planted fragments will in two
weeks grow to the size of filberts, and each of these will furnish
material to engraft upon ten mice. These new tumors are similar in
character to the original tumor, and really represent parts of it in
the same way that all the Baldwin apples in the world are parts of the
original tree which was found in Baldwinville many years ago, and as
all the Concord grape vines are really parts of the original vine. It
has been estimated that if all the growth capacity of this mouse tumor
were availed of by the successive inoculation of other mice, a mass of
tumor several times the diameter of the sun would grow in two years.
The condition of the individual seems to exert no influence upon the
growth of the tumor. Growth may be as rapid when the bearer is in a
condition of extreme emaciation as it is when the bearer is well
nourished and robust.

[Illustration: FIG 14.--PHOTOGRAPH OF A MICROSCOPIC PREPARATION FROM A
CANCER OF THE UTERUS. A large mass of cells is extending into the
tissue of the uterus which is shown as the fibrous structure. Such a
cell mass penetrating into the tissue represents the real cancer, the
tissue about the cell masses bear the blood vessels which nourish the
tumor cells.]

Those tumors which grow rapidly and invade and destroy the surrounding
tissue are called malignant tumors or cancers, but in a strict sense
no tumor can be regarded as benign, for none can serve a useful
purpose. A tumor after a period of slow growth can begin to grow
rapidly. Tumors may arise in any part of the body, but there are
certain places of preference particularly for the more malignant
tumors. These are places where the cells naturally have a marked power
of growth, and especially where growth is intermittent as in the
uterus and mammary gland.

Little is known in regard to the influence of inheritance on the
formation of tumors. Study of the tumors of mice show a slightly
greater susceptibility to tumor formation in the progeny of mice who
have developed tumors. Studies of human families seem to show that
heredity has a slight influence, but in the frequency of tumors such
statistical evidence is of little value. The question of inheritance
has much bearing on the origin of tumors. If the tumor is accidental
and due entirely to extraneous causes, inheritance is not probable;
but if there is some predisposition to tumor formation in certain
individuals due to some peculiarity, then inherita