CELL DIVISION, MORPHOLOGY, VIABILITY 809 



Kind of Radiation. Lasnitski (1948) discovered no striking differences 

 in the proportion of degenerate avian fibroblasts in vitro exposed to either 

 1000 r of X or j3 rays (Fig. 11-15). Degenerate cells made their appear- 

 ance slightly earlier, however, after /3-raying than after X-raying. 



A comparison of the efficiencies of fast neutrons and y rays in producing 

 cell degeneration of the developing rat retina led Spear and Tansley 

 (1944) to conclude that the former were 6.5 times as effective as the latter 

 per n unit. The degenerate cell count rises more rapidly after neutron 

 treatment than after y irradiation. Also after neutron treatment the 



IOO-, (0) 



5 10 15 20 



TlME.hr 

 Fig. 11-15. Effects of 1000 r of X rays (a) and /3 rays (b) on mitosis (heavy, solid 

 line), abnormal mitoses (broken line), and degeneration of cells (light, solid line) of 

 avian fibroblasts in vitro (after Lasnitski, 1948). 



time of appearance of the degenerate cells does not vary with the dose 

 as it does after y irradiation, when the larger the dose, the greater is the 

 delay in their appearance. This is interpreted to indicate that, while 

 after each irradiation the degenerate cells are made up of those killed 

 outright during mitosis and those that die later when they attempt, but 

 fail, to complete mitosis, in the case of neutrons a larger proportion of 

 the degenerating cells represent direct kills. The much greater effective- 

 ness of fast neutrons than y rays in killing mice by whole-body irradiation 

 is attributed by Mitchell (1947), at least in part, to degeneration of radio- 

 sensitive cells, such as bone marrow. He also suggests that neutrons 



