148 WHAT IS LIFE 



for seven years. "^ (These cultures have been 

 kept alive now — 1928 — for sixteen years.) Carrel 

 also found it true for tissues taken from various 

 mammals, including man. 



In experiments carried on by Dr. John M. 

 Wheeler and his assistant, Dr. Daniel Kirby, it 

 was shown that the iris section of live eye tissue, 

 if suitably placed, will grow outside of a living 

 animal body, the bulk of its cells doubling every 

 forty-eight hours. But since Leo Loeb first 

 demonstrated (1897) that, if placed in a suitable 

 medium, certain tissues will grow outside the 

 body, it has been fully established that in these 

 cultures in vitro repeated cell-division would con- 

 tinue forever. 



Raymond Pearl writes: "The experimental 

 culture of cells and tissues in vitro has now 

 covered practically all the essential tissue ele- 

 ments 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 locations in the body, kidney cells, and 

 others have all been successfully cultivated in 



vitro What I am leading up to is the broad 



generalization, perhaps not completely demon- 



' Scientific Monthly, December, 1919. 



