350 



Special Vertebrate Organogenesis 



rested in their course and remain abortive. 

 While the outgrowing axon branches roam 

 about the wound area, Schwann cells spill 

 from the cut nerve ends, notably from the 

 "degenerating" peripheral stump. When an 

 advancing axon tip meets such a Schwann 

 cord, it follows it and is thus guided into the 

 peripheral stump (Fig. 126C), and thi'ough it 

 to the peripheral tissues, where new trans- 

 missive connections can be established if the 

 arriving nerve branch is of the proper type. 



The highly irregular connective tissue that 

 seals the cut nei've ends, commonly referred 

 to as "scar," causes the dissipation of a 

 large proportion of the newly formed 

 branches, which may form dense tangles 

 called "neuromas." Nerve regeneration thus 

 involves a great deal of ovei-production and 

 wastage of sprouts. Eventually, a near-nor- 

 mal number of fibers may become collected 

 in the deserted channels of the distal stump. 

 The new branches, small at first (1m or less), 

 gradually gain in width and develop on their 

 surface a new myelin sheath {m, Fig. 126Z)), 

 which thickens proportionately. In this man- 

 ner, lines for the conduction of excitation 

 between centers and periphery are rees- 

 tablished. However, owing to the misdirection 

 of many fibers into wrong channels, the 

 physiological control restored by regenerated 

 nerves does not usually attain the original 

 perfection. 



In contrast to the practically vmlimited 

 power of regeneration observed in peripheral 

 nerves, regenerative growth of intracentral 

 nerve fibers declines with age and phyloge- 

 netic rank so as to be little more than abortive 

 in brain and spinal cord of adult mammals 

 under ordinary circumstances. Whether this 

 is due to an intrinsically lower growth po- 

 tential of the neurons or to less favorable 

 growth support, perhaps even greater active 

 obstruction, by the central, as compared to 

 the peripheral, environment, is still a matter 

 of debate. 



ANALYSIS OF THE DEVELOPMENT 

 OF A NEURON 



OUTGROWTH OF AXIS CYLINDER 



Mechanism of Elongation. As was indicated 

 above, it is now indisputably established that 

 the neurite (axon) develops as a direct proto- 

 plasmic extension of the nerve cell. At a 

 given point along the circumference of the 

 neuroblast, cytoplasm is protruded to form 

 a short thread with a highly mobile tip. 

 Presumably any breach in the cell surface 

 may serve as outlet. The fact that in the 



embryo the sprouts tend to emerge from the 

 same sides in all neuroblasts of a given group 

 must be ascribed to certain polarizing fac- 

 tors in the cellular environment, analogous 

 to the determination of rootlet formation in 

 Fucus eggs by the polar action of electric 

 fields, ultraviolet light, pH gradients, etc. 

 (Whitaker, '40). 



The young sprout of axoplasm has no rigid 

 axis skeleton, no firm sheath, nothing to 

 propel it in a predetermined direction. The 

 sprout continues to elongate by virtue of 

 forces residing chiefly within the cell of 

 origin, but the course of the elongation is 

 determined by extraneous factors. In his 

 classic observations on axon outgrowth in 

 tissue cultiue, Harrison ('10) correctly iden- 

 tified the mode of advance as of the amoeboid 

 type, which view has been fully borne out 

 by later observations in explants (Lewis and 

 Lewis, '12; Levi, '34) and in the living tad- 

 pole (Speidel, '33). Adopting and partly 

 amplifying Lewis' ('50) interpretation, we 

 may conceive of the sprout as a cylinder of 

 firmly gelated ectoplasm surrounding a core 

 of more fluid entoplasm streaming from the 

 cell body distad. At the free tip, this central 

 stream would erupt in numerous pseudo- 

 podial processes, which then compete among 

 one another hydrodynamically for the com- 

 mon axial current (Fig. 127). The branch 

 that succeeds in draining the inflow into its 

 own channel thus automatically obliterates 

 the weaker pseudopods, and as its surface 

 becomes gelated, it adds its length to the 

 already consolidated older parts of the fiber 

 lying behind it. Meanwhile, the tip bursts 

 forth anew, and thus the fiber advances in 

 a continuous series of steps of protrusion ol 

 pseudopods, competition, and consolidation. 

 Evidently, the protoplasm for the fiber is 

 produced in the cell body, but it is added 

 at the tip to which it is conveyed by the 

 central stream. The motive mechanism of this 

 convection is still obscure, but it may con- 

 sist of peristaltic contraction-relaxation waves 

 of the fiber surface. It provides some sort of 

 "pumping" action, which after the fiber ha^ 

 ceased to elongate, continues to supply proto- 

 plasm for its further growth in width (sec 

 p. 363). In contrast to true amoeboid loco- 

 motion, however, the rear end of the nerve 

 cell remains anchored to its svirroundings so 

 that instead of dragging the bulk of the cell 

 after it, the advancing tip merely spins ou' 

 a thread of increasing length between itself 

 and the cell body. 



The described active advance ends as soon 

 as the free tip of a fiber attaches itself perma- 



