HAROLD P. RUSCH 



sequence of events culminating in ceil E'. If the special func- 

 tion S is the first to be activated, as shown by arrow 2, the end 

 results are represented by cell E. The course of events asso- 

 ciated with the normal growth and differentiation of the mature 

 cell E will be described first. 



Cell B represents the primordial cell which is destined to 

 develop into specialized mature cell E. The first specialized 

 function, marked by F in the fertilized ovum, has disappeared 

 during the many stages between cells A and B. The products 

 of the reactions of the special function S have by this time 

 attained sufficient proportions to trigger and sustain the next 

 sequence of events, function L, and to provide energy and metab- 

 olites for other intra- and extracellular functions. This flow of 

 metabolites from circle S is indicated by arrows. The utilization 

 of cell food from the general pool by the mechanisms of re- 

 duplication and specialized functions at this stage is shown by 

 the heavier arrows. 



Cell C represents a further development, as indicated by 

 the increased size of the special function L. Besides contribut- 

 ing its own distinctive attributes to the cell, this function in turn 

 stimulates the third function, H, and may feed metabolites back 

 into the general pool of cell food. This stage clearly represents 

 how one reaction activates a second, and the second starts a 

 third, and so on ; thus, a series of mutually dependent, endogenously 

 induced enzyme formations leads to an orderly, sequential development of 

 dijff^erentiation. Without such provision, growth and maturation 

 would be chaotic. This mechanism for sequential develop- 

 ment is not idle speculation but is based on the fact that a num- 

 ber of interlocking metabolic reactions have already been shown 

 to initiate and sustain secondary chemical reactions in this 

 manner (42,49,52,75). It is also based on the observation that 

 some enzyme-forming systems in unicellular organisms can be 

 stimulated to produce the enzyme for a substrate by the intro- 

 duction or accumulation of suitable levels of the given substrate, 

 thereby producing profound changes in the metabolic patterns 

 of a cell without altering its hereditary makeup (16,66). This 



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