352 



Special Vertebrate Organogenesis 



a liquid, but can only proceed along inter- 

 faces, either between a solid and a liquid, or 

 between two immiscible liquids, or between 

 a liquid and a gas. The nearest analogon 

 among plants would be the clinging vine. 

 A nerve tip can traverse not even a small 

 liquid gap without an interfacial bridge. 



Interfaces capable of serving as the requi- 

 site substrata are furnished in the body by 



TOWING 



Fig. 128. Three phases in development of nerves 

 (diagrammatic). Top: Pioneering phase (free fiber 

 tip advances into surroundings). Middle: Applica- 

 tion phase (pioneering tip has become attached to 

 peripheral cell, younger tips apply themselves to 

 course of older ones). Bottom: Towing phase (shift 

 of peripheral cell produces corresponding displace- 

 ment of attached nerve fiber) . 



all the fibrous units (fibrils, fibers, filaments) 

 that pervade the liquid spaces in and between 

 cells and tissues and constitute the solid 

 framework of the "ground substances" (see 

 Section III, Chapter 1, by Schmitt). They 

 consist mostly of chains of filamentous pro- 

 tein molecules combined into btmdles and 

 networks of submicroscopic and microscopic 

 dimensions. Along such filaments the ter- 

 minal filopodia of the nerve fiber are drawn 

 out by interfacial forces of still unresolved 

 nature which cause protoplasm to spread 

 out along the interface, grossly comparable 

 to a "wetting" process. The linearity of the 

 fibrous units along which they extend is a 

 major factor in guiding the extending nerve 

 fibers. In a planar interface, they would 

 fuse to a sort of "terminal web." However, 

 the linear guide structures are the ones of 

 greatest practical importance, since even 

 planar surfaces commonly contain inhomo- 

 geneities that describe linear tracts within 

 the common plane (e.g., the fibrous constitu- 



ents of coats or membranes). The principle 

 according to which nerve fiber tips are 

 guided in their course by contact with siu:- 

 rounding structures has been designated as 

 "contact guidance^' (Weiss, '41c). 



In an irregular network, fibrils intersect 

 at countless places and angles. Nerve tips 

 advancing on such a trellis will be split at 

 each intersection, but, as was explained 

 above, competition will usually obliterate all 

 but one of these terminal branches, and only 

 this one will proceed. The decision of which 

 one of the multiple projections will endure 

 in any given instance may be essentially a 

 matter of the accidents of the local situation. 

 If so, the resulting nerve course will be 

 irregular and tortuous (Fig. 129^, e), as is 

 the case in the neuropil of the nerve centers, 

 in scar tissue (e.g., between severed nerve 

 stumps), and in the plasma clots of ordinary 

 tissue cultures. On the other hand, the more 

 the meshes of the fibrillar network are ori- 

 ented in a given prevailing direction, the 

 more the resulting nerve fiber course, tracing 

 the common directional component, will like- 

 wise become definitely oriented (Fig. 129at). 

 The extreme of this condition is attained when 

 the fibrous matrix consists of parallel guide 

 rails which leave the single-tracked nerve 

 fibers no alternative course (Fig. 129c,c?). 

 In this case, nerve orientation resolves itself 

 completely into a matter of the orientation 

 of the underlying svibstratum and can there- 

 fore be controlled by way of the latter, as 

 has been proved by a variety of observations 

 and experiments both in the living animal 

 and in vitro. The following examples may 

 serve as illustrations. 



When tension is applied in tissue culture 

 to a blood plasma clot, either during or after 

 coagulation, the meshes of the random net- 

 work of fibrin threads are drawn out in the 

 general direction of the lines of stress. Nerve 

 fibers allowed to grow out in such a medium 

 then move in the same prevailing direction 

 (Weiss, '34a). Thvis, by orienting the col- 

 loidal matrix, tension can indirectly orient 

 nerve growth. The immediate factor is the 

 orientation of the matrix, irrespective of how 

 it has been obtained. Fibrous tissue exudates 

 spreading along surfaces and being drawn out 

 in the direction of flow, for instance, act in 

 like manner (Weiss, '45). That this principle 

 of contact guidance is equally valid within 

 the living body has been substantiated in 

 numerous instances, most strikingly by the 

 directional control of nerve regeneration. 

 Without intervention, regenerating nerve 

 fibers commonly take random courses. But 



