The volumes of the dermal papilla, matrix and fully grown hair of vibrissal follicles of rats and mice were measured.

Dermal papillae were surgically removed from rats' vibrissal follicles and a small new dermal papilla was always reformed. The lower third of rats and mice follicles, containing the whole of the dermal papilla was amputated and very small dermal papillae were reformed.

Follicles containing small dermal papillae produced small hairs and a constant positive linear relationship of 7·0 was found between the volume of the dermal papilla and the volume of the full-grown hair for all cases studied.

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