DIFFERENTIAL GROWTH 1 77 



parable effects. Secondly, stress is decidedly not a determining factor in 

 cytodifferentiation. Though it seems to be prerequisite for some cell 

 types in order that they may be able to carry out terminal transforma- 

 tions for which they are already predisposed by previous differentiation, 

 it is not the cause of such differentiation. Without stretch, myoblasts 

 cannot transform into muscle fibers (39), but stretch itself neither 

 makes a cell into a myoblast nor ever transforms a non-myoblast into 

 a muscle fiber, as has once been claimed (6). 



An interesting combination effect of both contractile tension and 

 proteolysis has recently been observed in the healing of nerve stumps 

 (57). Tension transmitted from stump to stump through an intervening 

 blood clot, intensified by the syncretic contraction of the clot itself, ori- 

 ents the fibrin network in the latter in a prevailingly longitudinal direc- 

 tion. The oriented strands tend to fuse and thus become heavier than 

 their irregularly arranged cross links. Then proteolytic enzymes appear 

 on the scene and rout out the cross links, while leaving the straight linear 

 bundles standing. Thereby preferentially linear order becomes strictly 

 linear. 



The main value of these varied results is that they demonstrate in 

 principle how chemical activity (proteolysis, dehydration) can engender 

 spatial order of a high degree, although it has itself no space order. 

 Through its physical and chemical by-products, plain growth thus leads 

 to ordered growth. As the order, arrangement, and consequent exposure 

 of the growing tissue reciprocally affect the condition, manner, and 

 hence, rate of its further growth, we realize how complex and diverse 

 are the factors that contribute to the growth of a given tissue. Some of 

 the factual sequences are summarized in Figure 4. Many more are still 

 unexplored. Carrying this exploration of the "microtechnology" of de- 

 velopment, i.e. of "Entwicklungs-mechanik" in its original sense, for- 



GROWTH 



^ -^ \ ^ PROTEOLYSIS 



PROLIFERATION \ secretion 



7 SHIFTS 

 DE HYDRATION ^ 



\ <' 



CONTRACTION STRETCH EXPANSION 



TENSION 



ELECTR.POT.f?) T HfORTLOWf?) 



MICELLAR O RIENTATION 



r 



ORIENTED GROWTH 

 Figure 4. 



