194 NUCLEOCYTOPLASMIC RELATIONS 



be a non-nuclear eflFect and the action of the radiation is non- 

 photorecoverable (Brandt and Giese, 1956). The action spec- 

 trum for cihary immobihzation indicates that the ultraviolet radi- 

 ation inducing this effect is absorbed by protein. Blum, Cook, and 

 Loos ( 1954 ) have found four nonphotorecoverable effects in ul- 

 traviolet-irradiated sea urchin eggs. They have suggested that 

 these effects are non-nuclear rather than being indirect effects 

 following irradiation of the egg nucleus. 



We postulate that this effect of ultraviolet radiation is a general 

 one: Only injury to the nucleus from ultraviolet radiation can he 

 repaired by reactivating light; the cytoplasm is a nonphotore- 

 coverable system. If such a reaction system is truly a general one, 

 it would seem that the system could be profitably employed to 

 follow the interplay of the nucleus and cytoplasm during em- 

 bryonic development. Miss Skreb, working on enucleate amoebae 

 in Brachet's laboratory, has results which indicate that hastening 

 of death by ultraviolet radiation can be delayed by photoreac- 

 tivating light (M. Errera, personal communication). Whether 

 her system or ours is the exceptional one still remains to be deter- 

 mined. 



Conclusion and Summary 



Several questions have been asked that are concerned with 

 relations between the nucleus and cytoplasm. 



1. The four meiotic nuclei are apparently all genetically 

 equipotent. Since all four remain inside the egg, why does only 

 one take part in further development? By a cytological analysis 

 of oocytes from females that produced a high frequency of hap- 

 loid mosaic adults, it appears that meiosis occurring at the egg 

 periphery leaves three of the four meiotic nuclei susceptible to 

 destruction by some agent in the egg cortex. 



2. Is there any condition by which accessory sperm can take 

 an active part in development? In certain genetic strains of 

 Habrobracon and the honey bee, occurrence of partial andro- 

 genesis indicates that normally a zygote nucleus reacts with the 

 cytoplasm in such a way that the cytoplasm can block further 

 development of accessory sperm. The reaction system is incom- 



