Waldeyer[85] has suggested that the primitive basis for the development of the genito-urinary tract contains cells which are equivalent in their possibilities of ultimate development to the epithelium of the limiting germinal layers--a suggestion which is of importance in permitting the {118} epithelial tumors of the ovary to be brought under the general embryological laws of development. [Footnote 85: _Eierstock und Ei_, 1870.] As the growth of embryonal tissues is so defined that descendants are like their ancestors in all respects, so the development of tissues in the adult is regarded as defined with equal precision. Eberth and Wadsworth[86] have shown that the regeneration of corneal epithelium takes place from pre-existing epithelium. E. Neumann and others claim in like manner the development of muscular tissue from antecedent muscular cells. [Footnote 86: _Virchow's Archiv_, 1870, li. 361.] The relation of cancer to epithelial tumors is regarded as similar to that borne by sarcoma to tumors composed of connective tissues. The growth of the epithelial elements into the neighboring parts is through paths determined by pre-existing or new-formed connective tissue. The active element of the cancer lies more especially in its epithelioid cells, and its growth takes place in an atypical rather than a typical manner. Of the various epithelial tumors, there are those like the cutaneous horn or corn, the adenoma or cystoma, whose epithelial growth takes place in accordance with normal methods of production. The epithelioid constituent of the cancer, on the contrary, grows often with great luxuriance and with but little tendency to carry out the normal mutual relations of the epithelium and connective tissue of the part from which it proceeds. The epithelioid masses or sprouts are composed of cells whose relation to each other resembles that of normal epithelium in the absence of an intercellular substance, while the shapes of the cells correspond more or less closely with that of the epithelium in the region from which the tumor arises. The epithelioid cells of cutaneous cancers resemble those of the surface, the rete, or the glands of the skin. Cancers of the stomach or uterus contain epithelioid cells whose shape simulates the varieties in the stomach and uterus. Such resemblances are carried out in the degenerations which the cells of cancer undergo. The horn-like, keratoid, transformation of epidermoid cells in cutaneous cancers, the mucous degeneration of the epithelioid cells of cancers of mucous membranes, are sufficiently familiar. Notwithstanding these resemblances, which are also present in secondary tumors at remote parts of the body, the epithelioid growth advances without limit and without reproducing the normal type. Cancer is therefore defined as an atypical, epithelial new formation.