CELL DIVISION, MORPHOLOGY, VIABILITY 



783 



remained at zero for about 16, 18, and 24 hours after doses of 175, 420, 

 and 550 r, respectively. In the Chortophaga neuroblast the time from 

 irradiation to the reappearance of the first metaphases after mitotic 

 cessation approximates the curves shown in Fig. 11-7 for 26 and 38°C. 

 None of the tissues of Amby stoma larvae exposed to 15,000 r of X rays 

 exhibited any mitoses one week later (Rugh, 1949). 



lO.OOO-i 



1,000- 



UJ 



O 

 o 



100- 



10- 



1^ 

 10 



I 

 20 



30 



HOURS BETWEEN BEGINNING OF X IRRADIATION AND REAPPEARANCE 



OF MITOSIS 



Fig. 11-7. Graph showing the relation between dose and the time after the beginning 

 of X-ray treatment when mitoses reappear. Postirradiation temperature, 26° and 

 38°C; material, Chortophaga neuroblast. 



In contrast to the foregoing investigations, which point only to depres- 

 sion of mitotic activity as a result of treatment with ionizing radiations, 

 a few studies have indicated that certain exposures may actually speed 

 up the mitotic rate. Richards (1915) reached the conclusion that 

 exposure of the fertilized egg of the snail, Planorbis, to X rays during the 

 spindle formation period of the maturation or early cleavage divisions 

 stimulated the cell to complete its division much more rapidly than if 

 untreated; one experiment, for example, showed a change from 75 to 

 3 minutes for the time of the first cleavage division. The next succeeding 

 division was usually faster than normal, but following divisions showed a 

 progressive retardation. If the exposure was made, therefore, during the 



