388 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



cases, also by microorganisms. In principle, all these and still other more or 

 less accidental factors act in a similar way. Thus we can understand that 

 under certain circumstances these two sets of factors may approximately 

 balance each other and thus the oscillating growth which we have described 

 may be brought about. 



However, not only a depression in growth energy of tumor cells, but also 

 the opposite effect, can be obtained experimentally, namely, an increase in 

 growth energy in cells which possess either a normal or a very low level of 

 growth energy, or which may be retrogressing. Clowes and Baeslack observed 

 that in not very virulent tumors the growth energy may be stimulated through 

 a very mild exposure to heat; after subjecting tumor material for one hour 

 to a temperature of 40°-41°C, they noted a certain stimulation. Michaelis 

 also found such a stimulation under similar conditions, as well as after the 

 use of very low concentrations of otherwise poisonous substances. But, pre- 

 vious to these experiments, in our early serial transplantations of rat sarcoma, 

 we had produced stimulation in tumors in which, as a result of injurious 

 factors, the growth energy had been lowered, leading to a stationary state 

 or to retrogression. In a number of such tumors it was possible, by mechanical 

 means, such as pulling a thread through the cancer, making an incision into it, 

 or excising a piece of the tumor, to bring about a resumption of growth, 

 which occurred in certain cases even when transplantation of a tumor nodule 

 to a different place in the same animal had no or only a slight effect. But in 

 another experiment, transplantation of a stationary tumor into a second 

 animal led to a complete restoration of the growth energy of the tumor, which 

 subsequently could be further transplanted into other animals. Such a stimu- 

 lation was accomplished in stationary and retrogressing tumors only if 

 mitotic activity was still present in the tumor cells; if this had ceased, the 

 results were unsatisfactory. Thus it is seen that certain mechanical factors, 

 such as incisions, extirpation of pieces of tumor, removal of pressure exerted 

 by a fibrous capsule, or the process of transplantation, may stimulate growth 

 energy; but it may also be that, in some transplantations, the transfer to 

 hosts with strange individuality differentials may have had an additional 

 growth-stimulating effect. 



We have referred already to the increase in growth energy which occurs 

 quite commonly after transplantation of spontaneous tumors into other indi- 

 viduals of the same species and strains ; this was noticeable in our first trans- 

 plantations of rat sarcoma; it was very definite also in our transplantations 

 of a tumor which had developed spontaneously in a Japanese waltzing mouse, 

 and which could be successfully transplanted into all other Japanese mice. 

 Although in this instance the individuality differentials in tumor and waltzing 

 mice serving as hosts were sufficiently similar to allow takes in 100% of the 

 transplantations, still there was a marked increase in the growth energy of 

 the grafts in the early generations. This may therefore be attributed to a 

 stimulation of the tumor cells resulting from the process of transplantation, 

 as such. Similar effects of transplantation were observable also in the sub- 

 sequent transplantations of chicken sarcoma by Rous and Murphy. Here it 



