780 RADIATION BIOLOGY 



its application. First, the stage of blockage must not be among the 

 stages classed as mitotic. If any of the prophase cells that are being 

 included in the counts of mitotic cells are blocked by the X rays or caused 

 to revert mitotically (Carlson, 1940, 1941), the rate of fall in numbers of 

 mitotic cells after treatment will not be a true measure of the normal 

 mitotic rate of these cells. Second, the stage of blockage must precede 

 by only a short interval of time the stages included among those desig- 

 nated as mitotic stages. If the stage of blockage precedes by a long 

 interval the stage at which counts are being made, cells will enter mitosis 

 as fast as they leave it until the supply is depleted. By this time a certain 

 proportion of the blocked cells will have begun to recover and enter 

 mitosis, thus making it difficult to determine accurately the rate of fall of 

 mitosis. Third, the dose must be large enough to prevent all but an 

 inappreciable number of cells from leaking through the stage of blockage 

 or recovering early enough to swell the mitotic count in the postirradia- 

 tion period. Fourth, the dose must be small enough that delay is not 

 induced in the progress of cells through the mitotic stages in which cells 

 are being counted. A dose of 256 r, for example, as determined by 

 timing experiments on living Chortophaga neuroblasts in vitro will delay 

 appreciably the progress of cells through mid-mitosis (Carlson and 

 Harrington, unpublished). These cells were treated in very late pro- 

 phase, that is, just past the blockage stage. Such a delay decreases the 

 rate of fall of mitoses following irradiation. Fifth, the tissue must be 

 homogeneous with respect to the mitotic activity of its cells. If, for 

 example, only one-tenth of the cells in a certain tissue divide regularly, 

 then the intermitotic period of these cells must be one-tenth as long as 

 that calculated by this method, in order to account for the number of 

 mitotic cells present. If we were to apply the method of Knowlton and 

 Widner (1950) to the Chorto-phaga neuroblast, for which we have much 

 information on the conditions just discussed, we would proceed as follows. 

 Extensive series of counts of mid-mitotic cells at 22-min intervals after 

 treatment of neuroblasts in vitro with 64 r of X rays give average counts 

 of approximately 10, 7, 2, and at 0, 22, 44, and 66 min after treatment, 

 respectively. The slope of this curve is steepest from 22 to 44 min. A 

 straight line with the same slope drawn through 10 on the Y axis inter- 

 sects the X axis at approximately 44. Substituting this value for the 

 duration of mid-mitosis, 10 for the average number of mid-mitotic cells, 

 and 190, which has been obtained by averaging direct counts, for the 

 number of intermitotic 7 cells, we have 



10 44 



190 duration of intermitotic period 



7 For the sake of simplifying this analysis, "intermitotic" as used in this paragraph 

 refers to the period from the end of one to the beginning of the next mid-mitotic period. 

 This includes telophase, interphase, and prophase. 



