Such is the case in the nerve cells of the brain and spinal cord, a loss in which tissue is never repaired by the formation of new cells; and in the muscles the same is true.

The least differentiation is seen in those cells which serve the purpose of mechanical protection only, as the cells of the skin, and in these the formative activity is very great.

Not only must the usual loss be supplied, but we are all conscious of slight injuries of the surface which are quickly repaired.
Repair, other things being equal, takes place more easily in the young than in the old.

New formation of cells goes on with great rapidity in intra-uterine life, the child, beginning its existence as a single cell one two hundred and fiftieth of an inch in diameter, attains in nine months a weight of seven pounds.

The only similar rapidity of cell formation is seen in certain tumors; although the body may add a greater amount of weight and in a shorter time, by deposit of fat, this in but slight measure represents a new formation of tissue, but is merely a storage of food material in cells.