30(i UAUlvriON lUOLC^GY 



eiuu'leute halves subscMiiuMitly fertilized with unirrudiated .sperm. Blum, 

 R()l)iiis()ii, and Loos (1950, 19ol) carried out similar experiments with 

 ultraviolet-irradiated Arharia ejiigs and found the same results. They 

 also demonstratetl that other combinations, such as irradiated sperm with 

 unirradiated enucleate halves of egs^, result in dela3^ Their conclusion 

 was that the locus of the primary injury must he in the nucleus. Harding; 

 and Thomas (1949, 1950) found that centrifuged Arbacia eggs irradiated 

 unilaterally with ultraviolet through the fat cap were more affected than 

 were those irradiated through the pigmented end. They draw no final 

 conclusions from these results, but a nuclear effect seems to be fa\-ored 

 since the nucleus would be displaced toward the fat cap. Marshak 

 (1949b) suggests that the relative inefficiency of ultraviolet for the egg as 

 compared to the sperm favors a nuclear effect. Otherwise, the high pro- 

 portion of ultraviolet absorbed in the cytoplasm should make the egg 

 more, not less, sensitive. 



Thus most of the evidence clearly favors a nuclear effect. However, 

 there is certain evidence which is not in full agreement. Giese (1939b, 

 1947a) has shown that the action spectrum for delay by ultraviolet-irradi- 

 ated sperm resembles the absorption spectrum for nucleoprotein, whereas 

 that for ultraviolet-irradiated eggs resembles the absorption spectrum for 

 certain other proteins (Fig. 8-3). Giese (1939b, 1947a) discusses various 

 explanations among which is the possibility that the effect is partially 

 cytoplasmic in the case of the egg but entirely nuclear for the sperm. 

 However, in the egg, the primary absorption might be in the cytoplasm 

 with secondary effects on the nucleus or it might be by proteins, other 

 than luicleoprotein's, in the nucleus. Since the nuclei in the tw'o gametes 

 are not in the same state, such differences in importance between nucleic 

 acid and protein absorption would be possible. This emphasizes that 

 action spectra cannot be used to reach a clear decision between a nuclear 

 and a cytoplasmic site of radiation injury since nucleic acids and several 

 kinds of proteins are present in both. 



Blum and Price (1950) believe that the fact that recovery from ultra- 

 violet-induced delay is independent of the occurrence of cleavage suggests 

 a cytoplasmic locus, since the nucleus undergoes major changes at the 

 time of cleavage. However, Blum, Robinson, and Loos (1950, 1951) pre- 

 sent e\'idence that the primary absorption of the ultraviolet is in the 

 luicleus. On the basis of their belief that the sperm cannot be photore- 

 activated before fertilization, they conclude that recovery is a cytoplasmic 

 process. The argument that cytoplasmic rather than niu'lear processes 

 are suggested by a recovery independent of cleavage appears weak since 

 the cytoplasm at cleavage may well undergo changes (luite as profoiuid as 

 those in the nucleus. The evidence that sperm are not subject to photo- 

 reactivation has been called in cjuestion by the finding of Wells and Giese 

 (1950) of some photoreactivation of Stromiijloccntrotus sperm. Blum, 



( 



