THE RELATION OF CELLS TO ONE ANOTHER 211 



tissues of birds and mammals has revealed some of their 

 fundamental properties. 



1. Unlimited Proliferative Potentialities oj Tissue Cells. 

 When fibroblasts or epithelial cells are removed from the 

 body of an animal and kept in a nutrient medium under 

 proper conditions, their multiphcation goes on indefinitely 

 at the same rate. As long as waste products are ehminated 

 and food material is supplied, they synthesize new proto- 

 plasm from the constituents of their medium. A strain of 

 tissue cells is immortal, if maintained in a proper state 

 outside of the body. Within the organism, tissue cells 

 actuahze only a small part of their potentiaKties. But the 

 proHferative capacity always remains present, even in old 

 age, when the cells are still capable of unhmited multiph- 

 cation in vitro. 



2. Dependence of Cell Activity on the Composition oj the 

 Medium. Tissues taken from an embryo or from a pure 

 culture of embryonic cells and placed in a medium containing 

 inorganic salts and glucose, but no nitrogenous substances, 

 stop growing after a few days and die. In a medium contain- 

 ing inorganic salts and lacking glucose, death occurs almost 

 immediately. On the contrary, fibroblasts or epithehal 

 cells cultivated in embryonic proteins immediately increase 

 their rate of multiphcation. After a few days, the mass of 

 the tissue doubles in size every forty-eight hours and the 

 velocity of prohferation remains stationary. Under such 

 conditions, the cells accumulate reserves. Then, if they 

 are deprived of food, they go on multiplying for several 

 days. Connective tissue cells removed from an adult animal 

 rejuvenate at once and begin to multiply again when they 

 are placed in embryonic proteins, although they may have 

 been in a dormant condition for several years. After a few 

 weeks, they cannot be distinguished from embryonic cells. 

 These experiments led to the important conclusion that the 

 prohferation of a cell depends on the composition of the fluid 

 in which it is placed. The state of rest or of proliferation of a 

 tissue in the adult animal is a function of the quality and 

 quantity of the food material at its disposal. A tissue cell 

 has no spontaneous activity or energy. It is like a motor 

 which does not run when it lacks fuel. The reason for cell 



