MAMMALIAN CELL GROWTH IN TISSUE CULTURE 



193 



TABLE I 



The Amino-Acid and Sugar Constituents of Fetuin 



An especially intriguing aspect of the growth of diploid mam- 

 malian cells is a response to traumatic conditions which produces a 

 marked change in their growth pattern (Puck, Cieciura, and Fisher, 

 1957; Ham, 1960). Alteration of the physical or chemical environment 

 so as to depart from the optimal growth conditions often serves to con- 

 vert the cell population to a form which reproduces much more slowly. 

 The cell usually becomes enlarged, and the generation time, which may 

 be increased two-fold or more, can remain at this new value for weeks 

 or months, despite restoration of the optimal conditions for growth. The 

 nature of this poorly reversible change is not yet understood, and it 

 recalls the conversion of bacteria to their L forms, or of paramecia to a 

 new type of ciliary antigen. It is quite possible that changes which 

 characterize normal differentiation are related to this process. 



Possiblv diflferentiation is the kev mvsterv of the mammalian or- 

 ganism. By this time, a large number of model processes, many of which 

 have been studied in detail in microorganisms, are available for test in 



