286 Herbert M. Evans. 



Mackay went further and declared that in the Amniota generally 

 there occur these two distinct subclavians — the dorsal vessel, pos- 

 sessed by man and by most of the maminals, and by the Lacertilia, 

 the ventral vessel occurring in birds, Chelonia, Crocidilia, and the 

 Cetacea. Moreover in one form — in Chamseleo vulgaris — both kinds 

 of subclavians occurred, the ventral vessel, however, supplying chiefly 

 the shoulder muscles. 



The next advance was made by Hochstetter*^ who, in 1890, pub- 

 lished his paper on the "Origin of the Subclavian Artery in Birds." 

 Independently of Mackay, he also had reached the conclusion that 

 the dorsal subclavian of mammals was not represented in the adult 

 bird, but that the definitive wing vessels in the latter class arose 

 from the ventral segment of the carotid arch. In addition, Hoch- 

 stetter announced the discovery, that in still earlier embryos the 

 bir,ds possessed a subclavian which corresponded to that vessel in 

 the mammals. The secondary subclavian, the adult vessel of the 

 birds and the trunk which Mackay had discovered, did not arise, 

 said Hochstetter, till the beginning of the sixth day in the chick. 

 Preceding it, on the fifth day, the primary subclavian vessel had 

 arisen and extending into the early limb buds, had grown to trunks 

 of considerable size, furnishing a splendid arterial supply to the 

 growing extremities. This primary aortic subclavian was a branch 

 of the fifteenth dorsal segmental artery on each side and Hoch- 

 stetter was thus inclined to regard the subclavians as a modification 

 of the pair of segmental vessels. On the sixth day, the final vessel 

 began its downgrowth from the ventral portion of the third aortic 

 arch and could be seen anastomosing with the primary vessel derived 

 from the aorta. Thus, during the sixth and seventh days, the 

 chick's wing buds were supplied by a double source, a condition 

 corresponding to that seen by Mackay in the adult Chamseleo. 



Hochstetter's account established the fact that even in those am- 

 niotes where the definitive vessel is the ventral subclavian, there 

 arise in development, nevertheless,, typical dorsal subclavians which 

 correspond to those in the mammals, and in fact to the subclavian 



^Hochstetter, F. "Ueber den Ursprvuig cler Arteria Snbclavia der Vogel." 

 Morph. Jahrb. 1890. 



