In regard to the possible action of both injuries and parasites in
causing tumors, the possibility that their effects on different
individuals may not be the same must be considered. In addition to the
trauma or the parasite which may be considered as extrinsic factors,
there may be conditions of the body, intrinsic factors, which favor
their action in tumor development. The peculiar tissue growth within
the uterus called decidua, which occurs normally in pregnancy and
serves to fasten the developing ovum to the inner lining of the
uterus, may be produced experimentally. This growth depends upon two
factors, an internal secretion derived from the ovary and the
introduction into the uterus of a foreign body of some sort; in the
case of pregnancy the developing embryo acts as the foreign body. It
is not impossible that some variation in the complex relations which
determine normal growth may be one factor, possibly the most
important, in tumor formation.

Another theory is that the tumor is the result of imperfect embryonic
development. The development of the child from the ovum is the result
of a continued formation and differentiation of cells. A cell mass is
first produced, and the cells in this differentiate into three layers
called ectoderm, entoderm and mesoderm, from which the external and
internal surfaces and the enclosed tissues respectively develop, and
the different organs are produced by growth of the cells of certain
areas of these layers. The embryonic theory assumes that in the course
of embryonic development not all the cell material destined for the
formation of individual organs is used up for this purpose, that
certain of the embryonic cells become enclosed in the developing
organs, they retain the embryonic capacity for growth and tumors arise
from them. There is no doubt that something like this does take place.