376 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



noted also in the transfer of leukemic blood cells into other susceptible individ- 

 uals. In the mammary gland, where the development of cancer out of normal 

 tissue under the influence of hormones can be followed very well, it can be 

 seen that, step by step, the growth energy of the tissue on which the hormone 

 acts increases, and that as soon as a certain stage of intensity in this stimula- 

 tion has been reached, the transition into abnormal growth takes place, pro- 

 vided the conditions transmitted by the germ cells make the gland tissue 

 responsive to the action of the hormones. Furthermore, it is not a single cell 

 which is altered, but more complex structural units, the acini and ducts 

 of the mammary gland, undergo this cancerous change ; and the latter does not 

 depend upon the amount of newly formed tissue, but on the intensity of the 

 growth stimulation which the gland structures have undergone. There is, 

 thus, no indication that this process is caused by the occurrence of somatic 

 mutations and that the cancer-producing stimuli in general are effective be- 

 cause the right kind of somatic mutations are produced. Although, therefore, 

 the facts do not warrant the conclusion that this process of stimulation acts 

 by way of the genes, on the other hand, the conditions which determine the 

 degree of responsiveness of the tissues to the stimuli are transmitted by genes, 

 but by genes of the germ cells and not of somatic cells. Eisen found that in the 

 course of serial transplantations of a mammary carcinoma, which arose spon- 

 taneously in a rat belonging to a closely inbred strain, noticeable variations 

 in the growth energy were lacking in the different generations of transplants ; 

 he attributes the constancy in the slow growth rate in the course of serial 

 transplantations to the homozygous constitution of this strain and believes 

 that when an increase in growth energy is noted in the course of the first 

 generations of grafts, this is due to differences in the genetic constitution of 

 different members of the strain. However, it can be shown that this increase 

 in growth energy has in many cases been observed also in closely inbred 

 strains. It is certain that this phenomenon is not due to selective processes in 

 an impure strain of animals. But it is not observed in the case of all the 

 tumors; to some extent, it seems to depend upon differences in the stability 

 of the tumors used for serial transplantations. 



As already stated, the primary condition required for the development of 

 malignant tumors is an augmented growth momentum, and this augmentation 

 may continue to take place in the course of transplantations of the cancerous 

 tissue ; it is one of the principal causes for the additional number of successful 

 transplantations or "takes" which may take place during serial transplanta- 

 tion, and which accompanies the increase in growth momentum. But, omitting 

 here from consideration, differences in the receptiveness of the host for the 

 transplant, there are still other variable factors involved in the number of 

 takes, which are situated in the tumor cells; among such factors we have 

 referred above to differences in the resistance of the tissues to injurious condi- 

 tions, which is likewise not directly genetic in character; and a third factor 

 consists in the changes of an adaptive nature which can be seen sometimes 

 after continued transplantation of tumors, changes which also occur after 

 successive inoculations of bacteria and after longer continued exposure of 



