IT is well known that all organisms, plants as well as animals, are composed of small units—the so-called cells—in which a nucleus is surrounded by protoplasm and certain special structures. Each part of the animal body is formed by aggregations of different kinds of cells. The skin and the cavities within the body as well as the glands are composed of epithelial cells, the bones and muscles are produced by special bone and muscle cells. Uniting the various special structures and cell layers in the body, we find the so-called connective tissue consisting of special cells (connective tissue cells) and their product, the connective tissue fibers.
In the normal adult organism some kinds of cells are entirely or almost at a standstill as far as their growth (increase in number and size of cells) is concerned, while other kinds of cells are continually propagating. However, as for each newly formed cell an old one is cast off, no actual increase in the number of cells takes place during adult life under normal conditions. If, however, a small part of the body, e. g., the skin, is removed, the neighboring cells begin to proliferate, and soon fill out the defect. We call this kind of growth regenerative. Very energetic cell proliferation of course is necessary for the transformation of an ovum into the fully developed young organism. This growth, which ceases as soon as the definite organism has been formed and the various organs have been differentiated, we call embryonic growth. A very interesting rapid cell proliferation takes place also in the uterus after the insertion of the ovum, leading to the formation of the maternal placenta.
In the normal adult organism, a definite equilibrium exists between the different kinds of cell aggregations which we call tissues. Each kind of cell respects the territory of the neighboring cells. Not rarely it happens however that suddenly in a young or adult organism cells in a certain part of the body begin to proliferate in an unusual manner; they multiply more or less rapidly. This growth can not be called regenerative, because there was no primary defect to be filled out, and if there had been such a defect the proliferation does not limit itself to wound healing. This multiplication of cells leads to a definite swelling in a-certain part of the body. We call it a tumor. The aggregation of