331 



positions. But otherwise, the artery supplying blood to the wing in 

 the eight day chick, is identical with the Subclavian of the adult. 



Conclusions. — The results obtained from the present ob- 

 servations are, therefore, except as regards the independent origin of 

 the Subclavian and Segmental artery of the same segment, in exact 

 accordance with those of Hochstetter; that is, the final Subclavian 

 does arise from the ventral end of the third Aortic arch, as de- 

 monstrated by Mackay, but that this vessel is secondary as a blood 

 supplier of the fore-limb, a subclavian circulation having previously 

 existed, which took its origin from the dorsal Aorta. My observation 

 show the primary Subclavian in earlier stages than those of Hoch- 

 stetter; his earliest description of this artery is in embryos of 

 100 to 106 hours incubation. 1 was able to find it in 72 hours stages; 

 and Figs. 1 — 4 show it in the 80 hour stage. 



The primary Subclavian is an important part of the embryonic 

 circulation from the third to the beginning of the eighth day, supplying 

 the rapidly growing parts in the neighborhood of the wing. In the 

 early stages the vessels of either side arise independently of each 

 other, and also, according to my observations, they arise separately 

 from the Segmental arteries of the respective somites (Figs. 1 to 4). 

 Later, however, by a pinching up of the walls of the Aorta, resulting 

 from unequal growth, both the Segmental arteries and the Subclavians 

 of either side come to have a common origin from a short stalk from 

 the dorsal wall of the Aorta; and finally, through a continuance of 

 the same process, these two stalks are united to form a single, 

 median, dorsal branch from which the Subclavians and Segmental 

 arteries of both sides arise (Fig. 20). 



For a time (sixth to about the eighth day) the blood supply of 

 the region of the wing is derived from two sources, the dorsal Aorta 

 and the third Aortic arch. At the beginning of this period the greater 

 amount of the blood is received from the dorsal branch, but this con- 

 dition is soon changed, and by the end of the eighth day the dorsal 



Fig. 27, Camera sketch of dissection of heart and Aortic arches of a chick embryo 

 in the later part of the sixth day of incubation, showing the course of the primary 

 Subclavian from the Aorta, and of the secondary Subclavian from the third arch, and 

 their junction in the base of the wing. The Aorta was moved slightly backward from 

 its natural position, and the diameter of the secondary artery is slightly exaggerated. 



Fig. 28, Camera sketch of a dissection of the heart and Aortic arches of a chick 

 embryo between eight and nine days of inciibation. 



Fig. 29, similar drawing of a chick embryo of about thirteen days incubation. 



ow. auricles. a.S. third or Carotid arch. a. 4- fourth or Systemic arch. a. 5. fifth 

 or Pulmonary arch. c.cr. common Carotid. Other reference letters as in preeeeding 

 figures. 



