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A study has been made of some of the enzymatic mechanisms responsible for the previously reported fractionation of oxygen isotopes by whole organisms. The data indicate that the fractionation occurs through the activity of metalloenzymes.

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Ray Moree, who participated in the collection of the flies used in this work; to Miss Sophie Dobzhansky who has, in addition, prepared some of the slides for the chromosome examination in the salivary gland cells; to Professor M. M. Green, who raised the strains used for the genetic tests; and to the Division of Plant Biology of the Carnegie Institution of Washington, and particularly to. Dr. Je'ns Clausen, for their hospitality at Mather.