320 



Personal observations. — In the present observations, 

 attention has been confined to the question of the origin of the Sub- 

 clavian arteries in chick embryo. The subject was selected by Pro- 

 fessor William A. Locy, and the work carried on under his direction 

 in the Zoological Laboratory of Northwestern University. 



Surface study of living embryos. — Observations were 

 made on the living embryos, but little value, except of a negative 

 character, may be attached to them, only as controlled by the study 

 of sections. The primary Subclavian lies to^ deeply imbedded in the 

 tissues to be clearly seen from the surface, and it is likely to be con- 

 fused with the more superficially lying veins of the region; while in 

 the case of the definitive wing-artery, the density which the tissues 

 have assumed by the time this branch is established precludes ob- 

 servations on it from surface examinations. 



However, as regards the vessel, which in chicks of three to five 

 days incubation, appears crossing the ventral ends of the Aortic 

 arches, and which^ without doubt, is the one that Mackay describes 

 and figures as the Subclavian artery, some significant observations 

 may be made by a surface examination of the living embryo. This 

 vessel appears to take its origin from the ventral end of the third 

 Aortic arch, thence, crossing the other arches present in a posterior 

 and dorsal direction, it may be followed to the Ductus Cuvieri. The 

 flow of the blood in this vessel is from the third arch toward the 

 wing-bud. Moreover, since in young embryos the blood-flow in the 

 veins is intermittent, coordinating with the heart beats, it is easy, 

 from a superficial observation, to be led to the conclusion that this 

 is the artery supplying the fore-limb. But, if the neck of the embryo 

 be slightly stretched, the edge of the Hyoid arch pressed forward, 

 and the specimen rolled on its dorsal side, it can be seen that the 

 vessel in question continues in front of the third arch, and receives 

 branches from vessels lying ventral to the arch. Furthermore, one 

 can see that this vessel does not cross the Ductus Cuvieri, as claimed 

 by Mackay, and that its apparent continuation on the posterior side 

 of the Ductus is a part of another system. These observations were 

 fully confirmed by numerous repetitions and by a later study of 

 sections. In the latter, it became evident that the small vessel is a 

 vein, collecting blood from the ventral parts of the Hyoid and Mandi- 

 bular arches, and emptying into the superior Cardinal Vein just before 

 its junctions with the Ductus Cuvieri. The arterial circulation of the 

 wing-bud itself may be seen from the exterior, but can be traced 

 only as far as the base of the wing. 



