POTENTIAL IMMORTALITY OF TISSUES 191 



While, therefore, on theoretical grounds it is justifiable to extend the 

 conclusion as to the potential immortality of cells to ordinary tissue cells, 

 actually it has not been possible to demonstrate this characteristic by the same 

 method in normal tissues as in tumors, on account of the more severe injurious 

 effects produced by strange individuality differentials on normal tissues, 

 as compared to tumors, after their transplantation into new hosts. Our 

 attempts to transplant epidermis serially succeeded for only a relatively short 

 period. Normal cartilage seemed to be a much more favorable tissue for long- 

 continued transplantation, inasmuch as it is more resistant to injurious condi- 

 tions than are most tissues, and better able to withstand the unfavorable 

 effects of the homoiotoxins of the bodyfluids of the host and of the aggressive 

 host cells, especially the connective tissue cells and lymphocytes. In addition, 

 there is some reason for believing that transplanted cartilage gives off a 

 smaller amount of homoiogenous substance than do other more actively 

 metabolizing tissues. It was thus possible to transplant cartilage serially for 

 several years, and not only into young rats but also into very old animals 

 which were approaching the end of their life. In these experiments it was 

 the xiphoid cartilage of rats which was transplanted into dorsal subcutaneous 

 pockets of other rats. The length of time elapsing between consecutive 

 transplantations of a piece of cartilage, to a series of hosts varied between 

 one month and one year. On the average, a new transplantation was carried 

 out every five to six months. It was thus possible to keep the transplant 

 alive for several years, since at the time of the first transplantation the 

 cartilage had already reached an age varying between two and three years 

 and it could be transplanted serially for more than three years ; at the end 

 of the experiment the cartilage had reached an age of five to six years, a 

 period considerably exceeding the average length of life of the rat, which is 

 usually not more than three years. 



This relative success in the serial transplantation of cartilage is due to the 

 factors mentioned above. The lymphocytes of the host accumulate around 

 the transplant in smaller numbers, and, as stated previously, the lympho- 

 cytic reaction may even decrease in the course of time. The lymphocytes 

 were found in the largest number in the fourth week, and from then on 

 their number gradually decreased, until after five months the transplant 

 showed usually only a very weak or no lymphocytic reaction; but 20 days 

 after re-transplantation the lymphocytic reaction could again become distinct. 

 In the course of these transplantations the perichondrium may regenerate 

 and form new cartilage around a piece of this tissue, that had become 

 necrotic as a result of transplantation. Groups of very young perichondrial 

 cartilage cells may be found at the time of examination, but the new 

 cartilage does not penetrate into the surrounding tissue. The perichondrium 

 produces cartilage towards the inside, ensheathing or replacing the old 

 cartilage, but towards the outside it seems to produce a tissue that is transi- 

 tional between cartilage and fibrous tissue, and that resembles at the outer- 

 most border typical fibrous tissue. However, if the transplanted cartilage 

 becomes thick as the result of the growth activity of the perichondrium — 



