TUMORS AND NORMAL TISSUES 339 



method of measuring the individuality differentials; what is measured in 

 this way is not necessarily the degree of similarity or dissimilarity of the 

 individuality differentials of host and transplant, but the ability or lack of 

 ability of the tumor to overcome a limiting factor for the growth of the 

 transplanted tumor tissue in a certain host. It may be regarded, therefore, 

 as doubtful whether the data obtained in tumor transplantation can be directly 

 applied to the analysis of the individuality differentials of tumors; notwith- 

 standing these difficulties, there is much evidence that the organismal, and 

 in particular, the individuality differentials, are essentially the same in normal 

 tissues and in tumor tissues, and that the specific characteristics of cancer 

 tissues, which differentiate them from normal tissues, are not so much due 

 to changes in the organismal differentials as to certain other conditions. 



As has been said, in tumor transplantation the main concern is to deter- 

 mine whether or not a transplanted piece of tumor shows continued growth, 

 and it is customary to record the percentage of successful transplantations, 

 of "takes," as they are obtained under various circumstances. However, be- 

 sides the transplantability there are two other variable factors which should 

 be considered in evaluating the result of transplantation, namely, (1) the 

 growth energy of a tumor, by which is meant the rapidity of its growth, 

 and (2) the latent period intervening between the time of transplantation 

 and the first definite manifestation of an expansive growth of the grafted 

 piece. These data are obtained by measuring at certain periods the diameters 

 of the tumor, or better still, by determining, in addition, its weight at the 

 conclusion of the experiment. In some of our early transplantations, we gave 

 attention, also, to these last named factors. But as mentioned, only the per- 

 centage of "takes" was recorded by the majority of investigators, and the 

 lack of fineness of this test was not felt as a serious difficulty, especially in 

 the earlier period of tumor research in which the peculiar properties of 

 tumors were analyzed largely by means of transplantation. As a rule, the 

 growth of transplanted tumors was considered as something distinct from 

 the growth of various normal tissues. Only gradually, step by step, was the 

 great similarity in the behavior of normal and tumor tissues after transplan- 

 tation established, and at the same time the factors which differentiate tumor 

 and tissue growth were analyzed. 



The first successful transplantations of tumors in animals were carried 

 out by Hanau, Morau, Velich, Eiselsberg and Firket. They used for this 

 purpose, carcinoma of rat and mouse as well as sarcoma of rat. These ex- 

 periments established the fact that certain tumors can be transplanted to 

 other animals of the same species, at least for a limited number of genera- 

 tions. Incidentally also, some interesting observations concerning the factors 

 on which transplantation depended were made, especially by Morau in his 

 experiments with carcinoma of the mammary gland in the mouse. A new 

 motive was introduced into the experimental study of tumors in the begin- 

 ning of this century, in a series of consecutive transplantations of sarcoma 

 of the thyroid gland of rats by the writer (1901) and of a mammary gland 

 adenocarcinoma of the mouse by Jensen (1902). In these experiments, which 



