newly formed cells may form a well-defined mass which simply pushes aside the neighboring parts of the body. After some time the growth may entirely cease or it proceeds usually slowly. In this case we speak of a "benign" tumor. In many other cases, however, the newly formed cells not only multiply, but they also invade the neighboring tissues either as isolated cells or in clusters of cells. Each cell which migrates in the neighboring territory becomes again the center for the formation of a new tumor inasmuch as it has the power for indefinite propagation. Other cells even invade the lymph or blood channels of the body, and are carried with the "lymph or blood stream to different parts of the body. At certain places they can not pass the narrow passages, they remain attached to the vessel wall, begin again to multiply, to break through the vessel wall into the neighboring tissue and produce at various places, often distant from the original site of the tumor, secondary tumors which are called metastases. This infiltrating, metastatizing tumor we call a malignant growth or a cancer. According to the kind of cells which form the cancer and according to the proliferative and infiltrative energy of the cells composing the growth we distinguish many varieties of cancer. There are morphological as well as physiological differences between different varieties of cancer. Eoughly we may however classify the various cancers in those derived from epithelial tissue (for instance of the skin, stomach, uterus, the various glands) which we call carcinomata and those derived from proliferating connective tissue cells. Connective tissue in any part of the body may give rise to cancer. These connective tissue cancers are called sarcoma. The growth of a cancer exerts injurious influences on the organism as a whole. Through pressure on neighboring organs it often interferes with important functions of the digestive, respiratory, excretory organs, and leads to serious disturbances of metabolism. It often breaks through the epithelial membranes of the skin or of the inner cavities of the body; under those conditions parts of the tumor die, break down, and are cast off; an ulcerative surface is thus produced which serves as a place where certain bacteria find a favorable culture medium; putrefaction takes place and the absorption of the putrid material further weakens the organism. But even without the formation of an ulcer, parts of the tumor which are under unfavorable conditions of nourishment constantly die (become "necrotic"), while neighboring parts continue to grow. The absorption of the necrotic material may also exert an unfavorable influence on the metabolism. It is furthermore very probable that the living tumor cells give off certain substances which differ quantitatively or qualitatively from the substances produced in the normal organism, but whether these latter substances exert a toxic influence on the organism it is impossible to state at the present time. Cancer almost invariably progresses continuously and it is
