325 



the wing. In diameter, however, the secondary artery at the time of 

 its establishment is very insignificant. Especially in its middle course 

 it is irregular in outline, and so small, that it can be followed only 

 with the greatest difficulty. The primary Subclavian still continues as 

 the main source of blood supply to the region of the wing. 



In the chick of seven days incubation, the situation remains much 

 the same as in the six day stages, except that the beginning of the 

 retrogression of the heart into the thorax has slightly shortened and 

 made more lateral the course of the ventral artery. The dorsal 

 branch, which still is very large, has through the sixth and seventh 

 days, gradually changed its form of attachment to the dorsal Aorta. 

 In the earlier specimens the short vessel which gives rise to the Sub- 

 clavians and Segmental arteries of either side arise independently of 

 one another (Figs. 1 to 14), while now the Segmental and Subclavian 

 arteries of both sides are given off from a single, median, dorsal root 

 from the Aorta (Fig. 20). 



The secondary Subclavian in the seven day chick is even more 

 difficult to trace than in the sixth day, owing to the fact, that numer- 

 ous other small vessels of this region have made relatively so much 

 greater development. Figs. 18 and 19, drawings made from sections 

 of an embryo of the latter part of the sixth day, show the origin of 

 the secondary Subclavian from the third Aortic arch; and Figs. 15, 

 16 and 17, of the same series, show the juncture of the two Subclavian 

 arteries of the left side. 



During the last hours of the seventh day, and the first part of 

 the eighth, very rapid and extensive changes take place in the Sub- 

 clavian circulation. The primary artery, which up to this time has 

 been practically the only blood supply to the wing region, now com- 

 pletely atrophies, although in specimens of the early part of the eighth 

 day it may be found as a small spur extending dorsally into the base 

 of the wing from the secondary vessel. The direction and length of 

 the definitive Subclavian is also much changed, appearing now as a 

 comparatively short and laterally directed branch (Figs. 21 to 24). 

 This change results from the position which the heart has assumed 

 posterior to the fore-limb. 



By the ninth day the situation has reached a condition very 

 similar to that in the adult, the definitive Subclavian having become 

 relatively much shortened, and almost directly lateral in its course 

 (Figs. 25 and 26). 



An examination of sections of ten-day stage and older show that 

 this vessel is the definitive Subclavian. The changes that later take 



