260 L. H. Gray 



many different types of cell in interphase or early prophase 

 were the result of the passage of a single ionizing particle 

 either through, or in the immediate vicinity of, the chromo- 

 some thread. 



Zirkle and Bloom's experiments strongly support this view 

 but they do not, of course, prove that only one particle was 

 involved. They only irradiated a very small fraction of the 

 length of the chromosome thread in any one exposure, and in 

 order to secure a reasonable frequency of breakage had to use 

 10-12 protons, corresponding to an average dose within the 

 micro beam of perhaps 2,000 rads. The inference that breaks 

 are produced by single ionizing particles still rests on the linear 

 relation between breakage frequency and dose. Since this 

 relation was observed by Kotval and Gray (1947) to hold even 

 at such low doses (4 rads.) that few of the irradiated nuclei 

 were traversed at any time by more than one particle, there 

 can be little doubt about the validity of this inference in this 

 particular case. Similar considerations make it rather certain 

 that the induction of lysogenicity in bacteria observed by 

 Marcovich (1954) must be due to a single electron. 



3. Nuclear Transfers 



The fertilization of the ovum provides a natural means of 

 introducing an irradiated nucleus into unirradiated cytoplasm 

 and, in special cases, of the opportunity to study the converse, 

 namely the development of a cell containing an unirradiated 

 nucleus in irradiated cytoplasm. 



As remarked earlier (p. 257) developing Drosophila eggs are 

 more readily killed by nuclear than by cytoplasmic damage. 

 Opinion is still divided as to whether the killing of these eggs 

 at their most radiosensitive stage can reasonably be ascribed j 

 entirely to nuclear damage.* 



Direct evidence, however, has been provided by Whiting 

 (1949) through her studies of radiation damage in Habrohracon. 



* Note added in proof: Rather conclusive evidence in favour of the import-' 

 ance of nuclear damage at this stage has been presented by Ulrich (1955), who 

 compared the lethality among eggs in which either the anterior half only or ^ 

 the posterior half only had been exposed to X-rays. 



