430 



Dr. J. Y. Mackay. 



[June 16, 



and vein are dorsally placed as regards the artery, and the recurrent 

 laryngeal nerves turn round the ductus arteriosus or vestiges of the 

 fifth arches, a relationship which cannot be accounted for by supposing 

 with Rathke that the vessel takes its origin first from the aortic 

 root, because, if so arising, it would occupy a position dorsal to vein 

 and nerve, and it is impossible to imagine a method by which the 

 artery could pass from the dorsal to the ventral aspect of these struc- 

 tures without cutting them through in its course. 



n.V. Vh Jjjr 



D.vr 



W. Ventral vessel. S 1 . Subclavian of Mammals. 

 D V. Dorsal vessel. S 2 . Subclavian of birds. 



In these circumstances the author has undertaken an investigation 

 into the manner in which the subclavian artery first makes its appear- 

 ance in birds. He finds that it occupies from the first a ventral posi- 

 tion arising from the truncus arteriosus at the ventral end of the 

 third arch. The vessel may be seen in the freshly removed embryo 

 duck or chick on the third or fourth day, at a time when the pectoral 

 limb is merely a small projection from the body- wall, and on the fifth 

 day in the chick it may, while still filled with blood, be traced by the 

 eye from the ventral end of the third arch across the superior cardinal 

 vein to the limb. The presence of this ventral vessel is also demon- 

 strated during the third, fourth, and fifth days in the chick by micro- 

 scopic sections of hardened embryos, but, owing to the oblique course 

 which the artery holds in the body- wall, it is impossible in one series 

 of sections to trace its entire length, and this is probably the cause 



