. Analysis of development. Embryology; Embryology. 358 Special Vertebrate Organogenesis guided toward the peripheral stump, not by a chemical concentration gradient, but by a structural bridge of Schwann cells which has previously spanned the gap as a result of the orienting "two-center effect" (see p. 354) which the two proliferating cut surfaces exert upon the intervening blood clot. It is easy to demonstrate this effect directly in tissue culture (Fig. 131, bottom) by placing two fragments of degenerated (axon-free) peripheral nerve into a thin plasma clot (Weiss, '52b). Evidently
. Analysis of development. Embryology; Embryology. 358 Special Vertebrate Organogenesis guided toward the peripheral stump, not by a chemical concentration gradient, but by a structural bridge of Schwann cells which has previously spanned the gap as a result of the orienting "two-center effect" (see p. 354) which the two proliferating cut surfaces exert upon the intervening blood clot. It is easy to demonstrate this effect directly in tissue culture (Fig. 131, bottom) by placing two fragments of degenerated (axon-free) peripheral nerve into a thin plasma clot (Weiss, '52b). Evidently, if axons were to. 4-n Fig. 133. Deflection of peripheral limb nerve plexus toward transplanted limb buds (combined from Detwiler, '36b). The left half shows the plexus of a normal forelimb (contribution from segments 3, 4, 5), the right half nerve supply in two experi- mental cases in which limb buds had been trans- planted from their norma] site (n) to anterior or posterior levels, respectively, as indicated by arrows. grow from one of the stimips, the connecting strand of Schwann cells would automatically lead them over into the other stump. The chemical activity of the degenerated stump thus plays no part other than that of an accessory aid to structural orientation. In confirmation of this fact, degenerated nerve in a liquid medium leaves nerve growth wholly unaffected despite enhanced diffusion (Weiss and Taylor, '44), and conversely, oriented structural pathways are followed by nerve fibers regardless of whether or not they lead to supposedly "attractive" destina- tions. For instance, when a proximal nerve stump as fiber source is introduced into the stem of a bifurcated blood-filled tube, one branch of which contains degenerated nerve while the other ends blindly (Fig. 132), regenerating nerve fibers fill both branches equally well and abundantly (Weiss and Taylor, '44). In conclusion, the idea that remote tissues of destination can attract nerve libers direc
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