Injuries of tissue which are produced by the X-ray not infrequently result in tumor formation and years may elapse between the receipt of the injury and the development of the tumor.

These X-ray injuries are of a peculiar character, their nature but imperfectly understood, and the injured tissues seem to have lost the capacity for perfect repair.
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.