The caudal fin of Gambusia provides a satisfactory vertebrate system for the study of ultraviolet radiation damage in vivo. Regenerating fins exposed to ultraviolet radiation of 254 nm at energies between 1782 and <tex-math>$16,000\ {\rm ergs}/{\rm mm}^{2}$</tex-math> showed inhibition of growth. Regeneration at the ends of fin rays is not affected as much as regeneration between fin rays. Since these fin ray regions are 17 μm thicker than interradial areas, it appears that ultraviolet radiation inhibits growth in these thinner regions by destroying the less protected interradial blastema. At intermediate doses within the inhibition range, it is the comparatively undamaged cells in the more protected radial blastema that generate growth.

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