THE SCIENTIFIC INVESTIGATION OF CANCER 225
to increase and to decrease the growth energy of tumor cells at will by artificial means. If we expose tumor cells in test tubes to certain degrees of heat or cold before inoculating them into the new host, or if they are subjected to the action of certain injurious chemicals which are not sufficiently strong to kill the tumor cells, we find that the subsequent tumor growth following inoculation is very much impaired and weakened. These procedures, therefore, interfere with the action of those mechanisms which are responsible for cell multiplication. On the other hand, it is almost invariably possible to increase the virulence of tumor cells by subjecting them to mechanical stimuli, such as cut- ting into the tumor. Thus it comes about that tumors which recur after incomplete extirpation usually grow more rapidly than the original tumor. These observations make it very probable that the growth energy of somatic cells in general can be increased and decreased in a similar way and that the increase in growth energy in cancer cells is similar to that observed in normal cells in case of wound healing. In both cases mechanical stimuli increase growth energy.
There is a third class of observations of great biological interest that concerns the transmission of changes produced in one cell genera- tion to another which itself has not been exposed to these changes. Thus it has been found that it is possible through the action of certain chemical substances which, for instance, may be injected into a vein and which reach the tumor only through the circulation, to produce a remarkably inhibiting effect on the growth of the transplanted tumor and in some cases even to cause its retrogression and disappearance. But we also found that through such injections the tumor cells become immiinized against the action of these substances, that the effect, there- fore at present is only a temporary one, and that this immunity is trans- ferred to the next cell generation which had not been exposed to the action of the chemical. Thus in the case of somatic cells, as in the ease of protozoa and bacteria, changes produced by external agencies may be transferred to at least a certain number of the following cell generations. And the same seems to take place in the case of germ cells — ^as in the effect of alcohol for instance — an effect transmitted to cell generations not directly exposed to the action of the poison.
If we survey what we have said of the methods and results of cancer research, one conclusion especially impresses itself upon us, namely, that there is a very close connection between cancer research and general biology. In cancer research, whether we deal with the origin of cancer, the reaction of the body against it, or the conditions favorable to its growth, we have to do in each case with the properties of cells, and these properties as seen in the cancer cell can not be under- stood without a constant reference to the normal cells from which it was derived. And conversely, our knowledge of the life of the normal cell,
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