1. Races of fibroblasts that are functionally distinct have been isolated from the various tissues and organs of a single chick embryo.

2. Functionally distinct races of fibroblasts have also been isolated from corresponding parts of embryos of different ages.

3. Under the conditions of the experiments, and for the particular races of fibroblasts that have been studied, it has not been possible to demonstrate a gradual decrease, or a gradual increase, in the rate of multiplication of fibroblasts obtained from corresponding parts of embryos of gradually increasing ages.

4. Experiments made on strains of fibroblasts derived from the mesonephros and metanephros of a 16 day chick embryo have indicated that the rate of multiplication of these cells in a given medium reflects the physiological state, at the moment of isolation, of the particular part of the embryo from which they are obtained.

5. The rate of multiplication of a given race of fibroblasts in a particular medium does not serve, necessarily, as an index of the age of the individual from which the race is derived.

6. The functional differences that distinguish the various races of fibroblasts are permanent; they are retained by the cells from passage to passage indefinitely, despite such attempts as have thus far been made to change them.

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