ly
shut off (J.F. Cohnheim); (3) they are epithelial cells with a latent
power of unlimited proliferation which becomes active on their being
dislocated from the normal association (M.W.H. Ribbert and Borrmann);
(4) they are stimulated to unlimited growth by the presence of a
parasite (Plimmer, Sanfelice, Roncali and others); (5) they are
fragments of reproductive tissue (G.T. Beatson); (6) they are cells
which have lost their differentiated character and assumed elementary
properties (von Hausemann, O. Hertwig). The very number and variety of
hypotheses show that none is established. Most of them attempt to
explain the growth but not the origin of the disease. The hypothesis of
a parasitic origin, suggested by recent discoveries in relation to other
diseases, has attracted much attention; but the observed phenomena of
cancerous growths are not in keeping with those of all known parasitic
diseases, and the theory is now somewhat discredited. A more recent
theory that cancer is due to failure of the normal secretions of the
pancreas has not met with much acceptance.

Some generalizations bearing on the problem have been drawn from the
work done in the laboratories of the Imperial Cancer Research Fund. They
may be summarily stated thus. Cancer has been shown to be an identical
process in all vertebrates (including fishes), and to develop at a time
which conforms in a striking manner to the limits imposed by the long or
short compass of life in different animals. Cancerous tissue can be
artificially propagated in the short-lived mouse by actual transference
to another individual, but only to one of the same species. Cancerous
tissue thus propagated presents all the characteristic features of the
malignant growth of sporadic tumours; it infiltrates and produces
extensive secondary growths. Under suitable experimental conditions the
aggregate growth of a cancer is undefined, of enormous and, so far as we
can judge, of limitless amount. This extraordinary growth is due to the
continued proliferation of cancerous cells when transplanted. The
processes by which growing cancer cells are transferred to a new
individual are easily distinguishable and fundamentally different from
all known processes of infection. The artificial propagation of cancer
causes no specific symptoms of illness in the animal in which it
proceeds. Under artificial propagation cancer maintains all the
characters of the original tumours of the primary hosts.