HEREDITY AND TRANSPLANTATION OF TUMORS 377 



certain protozoa to various injurious conditions. In all these cases we may 

 perhaps have to deal with alterations in cytoplasmic or nuclear-cytoplasmic 

 mechanisms corresponding to the persisting modifications of Jollos ("Dauer 

 modifikationen"). Such processes of adaptation have been observed under 

 various circumstances; for instance, leukemic cells, which at first could be 

 transferred only to X-rayed individuals belonging to an unfavorable strain 

 of mice, could subsequently be transferred, also, to other individuals belong- 

 ing to the unfavorable strain which had not previously been X-rayed. We 

 shall discuss these processes of adaptation more fully in the next chapter. 



In accordance with this interpretation of apparently spontaneous changes in 

 growth momentum and takes which, as a rule, occur in the course of serial 

 transplantations of cancerous tissues, we may likewise interpret the differ- 

 ences in growth momentum and transplantability which have been observed 

 between spontaneous tumors originating in different mice of the same inbred 

 strains, or even in the same mouse, and which we have already mentioned 

 in this chapter. It should be expected that some differences may develop during 

 the process of cancerization of normal tissues. This process may be somewhat 

 farther advanced in some beginning tumors than in others, and there is no 

 reason for attributing such differences to somatic mutations. Changes of the 

 opposite kind take place during embryonal development ; here, associated with 

 a greater differentiation of the tissues, a gradual diminution in growth mo- 

 mentum and, correspondingly, in transplantability occurs; and these changes 

 taking place during embryonal life are irreversible. Bat they are not due to a 

 series of successive somatic mutations ; nor should we be justified in attribut- 

 ing typical changes in growth and differentiation in the granulosa of follicles, 

 previous to and during the process of maturation and corpus luteum forma- 

 tion, to a continuous series of somatic mutations. All these considerations 

 make it improbable that either the transformation of normal tissues into 

 cancers or the variations in growth momentum and transplantability of fully 

 developed cancers are due to somatic mutations. However, as stated, the 

 organismal differentials, and therefore also the genetic constitution of tu- 

 mors, are important factors in their transplantability, as well as in the produc- 

 tion of immunity against tumor transplants. 



After transplantation of tumors, as well as after transplantation of em- 

 bryonal tissues, processes of immunity can be more readily demonstrated in 

 the host than after transplantation of normal tissues. We shall discuss these 

 processes of immunity in tumor transplantation somewhat more in detail in 

 a succeeding chapter. Here, it may be stated merely that the genes in the 

 piece of tumor, which are strange to the host, are the precursors of those 

 constituents of the organismal differentials in the tumor, which may function 

 as antigens. The difference between the individuality and species differentials 

 of host and transplant not only gives rise to the primary local defense reaction 

 of the host against the tumor, but it also subsequently causes the transforma- 

 tion of these strange constituents into antigens and thus leads to the production 

 of immunity. It is especially when a tumor, following a period of growth in a 

 host, retrogresses that the host becomes immune against a second transplant 



