PHOTOKEACTIVATION 481 



Changes in the structure of the macronucleus after ultraviolet irradi- 

 ation consist in clumping of granular structures with subsecjuent vacuoli- 

 zation (Kimball, 1949). If the animals are exposed to the photoreacti- 

 vating light following ultraviolet irradiation, some clumping occurs, but 

 fusion and vacuolization do not ensue; the clumps produced are subse- 

 (j[uently dissolved. 



9. PHOTOREACTIVATION IN ECHINODERM ZYGOTES AND GAMETES 



Blum and coworkers have extensively studied the delaying effect of 

 ultraviolet on cleavage in Arhacia punctulata eggs, and the effect of photo- 

 reactivation on this delay (Blum . . . Robinson, 1949; Blum . . . Loos, 

 1949; Blum, Loos, and Robinson, 1950). In fertilized eggs irradiated 

 with a Hanovia intermediate pressure arc before cleavage, with filters 

 absorbing most of the visible spectrum, the time of onset of division is 

 delayed, the delay markedly affecting the first and less the second and 

 third division. In eggs exposed to the light of a fluorescent lamp after 

 ultraviolet irradiation, the delay is greatly reduced. 



Eggs irradiated with ultraviolet before fertilization show a similar delay 

 in the onset of the first few divisions following fertilization; treatment of 

 the unfertilized eggs with visible light after ultraviolet irradiation 

 decreases the delay. The delay is not affected if the eggs are illuminated 

 with visible light before ultraviolet treatment. 



Unfertilized eggs can be separated by centrifugation into two parts, one 

 yellow containing all the echinochrome pigment, the other white con- 

 taining the nucleus. The white halves are damaged by ultraviolet and 

 photoreactivated like whole eggs. This shows that the echinochrome pig- 

 ment is not required for this type of photoreactivation. If the enucleated 

 halves are treated with ultraviolet and then fertilized, no delay occurs, 

 suggesting that the site of action of ultraviolet is in the nucleus. 



The effective spectral range for photoreactivation of division delay is 

 between 3000 and 5000 A. 



In other experiments the effect of ultraviolet irradiation and photo- 

 reactivation on sperm has been studied (Blum, Robinson, and Loos, 1950, 

 1951). When ultraviolet-treated sperm is used for fertilization of non- 

 irradiated eggs or egg halves (both nucleated and enucleated ones), delay 

 in the time of cleavage occurs. Illumination of the ultraviolet-irradiated 

 sperm with photoreactivating light does not affect the delay, which is, on 

 the contrary, reduced if the eggs fertilized with ultraviolet-irradiated 

 sperm are exposed to the photoreactivating light. The authors point out 

 the similarity of this interesting observation with the situation found in 

 photoreactivation of bacteriophages in which the bacteriophage can be 

 reactivated only if adsorbed on the sensitive bacterium (see Sect. 3-3). 

 No photoreactivation was observed after X-ray treatment. 



