CELL DIVISION, MORPHOLOGY, VIABILITY 



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overlaps between certain of these doses shown in Table 11-2 are obviously 

 not real but due to the fact that counts were not made frequently enough 

 to bring out minor differences. After doses up to those barely sufficient 

 to cause a fall to zero in mitotic activity, the duration of the period of 

 decreasing mitotic activity of a given tissue is determined by the rate at 

 which the cells recover and begin to progress mitotically after treatment. 

 Since, after small doses, recovery begins sooner than after large doses, the 



a BT, body temperature; — , no information on temperature given. 

 6 See p. 775 and footnote 5. 



rise in mitotic activity that marks the end of the period of mitotic fall 

 occurs somewhat earlier afte? small than large doses. 



Prolongation of the period of mitotic decrease after doses larger than 

 those sufficient barely to reduce the mitotic count to zero is apparently 

 due to two factors: a radiation-induced retardation in the mitotic rate of 

 those cells that complete mitosis immediately after treatment and an 

 increase in the time required by cells to pass through anaphase, if the dose 

 is large enough to cause stickiness of the chromosomes. Jungling and 

 Langendorff (1930) found that in the Vicia root tip cessation of mitosis 

 occurred 18 hours after 420 r but 33 hours after 550 r. Increased doses of 

 X rays cause increased delay in the breakdown of the nuclear membrane 

 and increased stickiness of the chromosomes of the Chortophaga neuro- 



