Abstract
1. The ability of fibroblasts to mature and to manifest their various potencies in any particular medium is inversely proportional to the growth energy which they exhibit in that medium. Fibroblasts having access to high concentrations of food substances in their environment do not mature, regardless of their origin or the age of the animal from which they were derived. They behave as embryonic cells. 2. Fibroblasts cultivated in vitro are potentially able to produce cells with the structural and functional properties commonly attributed to macrophages. This is true regardless of their origin or the length of time which has elapsed since their isolation from the origin. 3. The fibroblast and the macrophage are considered to represent extreme functional and structural variations of the same cell type. 4. The structural and functional characteristics displayed by fibroblasts in vitro vary according to their origin and to the changes which take place in the composition of the medium in function of time.