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.