highly differentiated that all formative
activity is lost. 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. The remarkable repair
and even the new formation of entire parts of the body in the tadpole
will not take place in the completely developed frog.

Repair will also take place the more readily the less complicated is
the architectural structure of the part affected. When a series of
tissues variously and closely related to one another enter into the
structure of an organ, there may be new formation of cells; but when
the loss involves more than this, the complicated architectural
structure will not be completely replaced. A brick which has been
knocked out of a building can be easily replaced, but the renewal of
an area of the wall is more difficult. In the kidney, for example, the
destruction of single cells is quickly made good by new cell
formation, but the loss of an area of tissue is never restored. In the
liver, on the other hand, which is of much simpler construction, large
areas of tissue can be newly formed. For the formation of new cells in
a part there must be a sufficient amount of formative material; then
the circulation of the blood becomes more active, more blood being
brought to the part by dilatation of the vessels supplying it.

Repair after a loss can be perfect or imperfect. The tissue lost can
be restored so perfectly that no trace of an injury remains; but when
the loss has b