HEREDITY AND TRANSPLANTATION OF TUMORS 383 



represent gene hormones through which in the course of embryonal develop- 

 ment gene effects may be transmitted to the recipient tissues and organs. 



There are two experiments which indicate that substances of this kind 

 may be transmitted by means of parabiosis from one animal which is 

 genetically receptive to the growth of a transplanted tumor to the partner 

 which is genetically resistant to such a tumor. Thus Zakrzewski observed that 

 a Wistar rat, a strain not susceptible to the growth of the Jensen sarcoma, 

 could be made susceptible by the parabiotic union with a susceptible Warsaw 

 rat. Similarly Cloudman found that a hepatoma which originated in the C57 

 leaden strain, and which could readly be transplanted into mice belonging to 

 this strain, but which could not be transplanted as a rule into black C57 

 strain mice could be made to grow in the C57 black if the latter was united 

 by parabiosis with a C57 leaden strain mouse. 



However, as a rule it does not seem to be possible to change the inherited 

 strain receptivity or resistance of an animal by parabiotic union with an 

 individual belonging to a strain differing in these respects from the first 

 strain. Each of the two partners retains its own specific mode of reaction 

 against the transplant. It is perhaps necessary that the differences in the 

 constitution of the individuality differentials of the two parabiotic partners 

 do not exceed a certain limit if a differential favorable to tumor growth 

 shall exert its characteristic effects in the second partner. But as stated it is 

 possible that in addition special substances favoring the growth of certain 

 cancer cells may be involved in this effect. 



