HAROLD P. RUSCH 



pictured as the sequential deletion or segregation of some meta- 

 bolic patterns and the concomitant activation or induction of 

 other systems. Similar suggestions have been made by several 

 investigators (18,65,70,71,76). Such deletions are represented 

 by the absence from cell E of function S. The concurrent in- 

 crease in the remaining special functions is depicted by the 

 enormous size of function H. 



If the path to be followed is reduplication, the course of 

 events is in the opposite direction from that depicted for irrevers- 

 ible maturation. Cell D represents a mature cell with a po- 

 tential capacity for reduplication, but, in order to divide, it must 

 first retrace some of the processes that led to its maturation. 

 The mechanism for reduplication might be described as follows : 

 When a cell divides, the mechanisms for special functions and 

 reduplication are diluted in the daughter cells and must be 

 gradually rebuilt. For some unknown reason the patterns for 

 specialization recover most rapidly, but the mechanism for 

 growth also recovers, although at a slower pace. As the growth 

 mechanism slowly gains ascendancy, the special functions de- 

 crease in the reverse order in which they increased; function H 

 becomes inactive before function L, and so on. Eventually the 

 processes for reduplication become dominant and utilize an 

 ever-increasing share of the cell food until a time is reached when 

 the specialized systems no longer play a dominant role in the 

 over-all metabolism of the cell. At this time mitosis begins. 



Cell E' represents an irreversibly mature cell of a type 

 different from cell E. The sequence of events controlling its 

 growth and differentiation is similar to that just described. 

 The chief difference is that it acquires some special character- 

 istic metabolic patterns which differ from those in cell E and 

 retains some features identical with those in cell E. This is 

 shown by the dominance of function S in cell E' and function H 

 in cell E. The L system is common to both cells. During the 

 earliest stages in embryonic development some cells along the 

 path A-B' "induce" differentiation in cells along the path A-B 

 (31,63). The mechanism of induction might be similar to the 



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