A. Definitions of Transformation in vitro When normal tissues or organs are explanted to conditions favoring the growth of cells as individual units ("cell culture"), the original cell population undergoes a large variety of modifications. Only a minority of the cells will thrive and multiply and within a rather short period of time, the complex composition of the original explant is replaced by a much simplified one of only a few recogniz ably different cell types. With most organs fibroblast-like cells survive longest and outgrow other types. This is then a stable state of affairs for many...