Carcinoma is more common in women than in men. In women it is in the genital organs; in men, in the intestinal tract.

Irritation and injury seem to at least be of some importance as exciting causes, although in themselves it is doubtful if they can give rise to a carcinomathelial cells. The glands may be divided into groups a, b, c, d, and e, by the stems of stroma f, g, and h. The stems are covered by several layers of cylindric epithelium, while projecting into the gland cavities are long slender ingrowths of epithelium, devoid of stroma, as seen in i. Very delicate ingrowths consisting merely of two layers of epithelium are seen at k and k. At / the epithelium is several layers in thickness, and at m many layers with leukocytes. The arborescent character of the growth and peculiar gland grouping are characteristic of adenocarcinoma.

Loss of resistance of the connective-tissue stroma has been advanced, but does not seem logical. Many observers have tried to prove that these growths are infectious processes, the results of parasitic activity. Many cellular inclusions resembling protozoa have been found, but the general opinion at present is that these bodies are nothing more than degenerated cells or secretions of cells. Experiments to prove the infectious nature of carcinomata have not been generally successful. The transplantation of cancer tissue into a normal individual has failed. But when placed in another situation in the person from whom the tissue was excised growth has followed. This has frequently been considered proof of the parasitic nature. It probably means nothing more than that the pieces of tissue have found surroundings favoring their growth; a condition such as occurs in skin-grafting.

Fig. 66. - Syncytial Masses Invading a Venous Channel in a Case of Deciduoma Malignum (J. Whitridge Williams).

Bacteria, protozoa, sporozoa, gregarina', blastomycetes, amebae, and fungi have all been suggested as the cause. These claims, however, rest upon the form of the bodies and their staining properties, not upon cultivation and inoculation. Until these latter can be carried out the parasite theory must remain unproved.