R. C. VON BORSTEL 191 



kills the egg by damage to non-nuclear elements. The sm'vival 

 curve is exponential for nuclear killing and sigmoid with respect 

 to cytoplasmic killing ( Fig. 8 ) . Nuclear killing was discussed pre- 

 viously; damage to the cytoplasm, resulting in death, has certain 

 characteristics that make it highly instructive. ( 1 ) Eggs killed by 

 irradiation of cytoplasm die late in development — hatching time 

 is longer in many of those that do hatch, and of those that die, 

 most are characterized by having herniated gut or fragile endo- 

 derm or ectoderm tissues (Amy and von Borstel, 1955). (2) Sur- 

 viving embryos at high doses to cytoplasm have as adults irregu- 

 lar tergites as at least one nonlethal developmental abnormality. 

 (For a similar effect in Drosophila following x-irradiation, see 

 Ulrich, 1951, 1953.) These are gross defects indicating interfer- 

 ence with normal determination and synthesis. (3) The survival 

 curve is steeply sigmoid (von Borstel and Moser, 1956; Ulrich, 

 1955a,b,c). (4) The action spectrum for cytoplasmic killing 

 indicates that the energy is absorbed by nucleic acid or nucleo- 

 protein (Amy and von Borstel, 1955). (See Loofbourow, 1948, 

 for assumptions and limitations in action spectrum analysis. ) 



While this work was being done on Habrohracon, an independ- 

 ent study of ultraviolet radiation effects on Drosophila eggs was 

 carried out at Yale by Goldman and Setlow (1956). The research 

 can be considered as mutually confirmatory in most respects, 

 since Goldman and Setlow also observed gut herniation and an 

 action spectrum which they interpreted as indicating energy ab- 

 sorption in nucleic acid when the egg is irradiated. 



It has been calculated (see Bachem and Reed, 1931) that in 

 normal tissues ca. 80% of the energy of ultraviolet radiation is 

 expended in the first 30 microns. Most of the cytoplasm in 

 Habrohracon eggs is located in a 5- to 10-micron peripheral layer; 

 therefore, it appears reasonable to assume that the cytoplasm 

 comprises the substance sensitive to ultraviolet radiation. Cyto- 

 chemical methods show that peripheral cytoplasm is rich in 

 nucleic acid. This is consistent with action spectrum analysis. 



The steeply sigmoid shape of the dose-action curve for cyto- 

 plasm suggests a multihit survival curve of high order (von 

 Borstel and Moser, .1956). The shape of the curve appears to be 



