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.
There is a relation between malformations due to imperfect development of the embryo and tumors, the two conditions occurring together too frequently to be regarded as mere coincidence.