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Special Vertebrate Organogenesis 



fibrillar network automatically distorts the 

 meshes into a radial pattern converging upon 

 the proliferating center (Fig. 130, top). Sub- 

 sequent nerve growth, being guided over 

 these radial pathways toward the center, 

 naturally will give the illusion of having 

 been "attracted" by it. We may call this the 



structures, cogently explicable in terms of 

 demonstrable chains of physico-chemical 

 events. This may appropriately be called the 

 "two-center effect." 



Although tension has been revealed as 

 the most common effector mechanism in the 

 production of guide structures, it is conceiv- 



Fig. 130. Effect of local contraction on ultrastructure of a fibrous medium. Top: "One-center effect." 

 Shrinkage of area indicated by black circle from the dimensions of the left panel to those of the right panel 

 produces radial distortion of contiguous network. Bottom: "Two-center effect." Two "one-center effects" in a 

 common network yield resultant preferential orientation along connecting line between the two centers. 



"one-center effect." It is a concrete example 

 of one way in which localized chemical activ- 

 ity can translate itself into structural pat- 

 terns. 



In the presence of two separate centers of 

 proliferation (hence, two contracting foci), 

 the intermediate fibrous matrix is being 

 stretched, hence becomes aligned, along the 

 connecting line. There is thus established a 

 fibrillar bridge which any nerve fibers in 

 that area are bound to follow (Fig. 130, bot- 

 tom). Figure 131, top, shows, for example, 

 the straight tract of nerve fibers grown re- 

 ciprocally between two proliferating spinal 

 ganglia in a thin plasma lamella, guided 

 not by spurious "attractions" from the dis- 

 tance, but by contact with tangible guide 



able that other vectorial agents besides ten- 

 sion, such as hydrodynamic currents, high 

 electrostatic potentials, electrophoresis, or 

 perhaps still wholly unsuspected processes, 

 could effect fibi-illar orientation of the req- 

 uisite kind. 



On a strictly oriented substratum, nerve 

 fiber growth is thus fully determined by con- 

 tact guidance. On a substratum of random 

 configuration, that is, one not previously sub- 

 jected to orienting factors, nerve growth 

 would remain correspondingly random unless 

 additional factors became operative. Since 

 each one of the countless intersections of 

 an irregular pathway system presents the 

 nerve tip with alternative directions, any 

 factor that systematically favors one general 



