3 8 GROWTH 



pathologist of the Washington University, St. Louis. Leo Loeb 

 found that cancer cells, which are merely modified body cells, 

 can be grown indefinitely when transplanted successively from 

 old to young animals. This led to the conclusion that cancer cells 

 must be considered as potentially immortal, and Loeb general- 

 izes this for all forms of somatic cells of multicellular animals. 



Further experimental proof of this theory was furnished by 

 cultivating body cells outside of the body, or in vitro, as it is 

 called. Leo Loeb was the first to discover that it is possible to 

 grow tissues in vitro. To Ross G. Harrison of Yale University, 

 however, is due the credit for developing practical methods of 

 tissue culture in vitro, and to Alexis Carrel of the Rockefeller 

 Institute is due the credit for developing the method of culti- 

 vating tissues in vitro indefinitely by a process of continuous 

 irrigation. 



In 1 9 1 3 Doctor Carrel cut out a piece of tissue from the liv- 

 ing heart of a chicken and placed it in a continuously irrigated 

 nutrient medium in a glass dish. This strain of tissue is still 

 living, and it grows now as well as it did in 19 13, doubling 

 itself in bulk about every twenty-four hours. Now, since in the 

 chicken's body growth continues only for about a year and it 

 begins to slow down almost immediately after development of 

 the chicken's body begins, while in vitro it grew at an undi- 

 minished rate for thirteen years, the obvious conclusion is that 

 in vitro, where the nutrient medium is kept at a constant com- 

 position by continuous irrigation, cells are capable of indefinite 

 growth and that the decline and final cessation of growth in the 

 body are due to some changes with advancing age in the nu- 

 trient medium surrounding the cells in the body. The results of 

 Carrel's investigations thus furnish a complete proof of the 

 theory that body cells are potentially capable of indefinite 

 growth. 



Indefinite growth, of course, implies an indefinite life, that 

 is, physical immortality, for the phenomenon of aging or senes- 

 cence and consequently physical death, is no doubt associated 



