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he Subclavians, gave the place of origin of that artery as the dorsal 

 end of the third Aortic arch. This still left open the question as to 

 how the Subclavians came to their adult position, and it was pointed 

 out, especially by Brenner ('83) that, owing to the difference in the 

 relative position, the Subclavians in the birds and in the mammals 

 could not correspond in a dorsal mode of origin, 



Mackay ('88) met this difficulty by maintaining that the Sub- 

 clavian, while arising from the third arch, as claimed by Sabatier, 

 came, not from the dorsal, but from the ventral end of the arch. 

 Mackay describes this vessel as appearing about the third day, when 

 the anterior limb exists as a very small bud. According to his state- 

 ment, it takes its origin from the ventral end of the arch, almost at 

 its juncture with the Truncus arteriosus, ventral to the Vagus nerve 

 and the superior Vena cava, and almost in its adult position. From 

 its point of origin, he traces it backward past the aortic arches 

 present, and loses it at the Ductus Cuvieri. In older embryos he 

 follows it into the base of the wing-bud, and, on the sixth day, into 

 the wing itself. Mackay asserts that he found no other vessel enter- 

 ing the fore-limb , and that in the younger embryos he saw no 

 aortic branch of particularly large size. 



HocHSTETTER ('90) published the result of an investigation of 

 the history of the Subclavian of birds, which he completed before 

 becoming acquainted with the work of Mackay. He found that, while 

 the ultimate vessel arises from the ventral end of the third arch, as 

 affirmed by Mackay, there is also a primary branch from the dorsal 

 Aorta supplying blood to the wing- bud. This dorsal vessel makes its 

 appearance prior to the formation of the ventral Subclavian, and he 

 places the time of the appearance of the latter as the sixth, and not 

 the third day, as claimed by Mackay. 



According to Hochstetter, the primary Subclavian, arising from 

 the dorsal Aorta about the fifth day, continues as the only source of 

 circulation to the wing-bud up to the first half of the sixth day. At 

 that time a juncture is formed between it and a ventral vessel which 

 in the meantime has grown from the third arch, and has advanced 

 to the base of the wing-bud. The anterior limb continues to receive 

 its supply of blood from these two sources until some time in the 

 eighth day, when the primitive dorsal Subclavian atrophies and dis- 

 appears. The vessel described by Mackay as existing on the third 

 day, and passing in a posterior direction the ventral ends of the 

 arches, is considered by Hochstetter as a vein, which arises within 

 the tissues of the ventral aspect of the Hyoid arch. 



