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EMBRYOLOGY 



FlG. 88. Schematic representation of 

 three theories of the directive influence of 

 the limb bud upon the limb nerves. A: In 

 the limb a substance, S, diffuses through 

 the intervening tissues toward the neural 

 crest. A neuroblast differentiates and its 

 cone of growth encounters the substance. 

 5". at a low concentration, C„. (>> . The mi- 

 grating cone is affected by 5" in such a way 

 that it always turns toward the highest 

 concentration of S. In this way the cone 

 migrates to C„, the highest concentration 

 of S. B: The limb is assumed to set up 

 electrical differences in potential between 

 itself and the neural tube. The cone of 

 growth migrates along the lines of flow 

 of the electric current. C: The growing 

 limb exerts an influence on the long, nar- 

 row particles in the medium between it- 

 self and the neural crest. The migrating 

 cone of growth follows the oriented parti- 

 cles as it would in tissue culture. 



In the case of the chemical theory, some substance from the limb is 

 pictured as diffusing from the limb back toward the spinal cord (Fig. 88, A). 

 Since this substance originates at the limb, the concentration, C. would be 

 highest at the limb, C,„ and the farther away from the limb, the lower would 

 be the concentration. Finally, near the spinal cord it is very weak, C„q. The 

 chemical theory supposes that as the nerve fiber originates, the cone of 

 growth starts out at random. Its direction of growth, however, is soon 



