1. Fragments of skin taken from the back of the leopard frog were cultivated in a mixture of plasma and muscle extract of the same species.
2. A few hours after explanation, processes of activity are seen to arise, which finally lead to the formation of a compact epithelial rim around the fragment of skin.
3. These epithelial cells undergo gradual transformation into a spindle-shaped type of cell; in this form they resemble the spindle cells which have been described as connective tissue cells.
4. The growth of a rim of tissue around the explanted fragments of skin may be ascribed principally to the activity of a basal layer of epithelial cells, the units of which first advance into the medium as compact membranes by means of the so called epithelial movement, but which later become separated from the compact membrane, and having assume a spindle form, spread according to the manner of connective tissue cells.
5. In contradistinction to this basal layer, the cells of an upper and middle layer of epithelium upon separation from the fragment of skin and isolation in the nutritive medium, remain completely inactive.
6. The connective tissue of frog skin, for reasons not yet completely established, does not participate in the production of the rim of cells, possibly for the reason that it was early surrounded by the epithelium and was thus prevented from sending out cells into the medium.
7. The transformation of the epithelial cells into a type of cells of the spindle form is a result of the changes in the physicochemical conditions brought about by the life in vitro, which become similar to the physicochemical conditions normally characteristic of connective tissue.
8. The final stage of the explanted fragments of skin is a hollow epithelial sphere (cystic stage) filled with connective tissue.
9. During the cystic stage the epithelium again shows its normal polar differentiation, as a consequence of the physicochemical conditions which now approach normal.
10. These conditions also permit of the shedding of cells in a vertical direction, although previously the production of cells in a horizontal direction had already become impossible.