CELL DIVISION, MORPHOLOGY, VIABILITY 



797 



nificant difference in the extent to which mitosis was depressed, whether 

 irradiation was carried out in vacuum or in 100 per cent oxygen. It 

 seems, therefore, that the concentration of oxygen at the time of treat- 

 ment has no effect on the degree to which mitotic activity is reduced but 

 does determine the rate of mitotic recovery. The results were the same 

 whether 100 per cent carbon dioxide, 100 per cent nitrogen, or a vacuum 

 was used ; therefore it was clear that the oxygen concentration of the gas 

 mixtures determined the effectiveness of the X rays in these experiments. 

 Type of Preparation and Kind of Tissues. The effects on mitosis of y 

 irradiation of pre- and postcirculatory chick embryos have been com- 



HOURS AFTER IRRADIATION 



Fig. 11-12. Comparison of effects of approximately 130 roof y rays from radium on 

 mitosis in chick embryo tissues differing in age. Precirculatory chick designated by 

 broken line; postcirculatory chick by solid line (reconstructed from Spear, 1935; Wilson 

 el al., 1935). 



pared with the effects produced by comparable doses in the chick fibro- 

 blast culture preparation (Wilson et al., 1935; Spear, 1935). The mitotic 

 count of the embryo with blood circulation established fell to a minimum 

 in about the same length of time as the culture preparation, but recovery 

 was earlier and more abrupt (Fig. 11-12). In the precirculatory embryo, 

 mitosis reached a minimum sooner and recovery was greatly delayed. A 

 comparison of the effects of y radiation on malignant cells of the mouse 

 in vitro and in vivo showed no difference in the time required for the 

 mitotic activity to reach a minimum (Lasnitski, 1945). Mitotic activity 

 recovered, however, more rapidly in vivo than in vitro. 



The postirradiation picture of mitotic activity in a number of different 

 tissues in the mouse has been determined by Knowlton and Hempelmann 



