Earliest Blood Vessels in Anterior Limb Buds. 285 



II. Historical. 



Five important investigations have been made on this subject — 

 the researches of Mackay, Hochstetter, C. G. Sabin, Rabl, and of 

 Miiller; each paper containing a significant contribution and, with 

 the exception of Miiller's account, vi^hich does not include very 

 young embryos, each successfully carrying the subject to still earlier 

 stages of development. 



Twenty years ago Mackay^ published the first valuable account 

 of the origin of the carotid and subclavian arteries in birds, and 

 pointed out that the latter vessel in this class was entirely different 

 from the subclavian trunk in most mammals. 



It had been known for a long time that this vessel lay dorsal to 

 the superior caval vein and the vagus nerve in the mammals but 

 always ventral to these structures in the birds, and if the subcla- 

 vians of these two classes were regarded as identical this difference 

 in relations was quite unexplained. Moreover, the opinions of 

 embryologists here were not helpful, for they did not describe a ven- 

 tral but a dorsal origin for the bird's subclavian, Rathke figuring 

 it as a derivative of the dorsal end of the fourth aortic arch, and 

 Sabatier, following the earlier account by von Baer, as a dorsal 

 derivative of the third aortic arch. Embryology should have thrown 

 some light on the subject and Mackay set about to rework the embry- 

 ology. It is his chief contribution to have shown clearly that the 

 bird's subclavian had ample reason to be different in position and 

 in all its relations from the mammalian vessel, for it arose at an 

 entirely different point embryologically and in its growth, must 

 come into entirely different relations with the thoracic structures. 

 Mackay deserves the credit of being the first to recognize that the 

 definitive avian subclavian grows down from the ventral portion of 

 the third aortic arch and is thus from the beginning, and during 

 its entire development, a ventral vessel, in no sense homologous with 

 the dorsal outgrowth of the aorta which becomes the subclavian 

 artery of mammals. 



"Mackay, J. Y. "The Development of the Branchial Arches in Birds, with 

 Special Reference to the Origin of the Subclavians and C-arotids." Phil. 

 Trans. Roy. Soc., London, Vol. 179. 1888. 



