366 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



In the case of normal tissues we used as criteria of the suitability of the 

 transplant for the host, or of that of the host for the transplant, the changes 

 which the graft underwent in the host and the reactions of the host tissues, 

 especially the lymphocytes, connective tissue and blood vessels, against the 

 transplant. In this way we could show that there is a close correspondence be- 

 tween transplantability and the individuality differentials of host and trans- 

 plant, as determined by the genetic constitution of host and donor. In the case 

 of tumors, conditions are somewhat different. In general, in these experiments 

 tumors are used which are easily transplantable, which means that they grow 

 readily in a large number of individuals of the same species. Finer individual 

 differences in relationship, such as those between parents and children, and be- 

 tween brothers, and even between somewhat farther distant relatives such 

 as those tested by us in the case of normal tissues, cannot be distinguished if 

 such transplantable tumors are used. The behavior of the latter towards dif- 

 ferent individuals belonging to species and strains in which they are readily 

 transplantable, is to all appearances about the same. For this reason, only 

 very marked differences between different hosts can be discovered in this 

 way; they represent either strain differences or even differences as great as 

 those between subspecies, or those obtaining between hybrids which result 

 from the mating of two different strains. Furthermore, the standards used 

 for the determination of the degree of transplantability are different for 

 tumors and for normal tissues. For the latter, a more delicate grading of the 

 suitability of the individuality differentials of host and transplant is made 

 possible by the evaluation of the histologic changes which take place in the 

 cellular reactions of the host against the graft. These criteria are not com- 

 monly used or available in the case of tumor transplantations; instead, the 

 number of growing tumors (takes) and, less frequently, the growth energy 

 of the tumors are employed as criteria of the differences in individuality differ- 

 entials. In the investigations of M. S. Fleisher and the writer, a comparison 

 was made between the averages of growth energy and number of takes as 

 criteria of transplantability; in addition, we noted the duration of the latent 

 period, that is, the length of time which intervenes between the date of trans- 

 plantation and the appearance of the tumor visible to the naked eye. The 

 length of the latent period measures the growth energy in the first and most 

 critical period following grafting. It was found that the conditions deter- 

 mining the number of takes are in certain respects distinct from those de- 

 termining growth energy, while in other respects they are correlated with 

 each other. Both these factors depend primarily upon the relations of the 

 organismal differentials of host and transplant, and secondarily, upon the 

 growth energy of the graft and upon the adaptability of the transplant to 

 different hosts or of the host to different transplants. However, tumors which 

 may all be classed as transplantable, may still differ as to their growth energy ; 

 thus, in two parallel series of experiments, all inoculations may be followed 

 by growth of the graft and therefore be considered as takes ; yet, in one set 

 of hosts the growth energy may be greater than in the other. In general, differ- 

 ences in growth energy in a certain host represent finer gradations of the 



