329 



to make, owing partly to the fact that these arteries, at first, have 

 very thin walls, being, in fact, little more than spaces in the meso- 

 blastic tissue; and partly for the circumstance that the wing artery 

 in each case branches from the main vessel at such a sharp angle, 

 that it is exceedingly difficult to force an injection mass into them. 

 Nevertheless, satisfactory dissections were obtained in specimens of the 

 later part of the sixth day, the early part of the seventh, and later 

 (Figs. 27, 28 and 29). 



In the seven-day stages both the primary and secondary arteries 

 were found to exist simultaneously; one arising from the dorsal Aorta 

 in a position just dorsal to the ventricles of the heart, and passing 

 to the tissues at the base of the wing, where it is joined by the 

 ventral vessel, which, by a somewhat longer course, has reached this 

 point from its place of origin on the ventral end of the third arch 

 (Fig. 27). The definitive vessel springs from the Carotid arch on its 

 anterior margin, and at some distance from the ventral end of the 

 arch. This later condition may be accounted for by considering the 

 fact that the ventral part of the arch and its ventral conection with 

 the fourth arch are lengthening to form the innominate artery. The 

 course of the secondary vessel, as was seen in the sections, is quite 

 long, and the diameter of the artery is very small. 



The course of the artery is as follows : Arising from the anterior 

 side of the arch, it starts forward, but almost immediately turns 

 sharply, and proceeds in a backward course to the wing, where it is 

 distributed (Fig 27). This long backward course is necessitated by 

 the relative positions of the heart and the fore-limb. Later, as the 

 heart passes back into the thorax, the course of the vessel becomes 

 more lateral, passing almost directly outward from the third arch 

 (Figs. 28 and 29). 



The primary Subclavian proceeds at first in a dorsal direction, 

 then, making a sharp turn, as noted in the sections (Fig. 20), it passes 

 dorsally to the lung and joins the secondary vessel in the base of the 

 wing. 



Dissections made in the eighth day show no sign of the artery 

 from the dorsal Aorta, while the secondary Subclavian has become 

 much larger, straighter, and more lateral in its course, approaching 

 the condition in the adult (Figs. 28 and 29). 



From the eighth day on, no changes take place that would have 

 a bearing on the question of the origin of the Subclavian. The artery 

 undergoes a slight change of direction, and is shortened somewhat as 

 a result of the shifting of the neighboring organs to their adult 



