258 L. H. Gray 



The structure of B. subtilis has not yet been estabhshed by 

 staining methods but in Bacillus megatherium cytological 

 structure has been described by several workers (Robinow, 

 1953; Yuasa, 1956) which corresponds rather strikingly to 

 Hutchinson's (1955) description. 



Davis (1954) has studied the inactivation of Tl bacterio- 

 phage by slow electrons. She finds that the surface coat of the 

 phage particle, about 100 A units thick, is extremely insensi- 

 tive. If the surface coat is identified with protein in this case 

 and the core with DNA, then it is evident that when the 

 whole virus is exposed to radiation in the dry state the 

 inactivation is predominantly associated with energy de- 

 posited in the DNA. Energy deposition in the protein only 

 brings about inactivation at a dose level sixteen times higher 

 than that which suffices when energy is absorbed in the DNA. 

 The irradiation of oriented tobacco mosaic virus particles 

 leads to the same conclusion (Pollard and Whitmore, 1955). 



2. Micro Beams 



The irradiation of selected small regions of living cells by 

 protons and by pencils of u.v. radiation has been brought to 

 a high degree of technical perfection by Zirkle and Bloom 

 (1953). In the case of the proton beam 80 per cent of the 

 particles fall within a circle 2-5 [x in diameter and 96 per cent 

 within a circle 5 (jl in diameter. The convergent hetero- 

 chromatic u.v. beam intensely irradiates an approximately 

 isodiametric volume about 7 [x across. The cells principally 

 studied have been from adult newt heart cultured at 23° C 

 and observed by phase contrast illumination as they enter 

 and pass through division. 



Irradiation of the centromere region of a chromosome by 

 either radiation may result in the loss by that chromosome 

 of directed movement, so that it never moves to the equatorial 

 plate. Similar irradiation of regions of the chromosome not 

 including the centromere were never observed to produce this 

 effect. Somewhat larger exposure to u.v. resulted in a change 

 in refractive index which was at first confined to the length of 



