. Embryology. Embryology; Embryology; genealogy. 150 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 lim
. Embryology. Embryology; Embryology; genealogy. 150 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Barth, Lester G. (Lester George), 1905-1979. New York, Dryden Press
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