Abstract
The time of initial specific utilization of I has been detd. in the chick embryo to be at the end of the 10th day of incubation. I131 was admd. onto the chorio-allantoic membrane 24 hrs. in advance of determining uptake. This route of admn. was preferable to injn. into the yolk sac or into the air space, yielding both higher and more uniform values. The admn. of thyrotropic hormone (approx. 0.8 Waleszek-Koch units) did not hasten the onset of thyroid utilization of I but did increase the amt. collected.

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