Although certain tumors are admitted to be due to congenital peculiarities of tissue, and even to represent atypical growths from embryonal tissue, the theory of such an embryonal origin for all tumors seems unnecessary. The resemblance in symptoms as well as in appearance, and even in structure, of certain tumors to inflammatory products, and their frequent association with these, has led to the suggestion of an irritant as an exciting cause for the tumor, even in the absence of local peculiarities of tissue. {108} It is obvious that were the embryonal theory of origin, as extended by Cohnheim, universally applicable, the growth demands something more than a focus of embryonal cells. An immediate cause for their growth after a dormant period, extending even into old age, is required. Cohnheim finds such in a sufficient supply of blood. He attributes the development or rapid growth of the tumor to this feature, and supports his view by the usual appearance of exostoses when the skeleton is at its period of most vigorous growth, and of dermoid cysts at a time when the formation of the beard indicates active developmental conditions in the outer germinal layer. The growth of ovarian cystomata at and after puberty, and of these and mammary tumors during pregnancy, are also explained on the ground of a more abundant supply of blood at such periods. He and others find in physiological conditions a source for the abundant blood-supply--that is, the efficient nutrition for the growth of a tumor. The necessity of sufficient nutrition in the development of tumors is universally admitted, and its source may be looked for in pathological as well as physiological conditions.