CONDITIONS OF CELLULAR IMMORTALITY 67 



life of these cells in culture to any definite extent. It 

 is not to be expected, of course, that such tissues as hair, 

 or nails, would be capable of independent life, but these 

 are essentially unimportant tissues in the animal econ- 

 omy as compared with those of the heart, the nervous 

 system, the kidneys, etc. What I am leading to is the 

 broad generalization, perhaps not completely demon- 

 strated yet, but having regard to Leo Loeb's work, so 

 near it as to make little risk inhere in predicting the 

 final outcome, that all the essential tissues of the meta- 

 zoan body are potentially immortal. The reason that 

 they are not actually immortal, and that multicellular 

 animals do not live forever, is that in the differentiation 

 and specialization of function of cells and tissues in the 

 body as a whole, any individual part does not find the 

 conditions necessary for its continued existence. In 

 the body any part is dependent for the necessities of its 

 existence, as for example nutritive material, upon other 

 parts, or put in another way, upon the organization of 

 the body as a whole. It is the differentiation and spe- 

 cialization of function of the mutually dependent aggre- 

 gate of cells and tissues which constitute the metazoan 

 body ivhich brings about death, and not any inherent or 

 inevitable mortal process in the individual cells them- 

 selves. 



An examination of different lines of evidence has 



led us to two general conclusions, viz : 



a. That the individual cells and tissues of the body, 

 in and by themselves, are potentially immortal. 



b. That death of the metazoan body occurs, funda- 

 mentally, because of the way in which the cells and tis- 

 sues are organized into a mutually dependent system. 



Is there any further and direct evidence to be had 



