Introduction

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Abstract
Hayflick [1965] compared the life span of embryonic and adult human fibroblasts in vitro and found that the latter had a decreased growth potential. Hayflick''s model has since been employed for the study of aging, and at present is almost exclusively used in the field of tissue culture. This model is discussed in relation to the lifespan of chick, human, mouse and bovine fibroblasts, and human embryonic hemopoietic cells. The spontaneous appearance in culture, of cell types with limited growth potential, is considered. The fact that tumor cells seem to possess different methods of propagation depending on the cells of origin, is discussed. Wehther there is some selective advantage in these inter-species and tissues differences, or whether this was an accident of evolution, are considered.