1. A comparative study of the resting and regenerating epidermis of the rat, pigeon, and guinea pig shows the connection between the structure of the skin and the proliferating power of the epithelial cells. We may at present distinguish two types of epidermal cells: (a) cells with less proliferative energy (pigeon and rat), and (b) cells with a greater proliferative power (guinea pig); the former provides the element from which the type of a thin epidermis is built up, while from the latter the type of a thick epidermis develops.

2. The cell type with greater proliferative power in the normal skin shows also greater proliferative power during regeneration, and the regenerative stimulus causes approximately the same relative increase in the number and size of cells and nuclei in both kinds of skin during regeneration, provided that we base our determination of the number of mitoses on the same unit number of cells in both skins rather than on the same unit area. The absolute increase in the number of cells during regeneration is greatest in the type with the greatest proliferative power in the normal cell (guinea pig).

3. The number of mitoses and the size of cells and nuclei follow a similar curve in the different species during regeneration. They reach a maximum approximately on the 7th day; remain stationary or show some decline from the 7th day on until the time of the closure of the wound; following the closure a further fall takes place; but the figures remain higher than in the normal epidermis for a considerable period afterwards. There seems, therefore, to exist under the conditions of our experimentation a period of approximately 7 days, at the end of which the regenerative stimulus reaches its maximum effect.

The closure of the wound represents probably an additional factor determining the curve of cell proliferation. Failure of the wound to close at an early date seems to make the decline in the number of mitoses sharp after the maximum has been reached.

4. The curve representing the thickness of living regenerating epithelium declines much more slowly than the curve representing cell size and number of mitoses. This indicates probably that even after closure of the wound some push of epithelial cell towards the center of the former wound continues to take place.

5. From the second day the additional growth of the epidermal tongue decreases in each successive period, if we use the length of the tongue at the beginning of each period as the unit of measurement.

6. Retraction and contraction of the wound are factors additional to epithelial movements and epithelial proliferation determining the closure of the wound.

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