R. C. VON BORSTEL 193 



than nucleic acids. Indeed, Goldman and Setlow (1956) have 

 found a similar circumstance in Drosophila eggs and they explain 

 the effect as being caused by protein denaturation. This phe- 

 nomenon deserves careful analysis because it may reveal a great 

 deal about information content of cytoplasmic elements in de- 

 terminate eggs. Immediately apparent is the possibility that cyto- 

 plasmic protein is used in the determination of egg regions for 

 the first steps in differentiation and the nucleic acid utilized for 

 direction of later development. Alternatively, this phenomenon 

 may be an important clue in deciding between production of 

 toxic substances and inactivation of cytoplasmic particles as a 

 cause of death in these eggs. 



Photoreactivation 



Now, is there any general method by which one can distinguish 

 between action of the nucleus and action of the cytoplasm in 

 bringing about an embryonic event? 



It has already been pointed out that one can distinguish be- 

 tween eggs killed by ultraviolet radiation when the nucleus and 

 cytoplasm are separately exposed. In 1949, Kelner and Dulbecco, 

 working on fungi and viruses respectively, independently dis- 

 covered the phenomenon of photoreactivation. This is the coun- 

 teraction of ultraviolet radiation damage (ca. 2600 A) by radia- 

 tion of a longer wavelength (ca. 3600 A). Photoreactivation is 

 possible after ultraviolet irradiation of a wide variety of living 

 material from protozoa to vertebrates, from viruses and bacteria 

 to the cells of higher plants (see Dulbecco for review, 1955). 

 After ultraviolet irradiation of the nucleus or cytoplasm of the 

 Habrohracon egg, eggs were subjected to reactivating light ( von 

 Borstel and Wolff, 1954, 1955 ) . The results were striking. Injured 

 nuclei can be repaired by light of a longer wavelength, but in- 

 jured cytoplasm cannot (Fig. 8). In both the photorecoverable 

 nucleus and the nonphotorecoverable cytoplasm the energy is 

 absorbed into nucleic acid or nucleoprotein ( Amy and von Borstel, 

 1955). 



A similar situation has been reported in Paramecium, where 

 ciliary immobilization from ultraviolet radiation is adjudged to 



