Pulse and continuous labelling with tritiated thymidine are used for a quantitative study of cell division rates in regeneration blastemas. Proliferation is initially uniform; later a proximodistal gradient develops in the mesenchyme, with the highest labelling index at the tip, where practically all cells are shown to be dividing. In the ectoderm there appear to be two growth bands, one close to the stump and the other close to the tip. The results are consistent with the progress zone theory, and agree well with the numerical estimates of growth rates used in our previously reported simulation.

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