78 



RADIATION BIOLOGY 



blast, both of which tend somewhat to delay the completion of mitosis 

 (Carlson, 1941). The amount of delay is directly related to the nearness 

 of the neuroblast at the time of treatment to the stage of mitotic stoppage ; 

 the more recently the treated cell has passed this stage, the greater is the 

 delay. 



The interval of time elapsing between treatment and the virtual dis- 

 appearance of mid-mitotic stages 6 is considerably greater than the average 

 interval of time required by the untreated cell to complete mitosis from 



180 



12 3 4 5 6 



DURATION OF INCUBATION IN HOURS SUBSEQUENT TO IRRADIATION 



Fig. 1 1-5. Mitotic effects of y rays from radium on chick fibroblasts in vitro. Curves 

 from top to bottom represent effects of 50, 83, 300, 1000, and 2000 r, respectively, 

 given at 33 r/min (after Cade, 1948, after Canti and Svear, 1929). 



the stage just following that at which radiation blockage occurs. The 

 untreated chick fibroblast requires about 35 minutes on the average to 

 pass from the beginning of prophase to the formation of the daughter 

 nuclei (Strangeways, 1922) or about 20 minutes to complete metaphase, 

 anaphase, and telophase (Simon-Reuss and Spear, 1947), but it is not 

 until 2 hours after irradiation that mid-mitotic stages have virtually dis- 

 appeared (Canti and Spear, 1929). The root-tip cells of Vicia require 



6 Mid-mitosis is used in this chapter to include prometaphase, metaphase, and 

 anaphase stages, i.e., the period between breakdown of the nuclear membrane and the 

 loss of the smooth form of the chromosomes after they have reached the poles. 



