TUMOR GROWTH 437 



also the more recent work of Lumsden concerning the adaptation of tumors to 

 the action of heterogenous serum, resulting from a temporary growth in a 

 strange species, was in conformity with this view. While there is much evi- 

 dence for the conclusion that tumor cells may display a remarkable ability of 

 adaptation to new environments, the transplantability of tumors is determined 

 above all by the relation of their organismal differentials to those of their 

 hosts. Haaland and Woglom were struck by the observation that in the same 

 individual one tumor, a spontaneous cancer, may continue to grow, while 

 another, a transplanted tumor, retrogresses. However, such an occurrence 

 is to be expected if we consider the great similarity or identity of the in- 

 dividuality differentials in the host tissues and in the spontaneous tumors and 

 their differences from those of the strange transplanted tumors. The impor- 

 tance of the relation between the individuality differentials of host and 

 transplant had not yet been recognized by Haaland, who attributed the dif- 

 ference in the fate of the two tumors to local conditions residing in the 

 tumor cells. Indeed, the sharp distinction between autogenous and homoiog- 

 enous tumors which the theory of the individuality differentials implies 

 had not yet been made by the majority of authors. Thus as late as 1916, 

 Tyzzer applied the findings concerning the growth of homoiogenous tumors 

 to the explanation of the origin of spontaneous tumors. He compared the lack 

 of development of a spontaneous tumor with the non-take of a homoiogenous 

 tumor and defined the factors which prevent a spontaneous tumor from 

 developing or from expanding as immunity; the means for regulating the 

 growth of autogenous tissues were considered analogous to those which de- 

 termine immunity against transplanted tumors. He further concluded that 

 spontaneous tumors must have feeble antigenic power and offer effective 

 resistance to the normal influences which inhibit undue tissue growth ; in this 

 way the continuous growth of a tumor is made possible in the animal in which 

 it originates. Otherwise reactions sufficient to destroy spontaneous cancerous 

 growths would occur more frequently. A spontaneous tumor, according to 

 this investigator, is therefore a parasite strange to the host and it owes its 

 origin to a somatic mutation. Similarly, L. C. Strong and his associates ex- 

 pressed the opinion that a genetic analysis of the factors underlying tumor 

 transplantation will explain also the origin of spontaneous tumors. Inasmuch 

 as according to these authors it is possible by means of transplantation to 

 determine the specific number of factors (genes) which each tumor requires 

 for its growth in a strange host, it was perhaps tacitly assumed that the 

 number and character of these genes explain also the development and peculi- 

 arities of a spontaneous tumor. 



However, it follows from the concept of organismal differentials that an 

 analysis of the factors underlying transplantability of tumors can give an 

 insight only into the difference between the genetic constitution of the host and 

 the tumor graft, and that there is no reason for assuming that the hereditary 

 conditions which favor the development of a spontaneous tumor are identical 

 with the genetic factors which would be required for the growth of a trans- 

 planted tumor, when these factors are determined according to the procedure 



