CELL DIVISION, MORPHOLOGY, VIABILITY 775 



the "appearance of chromosomes to the beginning of the journey to the 

 poles." The fact that their "prophases" are about half as abundant as 

 their "metaphases," as determined by counts in untreated preparations, 

 suggests that their "prophase" represents a portion of the mitotic cycle 

 comparable in extent to that in the chick fibroblast. Regardless of the 

 terminology adopted by the particular worker, until contradictory evi- 

 dence from observations of the mitotic history of living, irradiated cells 

 other than grasshopper neuroblasts is forthcoming, the conclusion seems 

 justifiable that the mitotic radiosensitivity is greatest shortly before 

 breakdown of the nuclear membrane when the chromosomes are clear 

 and well formed, and that whether or not an irradiated cell will continue 

 through mitosis or be stopped mitotically depends on whether or not it 

 has passed this stage. 



Comparison of the results obtained from studies by different investi- 

 gators of the effects of ionizing radiations on the mitotic activity of differ- 

 ent tissues is difficult, especially in certain of the earlier studies, because 

 investigators (1) failed to control the temperature following treatment, 

 (2) used the term "mitosis" to include different and often undesignated 

 division stages, (3) made mitotic counts at very infrequent and irregular 

 intervals, or (4) did not make accurate measurements of the doses given. 

 Nevertheless, some general conclusions can be reached from an analysis of 

 the various studies. 



The rate of decrease of mitotic activity immediately after irradiation is 

 greater in animal than in plant tissue. This is probably correlated with 

 the length of the mitotic cycle, which apparently is usually shorter in 

 animals (Lewis and Lewis, 1917; Strangeways, 1922; Wright, 1925; Carl- 

 son, 1941; Carlson and Hollaender, 1948) than in plants (Gray and 

 Scholes, 1951). The times after treatment for the number of dividing 

 cells to reach a minimum are shown in Table 11-1. The mitotic activity 

 of the animal cells reaches a minimum 3^-6 hours and that of plants 9-21 

 hours after treatment. An exception is the value of 3 hours found by 

 Marshak (1937) for root tips of several plant genera and by Gray et at., 

 (1940) for root tips of Vicia after small doses. The former paper contains 

 so little information on this particular point that it is not possible to 

 evaluate the evidence. In the latter paper it is actually stated that 

 "mitosis is not far from its minimum 3 hours after irradiation." 5 The 

 small dose used may also have some bearing on this low value (see Table 

 11-2). 



When the exposure time is short, the duration of the postirradiation 

 period of mitotic decrease is positively correlated with the dose; the larger 

 the dose, the more extended is the time interval between treatment and 

 the period of minimum mitotic activity (Table 11-2, Fig. 11-5). The 



6 Gray in a personal communication to me states that "the true value could easily 

 have been as late as 6 hours, since the minimum tends to be rather flat." 



