174 LEO LOEB - 



cepted, and as it seems to us, with full justification, inasmuch as 

 no facts were known at that time which suggested the immor- 

 tality of somatic cells. It was the apparently endless transplan- 

 tation of tumor cells which proved the contrary view. 



To recapitulate what we stated above : tumors are merely trans- 

 formed tissue cells. All or the large majority of adult tissues are 

 potential tumor cells. Tumor cells have been shown experi- 

 mentally to be potentially immortal, therefore tissue cells are 

 potentially immortal. 



This wider conclusion I expressed nineteen years ago. Quite 

 recently the immortality of certain connective tissue cells has 

 been demonstrated by Carrel through in vitro culture of these 

 cells. Under those conditions the tissue cells escape the mechan- 

 isms of attack to which the homoiotoxins expose the ordinary 

 tissue cells in other individuals of the same species. Under these 

 conditions the reactions of the host tissue against homoiotoxins 

 which would have taken place in vivo, are eliminated. We must, 

 however, keep in mind that this method of proving the immor- 

 tality of somatic cells applies only to one particular, very favor- 

 able kind of cells and it is very doubtful, if by cultivation in vitro 

 the same proof could be equally well supplied in the case of other 

 tissues. On the basis of tumor transplantations on the contrary 

 we were able to show that a considerable variety, perhaps the 

 large majority of all tissue cells possess potential immortality. 



GROWTH CURVES AND SPECTRUM OF RELATIONSHIPS. 



We may approximately represent the effect of syngenesio, 

 homoio, and heterotoxins on various kinds of tissues in the form 

 of curves, where the base lines indicate the spectrum of relation- 

 ships and the ordinates growth energy of the tissues in the various 

 hosts. We find then that embryonic and adult invertebrate 

 tissues and the embryonic tissues of lower vertebrates from one 

 class (Curve i). This, however, does not necessarily imply that 

 all these tisues behave in an identical manner, but that there exist 

 some essential similarities. Our data are as yet by no means 

 complete in this respect. 



Very similar to this curve of the primitive tissues is that repre- 

 senting the growth of the transplantable tumors (Curve II). 



