1. Under appropriate conditions, fibroblasts are able to multiply in serum, at a slow rate, for very long periods.

2. The rate of multiplication of fibroblasts in a given sample of serum depends entirely upon the nature of the strain. Cell races endowed with a high residual growth energy multiply more rapidly in serum than those whose growth potencies are of a lower order.

3. Fibroblasts, in the beginning, multiply more abundantly in plain serum than in heparinized serum, and also more abundantly in heparinized serum than in herparinized plasma. Later, these differences become less pronounced.

4. The first effect of serum on fibroblasts is invariably injurious, the degree of injury differing according to the nature of the cell strain and the age of the animal from which the serum is derived. With the passage of time, however, the colonies undergo gradual improvement, both in the appearance of the component cells and in their rate of proliferation.

5. In media containing embryonic tissue juice, or other growth-activating substances, fibroblasts form colonies that are isomorphic and composed of isomorphic cells. In serum, fibroblasts form colonies of heteromorphic appearance. Each colony becomes composed of cells that differ from one another, to a more or less marked degree.

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