Various experiments are described which show the locus of action of ultraviolet radiation and of x-ray in delaying cell division of the fertilized egg of Arbacia punctulata to be in the nucleus.

Photorecovery following ultraviolet radiation has its locus in the egg cytoplasm.

Analogy is drawn with the action of ultraviolet radiation and photorecovery on the E. coli-bacteriophage system, and certain suggestions are made regarding a common mechanism.

Photorecovery after x-ray exposure could not be demonstrated.

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