IMMEDIATE EFFECTS OF X-RAYS ON LIVING CELLS 137 



tissue, for instance, is varied. Dr. Ivutli Itzliaki has been carj-ying out 

 work of this sort, and has found that the descri])tion of extraction 

 methods given in the literature are often quite inadequate because such 

 details are not given. For instance, when homogenizing mouse spleen 

 at fairly high speed in normal saline, she found that insoluble and prob- 

 ably denatured nucleoprotein fibres were formed, while at slow speeds 

 whole cells were still present and no fibres. Reproducibility was difficult 

 because the resistance of the homogenate reduced the speed of the 

 homogenizer by an unknow^n amount. These facts are mentioned merely 

 to illustrate the difficulties of such work with nucleoprotein complexes. 

 Many experiments reported in the literature give no information 

 about the initial lesion produced by the irradiation, since the nucleo- 

 protein extraction is not made immediately after irradiation. 



Perhaps the most definite evidence of damage to this structure by 

 irradiation has been given by the work of Ord and Stocken (1960), who 

 used the Bendich method of separating different fractions of the DNA 

 and found that irradiation caused an increase in the amount of the 

 more soluble fractions at the expense of the less soluble. 



Irradiation in vitro of already extracted nucleoprotein must give 

 different results according to the method of extraction used and one is 

 then faced with the difficulty of deciding which preparation most 

 nearly resembles the substance as it occurs in the living cell. To give a 

 rather extreme example Anderson and Fisher (1960) found that the 

 extraction of rat thymus brei with 2 volumes of 1 -4 M sodium chloride 

 produced a sample of very high viscosity. The authors considered that 

 linear aggregates of DNA were present, other linkages having been 

 broken by the strong salt. As expected by Anderson, these complexes 

 were very sensitive to irradiation and a definite fall in viscosity was 

 noted after a dose of 50 r. 



The kindness of Professor J. S. Mitchell allows me to present to you 

 some partly unpublished data of his own (Mitchell, 1959) which deal 

 wdth a true initial effect and possibly one in which the deoxyribo- 

 nucleic acid is involved. The Walker rat carcinoma has been irradiated 

 in vivo in an anaesthetized animal with 2,000 r given in 2 min 16 sec. 

 By a special device, liquid nitrogen is applied to the tumour less than 

 1 sec after irradiation. On w^arming the excised tissue high energy u.v. 

 quanta may be released mainly at - 5°C and 50 to 70°C. In 35 recent 

 experiments definite results have been observed in 5 cases and possible 

 ones in a further 6 cases. The long-lived excited states must last for 

 some seconds or even minutes at body temperature and might be 

 associated with reactive chemical intermediates, but the experimental 

 results are not inconsistent with an excitor mechanism. If the DNA is con- 



