CELL DIVISION, MORPHOLOGY, VIABILITY 



787 



be twice as effective as 7 rays (Spear and Glucksmann, 1938; Tansley 

 et al., 1948). A dose of 134 energy units of a radiation reduced the 

 numbers of eye and brain cells in mitosis, to approximately the same 

 level as 268 r of 7 rays (tissue exposed to 1 energy unit has received the 

 same increment of energy/g as water exposed to 1 r of 7 rays). 



Miwa et al. (1939b) observed in Pseudocentrotus that after ft irradiation 

 the cleavage delay of all the treated eggs was about the same (with a 

 spread of 30 minutes), while after a irradiation the spread was much 

 greater, some eggs being delayed hardly at all and others 3 or more 



2.0- 



X 



UJ 

 Q 



o 



h- 

 o 



h- 



o 

 o 



_) 



10 



NEUTRON 



100 200 



DOSE, r or eu 



Fig. 11-8. Relative efficiencies of a rays (solid circle), 7 rays (open circle), and fast 

 neutrons (square) in reducing the mitotic index in Vicia root tip meristems 3 hours 

 after irradiation (after Gray and Reed, 1950). 



hours. Because their study did not include careful measurements of the 

 doses used, it is difficult to relate these results to those of other a-ray 

 studies. 



Lasnitski and Lea (1940) compared the efficiencies of 7 rays with 

 medium X rays, and hard X rays with soft X rays in reducing the pro- 

 phase, metaphase, and anaphase-telophase counts in chick fibroblasts 

 in vitro. For several doses within the range of 20-200 r, the hard and 

 soft X rays were found to be equally effective, but the 7 rays were found 

 to be less effective than medium X rays by a factor of about 2:1. These 

 conclusions are based on the assumption that the low point is reached a' 

 80 minutes after each kind of radiation, when all counts were made; if it is 

 not, the different values may be measures of the rapidity at which the 

 mitotic count falls or at which recovery takes place. 



