1. Limb-bone rudiments from 6-day-old chick embryos were explanted in a medium of fowl plasma and embryo extract and the effect of various concentrations of T3 on the growth in length and wet weight of the rudiments was studied.

  2. Treatment with 0·01 to 0·32μg.T3/ml. produced a progressive retardation in the growth in length of the tibia, and a stimulation in the growth of the radius, which was maximal at intermediate concentrations. The growth in length of the ulna was increased by low levels of added hormone and retarded by high levels. The final wet weight of the tibia was slightly increased by low T3 concentrations and reduced by high doses. The final wet weights of the radius and ulna were greater than the controls at all T3 concentrations, but the greatest increase in the size of the ulna was reached at a lower dose level than that needed for the radius. These results suggested that the growth stimulation of the radius and retardation of the leg bones when exposed to the same concentration of T3 reflect a difference in sensitivity of the rudiments which may respond in a fundamentally similar way.

  3. The response of the ulna to 0 ·16μg. T3/ml. was a continuous process of growth stimulation followed by retardation.

  4. A difference in the threshold of response between the third metatarsus and radius was found at low concentrations of T3: the flattened cell zone of the metatarsus was affected by 0·0005 μg./ml. to which the radius was insensitive.

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