252 H. J. CURTIS AND CATHRYN CROWLEY 



above method should provide a valid estimation of changes in somatic 

 mutation rates, but should give no indication as to absolute numbers of 

 mutations. 



Previous work (Stevenson and Curtis, 1961) has shown that the aberra- 

 tions in liver cells increase approximately Knearly with age up to 12 months. 

 Also, mice given a single large dose of X-rays show aberration percentages as 

 high as 70%, which decrease very slowly with approximately half of them 

 present 7 months later. The disappearance of the aberrations might be 

 ex|Dlained by cell division which occurs slowly in the liver. 



It is well-known that chronically administered X- or y-radiation is much 

 less effective in causing shortening of the life-sj^an than a single massive dose. 

 For neutron irradiation, the shortening of the life-span is independent of the 

 dose-rate. The present work was undertaken in an attempt to reconcile and 

 explam these facts on the basis of the somatic mutation theory of agemg. 



METHODS 



The mice were of the strain "Charles River CDI" and were either bred in 

 the laboratory or obtained from Charles River Farms. The experiments were 

 begun when the mice were 8 weeks old. 



The technique used for preparing liver cells for observation is essentially 

 as previously described (Stevenson and Curtis, 1961). The mice were given 

 subcutaneous injections of 0-005 ml/g of CCI4 and 72 hours later, at the -peak 

 of mitotic activity in the liver, the animals were sacrificed and squash 

 preparations were stained with acetic orcein. The slides were scanned and 150 

 anaphases and telojDhases were counted for each mouse. Abnormal mitotic 

 figures were scored as either bridges or fragments or both. 



The X-ray exposures were performed with 250 kVp; 30 niA; | mm Cu and 

 1 mm Al filtration. ) -Rays were from a small ^°Co source and the 7-5 rads 

 was given in 7-5 hours. The neutron irradiations were performed at the Brook- 

 haven reactor (Curtis et al., 1956) in which a converter plate was used to give 

 fast neutrons with a fission spectrum. 



RESULTS 



The experiments described here are time-consuming and are incomplete. 

 However, there are several important facts revealed which make it worth 

 while to issue a progress report. 



The appearance of the abnormalities is essentially as previously described 

 (Stevenson and Curtis, 1961). The first experiment involved a determination 

 of the increase in aberrations with age for mice subjected to chronic y- 

 irradiation at a rate of 7-5 r/day starting at the age of 2 months. Control 



