The "warts" which tar elicits on rabbit skin (papillomas, carcinomatoids, frill horns) are true tumors, benign growths expressive of slight yet irreversible deviations of epidermal cells from the normal. The neoplastic condition gives the cells a superiority over their neighbors when both are submitted to the same encouraging influences, and then they proliferate into tumors. Their state entails such disabilities, though, that they are unable to maintain themselves under ordinary circumstances, and consequently growths composed of them disappear when no longer aided. Often the neoplastic cells resume the normal aspect and habit of life long before the tumor mass is gone; and they may persist as part of an apparently normal epidermis, retaining their neoplastic potentialities for months after all signs of the growth have disappeared. In these instances it can be made to appear again, sometimes repeatedly, by non-carcinogenic stimulation of the skin (wound healing, turpentining). There is reason however to suppose that in the end the tumor cells, unless helped, die or are cast off. It is plain that the neoplastic state does not necessarily connote independence of behavior or success in tumor formation. On the contrary it may render cells unable to survive or endow them with powers which they can exert only under favoring conditions. This is the case with the cells composing the tar warts of rabbits. In the lack of such conditions the cells of these growths do not manifest themselves but remain in a subthreshold neoplastic state, whereas if aided they form neoplasms. The deviations from the normal represented by the benign tar tumors of rabbits are slight and limited in character, but further deviations in larger variety may be superimposed upon them, with result in malignant tumors, growths possessed of a greater, though not always absolute, independence. Tar cancers usually come about in this way, by successive, step-like deviations from the normal, and so also do the cancers which derive from virus-induced papillomas as well as many human carcinomas. After cells have become cancerous they frequently undergo further changes, some apparently step-like in character, and all taking the direction of greater malignancy. The hypotheses that tumors are due to somatic mutations and to viruses respectively are discussed in the light of these phenomena.

A squamous cell carcinoma derived from a virus-induced rabbit papilloma has been propagated in fourteen successive groups of animals. It grows rapidly now in most individuals to which it is transplanted, killing early and metastasizing frequently. The original cancer was the outcome of alterations in epidermal cells already rendered neoplastic by the virus, and the latter, or an agent nearly related to it, has persisted and increased in the malignant tissue, as a study of the blood of the first ten groups of cancerous animals has shown. An antibody capable of specifically neutralizing the virus in vitro appeared in the blood of every new host in which the tumor enlarged progressively, and reached a titer comparable with that obtaining in animals which had long carried large papillomas. The antibody was absent from normal rabbits and those in which the cancer failed to grow. The implications of these facts are considered.

In the experiments here described transplanted lymphomas of two kinds regularly regressed following repeated injections of normal guinea pig serum intraperitoneally into mice carrying them, the animals meanwhile remaining lively and devoid of signs of illness or wasting. The lymphomas of untreated control mice, by contrast, usually grew progressively and killed their hosts within 20 to 30 days, and the same was true of the growth of other mice given repeated injections of horse serum or rabbit serum. In similar experiments, the cells of a transplanted lymphosarcoma of rats were temporarily kept from proliferating by repeated intraperitoneal injections of guinea pig serum, though the cells of two transplanted mammary carcinomas of mice, and those of fibrosarcoma, grew unimpeded in hosts likewise treated. Additional experiments related to the phenomenon here described, and a discussion of the findings as a whole, are given in an associated paper.

In an extension of the experimental studies recorded in an associated paper; attempts were made to isolate and characterize the constituent of guinea pig serum responsible for inducing regression of transplanted lymphomas in vivo. The active material was precipitated readily from the whole serum, along with some of the globulins, by means of ammonium sulfate in concentrations of 2.0 molar or greater; it withstood heating at 56°C. for 20 or 30 minutes, but was inactivated upon heating at 66°C. for similar periods; it was completely inactivated by chymotrypsin in concentrations of 1 or 2 mg./cc. during 6 hours at 37°C. Furthermore, the inhibitory effects of small amounts of the guinea pig serum in vivo were enhanced upon admixture with immune sera prepared by injecting the lymphosarcoma cells into rabbits. The facts as a whole suggest that the active material is a protein, and that it may be one or another of the components of complement; yet they do not suffice to establish its identity. Microscopic studies showed that the cells of subcutaneous lymphomas rapidly died and were resorbed following injections of relatively large amounts of guinea pig serum intraperitoneally into mice carrying them, while similar changes followed more gradually after repeated injections of smaller amounts of guinea pig serum. No changes referable to the guinea pig serum were seen in the normal tissues or organs of mice receiving it. Mouse lymphoma cells, suspended artificially as individuals in a physiological saline solution, regularly remained viable following incubation in vitro in mixture with guinea pig serum during 6 hours at 37°C. The finding provides strong evidence that the regression of lymphomas that follows injection of guinea pig serum in vivo is brought about through some reaction in which the guinea pig serum and the host both participate. Some of the implications of the findings are discussed.