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The manner in which the large arteries springing from the aorta in the higher Vertebrates have been developed from a set of arches comparable to the branchial vessels of Fishes has been clearly explained by Rathke, and since his time the views which he first set forward in the year 1857 have been almost universally accepted and taught. I have been led to believe, however, from the observation of certain facts in the anatomy and development of Birds, that the deductions which Rathke made, drawn chiefly from his studies in Mammalian and Reptilian embryology, are not applicable to this class in so far as concerns the origin of the carotid and subclavian arteries. I am inclined to think also that the theories now generally held will be found inadequate to explain the origin of these vessels in the case of Chelonian and Crocodilian Reptiles, and to a less extent probably in Mammals.