To sum up, the tissues from which cancer grows, in their normal process of repair tend to reproduce themselves more or less exactly, or if the injury is too gross, they are replaced by scar tissue; but when we reach the age at which their biological work is done, there is a tendency to atypical reproduction, in which an atypical cell continues to reproduce itself atypically and grows at the expense of the organism, eating into or eroding it as it enlarges, till it finally kills the host on which it preys. This will serve as a general definition, but, if we wish to be a little more concrete, we must plunge for a while into the realms of pathology, in order to get a clearer idea of what cancer means. Our body is made up of three layers of tissues; each of these has its separate function, and, within small limits, its own way of reacting to long continued injury. Early in our prenatal development, these three layers can be distinguished, and each of these later produces its own type of tissue, and under appropriate conditions, its own type of malignant tumour. From the outer and inner layers develop the cells which actually touch the outside world, that is to say, which cover the exterior of our body and provide our inner lining, or mucous membranes. From the inner layer is developed glands which are, so to speak, ingrowths from this layer, and it is the tumours arising from this latter tissue layer which mostly concern us now, and which are the cause of so much human suffering. These Carcinomata, as they are called, all have something in common, alike from the point of view of their recognition, pathology and onset. They begin in some tissue which has previously been the seat of disease, usually some chronic inflammatory process which has been present for years, and which may have healed up and broken down many times. When this occurs on open surfaces, such as the tongue, intestinal mucous membrane, or lip, we can watch the gradual transformation of the disease from a simple chronic inflammatory process to that of a malignant growth.