The extension in the vicinity may be continuous or the reverse, the latter through the formation of secondary nodules, which may {110} eventually become fused with the primary mass. The continuous growth takes place, as has been more particularly shown by Köster, along the lymph-channels surrounding the tumor, which may become filled, distended, and eventually obliterated by projections from the neoplasm. Both methods of peripheral growth, by secondary nodules and continuous extension, represent an infection of the surrounding tissues, especially if it be admitted that the cells through which the increase is accomplished are direct descendants of the pre-existing cells of the part. Not only does the extension take place through the lymphatic vessels about the tumor, but blood-spaces as well as lymph-spaces may be invaded. Thrombi are then found whose structure is frequently that of the tumor, and whose connection with the same is direct through the perforated wall of the vessel. These features in the growth of tumors lead directly to the consideration of the means by which multiple tumors appear in remote parts of the body after a single tumor has appeared in a given locality, and after the removal of such a primitive growth. The distinction between primary and secondary tumors is now so obvious that one is inclined to forget that the presence of numerous tumors at various parts of the body was at one time regarded as evidence of the constitutional or dyscrasic nature of the morbid growth. Such a multiplicity seemed to indicate that the blood was charged with the constituents of the tumor, which were deposited at various parts of the body. Although certain multiple tumors may be present in different localities without an apparent relation between an antecedent and a subsequent growth, such tumors are usually limited to certain systems of the body. Multiple bony tumors are found growing from bones, fibrous and warty tumors from the skin, and fibro-myomata from the uterus. Cohnheim's theory of the embryonal origin of tumors may seem applicable in such cases, but the frequent association of the osteomata with chronic inflammatory conditions, of cutaneous warts and fibrous tumors with local irritative processes, makes such a hypothesis unnecessary. Those tumors whose multiplicity is of the greatest clinical importance are the rapidly growing forms terminating fatally. Such are those which reappear in the scar after the removal of a cancer, or in the adjoining chain of lymphatic glands or at remote parts of the body. The most satisfactory explanation of their presence, and of the generalization, recurrence, or metastasis of tumors, is derived from what has already been stated with reference to the manner of the growth of the latter.