768 



RADIATION BIOLOGY 



the proportion of dividing cells in the tissue under consideration is greater 

 under one set of experimental conditions than another, the mitotic 

 activity has been increased or mitosis has been stimulated. 



Nature of Effect. Direct observations of mitosis in hanging-drop 

 preparations of living Chortophaga neuroblasts following X irradiation 

 have demonstrated that there exists in these cells what we may term a 

 critical period between late and very late prophase (Fig. 11-1) (Carlson, 



SUPH 5 ! 



Fig. 11-1. Mitotic cycle of Chortophaga neuroblast in vitro at 38°C (from Carlson and 

 Hollaender, 1948). 



1941, 1950). Irradiation of a cell before it has reached this period usually 

 causes it to stop or even to revert mitotically; irradiation after it has 

 passed this period affects its progress through mitosis little if at all. The 

 cell passes through this critical period about 5 minutes before the nuclear 

 membrane disappears. At this time the chromosomes have almost 

 reached the end of prophase contraction, the shape of the cell is changing 

 from concavo-convex to spherical, the nucleoli are disappearing, and the 

 cytoplasmic viscosity is falling rapidly (Carlson, 1946). Neuroblasts that 

 have passed this stage at the time of treatment complete mitosis with 



