In a consideration of the general pathology of morbid growths the first question which suggests itself relates to the method of origin of the tumor. The tendency of the present seeks for a local cause, and the most recent theory, that of Cohnheim, demands an accumulation of dormant embryonal cells as such a cause. Cohnheim supports this view by the experiments of Zahn and Leopold, which show that foetal cartilage transplanted into the tissues of a mature animal may grow so rapidly as to present the characteristics of a cartilaginous tumor, while tissues transferred from the animal after birth do not increase in size, but are usually absorbed. As the active elements of the growth are cells, and all cells admissibly arise from pre-existing cells, it follows that the primitive cells of a tumor are derived from those resulting from the segmentation of the ovum or are introduced from without. Numerous experiments have been made with a view to the inoculation of tumors, the transplantation of living fragments of the latter to the living tissues of a healthy individual, for the sake of producing a tumor, but hitherto almost invariably without success. The alternative remains that the embryonal cells are those whose derivatives are present in, and form the essential element of, the morbid growth. All tumors may thus be said to have an embryonal origin. As the segmentation of the ovum eventually results in the production of normal tissues and groups of tissues whose structure and function are wholly different, so the possibility of the production of abnormal groupings of tissue with corresponding irregular manifestations of function is obvious.