Regeneration

In biology, an organism is said to regenerate a lost part, if a substitute for the loss grows from the rest of the organism, and the substitute is a copy or almost a copy of the old lost part. Regeneration of a lost limb occurs in two major steps, first de-differentiation of adult cells into a stem cell state similar to embryonic cells and second, development of these cells into new tissue more or less the same way it developed the first time. Some animals like planarian instead keep clusters... full article at wikipedia