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bination of several sections in this region. The wing-artery is here 

 formed by a latero-dorsal derivative (scl.a.p.) of the Aorta (d.ao.) and 

 is, in the early stages, entirely separate from the dorsal portion of the Seg- 

 mental artery of this somite. The course taken by the primary Sub- 

 clavian is first an upward and a lateral one, when, bending in a broad 

 curve, as shown in Fig. 4, it sweeps downward into the wing. This 

 broad angled turn in the course of the artery remains about the same 

 throughout the existence of the primary Subclavian. 



During the next twelve hours the primary Subclavian, as well as 

 the Segmental artery, assume much more definite proportions, and, 

 while they arise independently of each other, their roots have a similar 

 origin from a latero-dorsal dilatation of the wall of the dorsal Aorta. 

 It is by a later constriction of this dilatation that the Subclavian and 

 the Segmental artery come to have a common root (Figs. 3 and 4). 



In specimens of one hundred hours of incubation, when the wing- 

 bud has attained a relatively large size, the primary circulation is 

 well established. The wing vessel of each side is given otf in common 

 with the Segmental artery of that side from a short dorsal branch of 

 the Aorta. Although about fourty-eight hours will elapse before the 

 secondary artery will be established, nevertheless, at the point where 

 the juncture between the two is later to take place, there is some- 

 times an enlargement of the primary vessel, and an irregular branch 

 of small diameter may be seen in some specimens extending in a 

 ventral and dorsal direction (Fig. 5 scl.a.s.). This is, perhaps, the 

 first indication of the ventral vessel which appears to develop from 

 both ends. The circulation in the wing-bud itself is at this time very 

 well established, but no arterial vessels are found in the wing that 

 cannot be traced to a connection with the dorsal Aorta. 



In the specimens of this age very good sections were obtained 

 of the small vessel, supposed by Mackay to be the Subclavian. It is 

 undoubtedly a vein, as previously noted, and it passes very close to 

 the third arch at the point where the ventral Carotid is later given 

 off. In the neighborhood of the fourth arch it becomes somewhat en- 

 larged, and receives a branch which gives it the appearance of the 

 branched vessel shown in Mackay's figures as the Subclavian. 



From the fourth to the beginning of the sixth day very little 

 structural change takes place. The wing artery from the Aorta in- 

 creases in size, commensurately with the development of the parts which it 

 supplies (Figs 5 to 14). Up to the sixth day there exists no branch 

 from the third arch except the Carotids; but in the early part of this 

 day a very small vessel may be found springing from the ventral end 



