DIFFERENTIAL GROWTH I4I 



cell is an unstable system which can have neither existence nor form 

 except in relation to its environment. Few animal cells (e.g. red blood 

 cells) freeze their shape by assuming a rigid envelope. For the rest, 

 shape is variable, merely an index of substance disposition attained in 

 given surroundings and valid only for these particular surroundings. 



Obviously, then, the misleading mental picture of absoluteness and 

 static rigidity of cell shape, conjured by the microscopic picture of 

 fixed cells on slides or in textbooks, must be replaced by a dynamic 

 definition based not on the momentary expression of the cell but on 

 all the possible reactions of which it is capable. The variety of expres- 

 sions that thus can be assumed by the cell on any level of differentiation 

 I have called "modulations" (4, 47). If we may draw a chemical anal- 

 ogy, differentiation would be comparable to the synthesis of a new 

 compound by an irreversible reaction chain, while modulation would 

 correspond to the isomerism and allotropy of intermediate or terminal 

 products. The former involves a change of composition, the latter merely 

 of disposition in space, of the constituent elements. The implications of 

 this simile for the concept of the cell will be dealt with later. 



It can be plainly recognized now how past concepts of differentia- 

 tion have been vitiated by undue reliance on visual criteria. Cells that 

 looked alike were thought to be essentially of like character, although 

 they often were radically different; and conversely, cells that looked 

 very different were considered as very disparate, although they actually 

 were often of the same kind. The literature of histology, embryology, 

 and pathology abounds with notable examples of victims of this "visual 

 illusion." 



Differentiation and Dedifferentiation 



If the test of differentiation, in contradistinction to modulation, is 

 the unidirectional, progressive, and irreversible trend of the former, 

 how safe is the evidence that differentiation in this strict sense ever 

 really occurs? Might not all differentiations be cases of modulation, 

 merely obscured by the difficulty of always finding the right conditions 

 to reverse the fate of the cell ? 



As long as we confine ourselves to normal development, where 

 different cell behavior is generally associated with different local 

 conditions, the answer remains in doubt. For a crucial test of whether 

 different cells differ in character, we must remove them from their 

 different environments and observe them (a) in identical environ- 



