EMBRYOLOGY AND HUMAN MORTALITY 147 



consequence of life. Many cells are potentially immor- 

 tal and the potentiality is actually realized if appropriate 

 conditions are provided. Protozoa are immortal. Germ 

 cells are immortal. Various somatic cells, and even tis- 

 sues have been proved to be potentially immortal by 

 demonstrating in a variety of ways that under appro- 

 priate conditions they continue to live indefinitely. This 

 is the lesson taught us on the one hand by successive 

 transplantations of tumor cells, which are only modified 

 somatic cells, and on the other hand by successful cul- 

 ture of many sorts of somatic cells in vitro. 



Analytical consideration of the matter shows very 

 clearly that the somata of multicellular organisms 

 die because of the differentiations and specializations 

 of structure and function which they exhibit in their 

 make-up. Certain cells are differentiated to carry on 

 certain specialized functions. In this specialization they 

 forego their power of independent and indefinitely con- 

 tinued existence. The cells lining the lungs, for example, 

 must depend in the body upon the unfailing normal ac- 

 tivity of the cells of the alimentary tract and the blood in 

 order that they, the epithelial cells of the lungs, may get 

 proper nutrition. If in such an interlocking and mu- 

 tually dependent system any one part through accident 

 or in any way whatever gets deviated from its normal 

 functioning, the balance of the whole system is upset. If 

 the departure of any part from its normal functional 

 course is great enough to be beyond correction promptly 

 through the normal regulatory powers of the organism, 

 death of the whole will surely ensue. 



What I have tried to show in this and the preceding 

 chapter is a quantitative picture of how the different 

 organ systems get out of balance, and wreck the whole 



