appear only at the last. A condition of
malnutrition and emaciation often results due to the passage into the
blood of injurious substances formed in the tumor, or to the
destruction of important organs by the growing tumor. The growth of a
tumor in the intestine may obstruct or close the canal and thus
interfere with nutrition.

The cause or causes of tumors are unknown. We know that the tumor
represents essentially an abnormal growth, and that this growth is due
to new formation of cells. In certain cases the tumor repeats the
structure of the organ or tissue in which it originates, in others it
departs widely from this; always, however, its structure resembles
structures found in the body at some period of life. The tumor cells,
like all other cells of the body, grow by means of the nutriment which
the body supplies; they have no intrinsic sources of energy. The great
problem is what starts the cells to grow and why the growth differs
from that of normal tissue, why it is not regulated and cooerdinated as
are other forms of growth. When a small piece of the skin, for
instance, is cut out growth as rapid as that in tumors takes place in
the adjoining cells, _but it ceases when the loss is restored_.
The same is true when a piece of the liver is removed.

Various hypotheses have been formed to explain the tumor, all of them
of interest, and they have had great importance in that the attempt to
prove or disprove the hypothesis by continued observation and
experiment along definite lines has produced new knowledge. The
various theories as to cause may be divided into three heads.

The parasitic theory. This supposes that a living parasite invades the
body, and by its presence excites the cells of certain tissues to grow
in tumor form. It is known that active growth of the cells of the body
can be excited in a number of ways, by chemical substances such as
certain of the coal tar products, and that it often takes place under
the influence of bacteria. It is further known that parasites can
produce tumor-like growths in plants. The large, rough excrescences on
the oaks are produced by a fly which lays its eggs in or beneath the
bark, and the larva which develops from the egg secretes a substance
which causes the cells about it to multiply, and a huge mass is formed
which serves the developing insect for both food and protection. Large
tumor-like masses are formed on the roots and stalk of cabbages as the
result of the i