360 



Special Vertebrate Organogenesis 



the border zone between fibrous colloids of 

 different concentrations, and model experi- 

 ments in tissue culture have verified the 

 tangentiaf deflection of radial nerve fiber 

 growth afong sucli borders (Weiss, ':)4aJ. 

 ijiimifar cross patterns may arise in the body 

 at the boundary between masses or layers 

 of cells of different kinds which exert ditfer- 

 ent effects on the surrounding colloids. Many 

 other processes that would lead to the same 

 end are imaginable, all of them thus far 

 untested. 



Striking examples of angular deflection 

 in the embryo are the dorsal roots of the cord, 

 the various central and peripheral plexuses, 

 and the partially decussating systems in the 

 brain. Dorsal root fibers, after entering the 

 cord laterally, turn abruptly into a longi- 

 tudinal course (with or without branching), 

 forming thus the dorsal funiculi. Evidently 

 the switch is produced by the encounter with 

 longitudinal pathways in the marginal veil 

 oriented lengthwise by the passive elongation 

 of the tube mentioned before (p. 359). Plexus 

 formation is to be expected wherever layers 

 with predominantly radial structure alter- 

 nate with tangentially oriented ones, as in 

 the retina or in the various strata of the 

 cortex (see Fig. 134). Thus the horizontal 

 interconnections among the vertical projec- 

 tion systems, which are such an important 

 functional feature, are presumably antici- 

 pated by lamination in the texture of the 

 ground substance, which in turn might be 

 due either to the differential growth expan- 

 sion of the various cell strata or to differential 

 impregnation of the grovmd substance from 

 different concentric cell layers (see Weiss, 

 '39, p. 509). Peripheral nerve plexuses are 

 probably likewise caused by "crossroads"; 

 the brachial and pelvic plexuses, for instance, 

 by the tangentially disposed girdle mesen- 

 chyme which lies across the nerve paths 

 radiating toward the limb base. A cross 

 structure in the optic chiasma, whose angle 

 of intersection changes with the relative 

 shifts between eyes and brain, could account 

 for the ipsilateral deflection of optic fibers 

 in forms with partial decussation, with the 

 probability of diversion, hence the proportion 

 of imcrossed fibers (important in binocular 

 vision) being perhaps a function of the 

 chiasmatic angle during the growth phase. 



Hypothetical though many of these de- 

 tailed applications of the principle of contact 

 guidance to concrete embryological problems 

 may be, they at least formulate the problems 

 involved for practical experimental attack. 

 For final judgment, the results of the latter 



must be awaited. At any rate, it appears 

 clear that plexus formation as such, with 

 fibers turning off their former course at a 

 sharp angle and intermingling in a common 

 plane, otten to emerge again later as inde- 

 pendent bundles, defeats any but a structural 

 concept of guidance. 



Branching. Individual neurons may remain 

 essentially unbranched, as in many sensory 

 types, or they may branch more or less pro- 

 fusely, as do the motor neurons. Since, ac- 

 cording to the all-or-none principle, the 

 neuron can only act as a unit, the extent of 

 branching has great functional significance 

 and must be either preadapted to, or actively 

 regulated by, functional needs. Extensive 

 branching, economical in the motor field 

 where it enables a single neuron to engage 

 several hundred muscle fibers at a time, 

 would be undesirable in the sensory field, 

 where it would blur discrimination. Despite 

 its biological importance, however, the prob- 

 lem of branching has not yet been system- 

 atically studied (Simderland and Lavarack, 

 '53). 



Branching occurs either at the tip of a 

 growing fiber by dichotomy (terminal 

 branching) or along the stem some distance 

 behind the tip which is either still free or 

 already connected (collateral branching). 

 Terminal branching results whenever two 

 simultaneous terminal pseudopods (see Fig. 

 127 e) are of equal strength so that they can 

 divide the inflow of protoplasm between 

 themselves and continue to advance with in- 

 dependent tips (see Speidel, '33). The fre- 

 quency of this occurrence depends on the 

 structure of the pathway system; the more 

 intersected the latter, the higher the inci- 

 dence of branching (Fig. 129). Accordingly, 

 terminal branching is profuse in the maze 

 of central neuropil, as well as in peripheral 

 scar tissue (e.g., after nerve severance), but 

 is infrequent along well-oriented pathways. 



Collateral branches arise as side sprouts 

 from already consolidated fiber stems, pre- 

 sumably in response to local irritations, 

 mechanical, chemical or electrical (Peterfi 

 and Kapel, '28; Speidel, '33; Edds, '53). The 

 repeated branching of motor fibers, for in- 

 stance, could be ascribed to a seriation of such 

 irritations as would attend consecutive divi- 

 sions of young muscle fibers. The size of a 

 "motor unit" (number of muscle fibers at- 

 tached to a single neuron) would then simply 

 reflect the degree of ulterior growth of that 

 muscle after receiving its primary quota of 

 fibers. The systematic occurrence of similar 

 irritations near certain layers or nuclei of the 



