66 BIOLOGY OF DEATH 



by experimental work in this field, probably warrants 

 the conclusion that this potential longevity inheres in 

 most of the different kinds of cells of the metazoan body, 

 except those which are extremely differentiated for par- 

 ticular functions. To bring this potential immortality 

 to actuality requires, of course, special conditions in 

 each particular case. Many of these special conditions 

 have already been discovered for particular tissues and 

 particular animals. Doubtless, in the future many more 

 will be worked out. We have furthermore seen that in 

 certain cases the physico-chemical nature of the condi- 

 tions necessary to insure the continuance of life has been 

 definitely worked out and is well understood. Again 

 this warrants the expectation that, with more extended 

 and penetrating investigations in a field of research 

 which is really just at its beginning, we shall understand 

 the physics and chemistry of prolongation of life of cells 

 and tissues in a great many cases where now we know 

 nothing about it. 



One further point and we shall have done with this 

 phase of our discussion. The experimental culture of 

 cells and tissues in vitro has now covered practically all 

 the essential tissue elements of the metazoan body, even 

 including the most highly differentiated of those tissues. 

 Nerve cells, muscle cells, heart muscle cells, spleen cells, 

 connective tissue cells, epithelial cells from various loca- 

 tions in the body, kidney cells, and others have all been 

 successfully cultivated in vitro. We may fairly say, I be- 

 lieve, that the potential immortality of all essential cel- 

 lular elements of the body either has been fully 

 demonstrated, or else has been carried far enough to 

 make the probability very great that properly conducted 

 experiments would demonstrate the continuance of the 



