood supply 

 from two separately derived vessels until about the eighth day, at 

 which time the primary subclavian atrophies and finally disappears. 

 Confirmatory studies on the origin and development of the subclavians 

 in the chick were carried out by C. G. Sabin (1905). He reports that 

 the primary subclavian begins to make its appearance at about 72 

 hours of incubation, and that by the first half of the fourth day the 

 primary circulation is well established. He points out that the wing 

 vessel is given off in common with the segmental artery on each side 

 from a short dorsal branch of the aorta. Development of the second- 

 ary subclavian appears to take place from the primary subclavian 

 forward and from the carotid arch backward. During the early part 

 of the sixth day, Sabin reports the beginning of the formation of the 

 ultimate subclavian from the carotid arch, where it arises from the 

 anterior surface. 



At the time of junction of the two subclavians the forelimb occupies 

 a position posterior to the heart, with the result that the secondary 

 subclavian has a comparatively long course to the limb. The major 

 blood supply to the wing is still provided by the primary vessel until 

 about the seventh day of incubation, at which time the heart begins 

 to retrogress into the thorax, thus shortening the course of the second- 

 ary subclavian. During the latter part of the seventh day and early 

 part of the eighth day of incubation the primary subclavian atrophies, 

 although a distal vestige may remain for a short time as a small spur 

 extending dorsally into the base of the wing from the secondary sub- 

 clavian. 



As the heart migrates posteriorly it gradually comes to lie in a 

 position posterior to the wing. Consequently, the definitive subclavian 

 becomes shortened and laterally directed. By the ninth day the 

 condition in the embryo is similar to that in the adult. 



