THE SCIENTIFIC INVESTIQATION OF CANCER 221
other of that species, and that in its chemical constitution an animal of one species differs still more from an animal of another. Every cell of the body has a chemical character in common with every other cell of that body and also in common with the body fluids; and this particu- lar chemical group differs from that of every other individual of the species and to a still greater degree from that of any individual of another group or species. Thus it happens that cells belonging to the same organism are adapted to all the other cells of that organism and also to the body fluids, but when transplanted into another organism, that specific substance which is peculiar to the individual is more or less toxic, or poisonous, for the new host, and causes the latter to react defensively against it by producing new substances which appear in the circulation and change the transplanted tissue. The transplanted tissue thus modified seems to exert an attraction for certain cells of the new host which hasten to the place in order to attack it. Or perhaps the transplanted tissue is directly altered by the foreign-body fluids and as a result defensive cells of the host are called into action.
Conditions are very similar when we deal with tumors instead of with normal tissue. Tumors which originate spontaneously in. a cer- tain individual can usually be transplanted without diffictdty into another place in that individual. The cells remain alive and continue to multiply at the new place; but cells of a tumor when transplanted into another individual of the same or different species are destroyed, just as we saw was the case with the normal tissue. At this point, however, there is a difference between normal and cancerous tissue. While normal tissue in all cases is destroyed sooner or later after transplantation into another individual, there are certain tumors which in contradistinction to the large majority of tumors will grow in another individual of the same species after transplantation and some- times even in an individual of a nearly related species. While struc- turally these transplantable tumors behave like other tumors, they differ from the large majority and from normal tissue in that they show a decreasing sensitiveness to the toxic action of those substances which differentiate one individual from another. These are the tumors which are used for experimental purposes. But, while it is true that these tumors will grow in a new host, it is not true that the cells of the new host are in all cases chemically like those of the tumor. Indeed, there is a difference which again makes it possible for the host to call into action certain defensive mechanisms against the invader, but these, xmder ordinary circumstances, are not strong or active enough to de- feat it. There are, however, conditions which make the defensive action of the new host victorious over the inoculated tumor, and then it is destroyed. It is possible experimentally to increase the produc- tion of these defensive mechanisms.
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