CELL DIVISION, MORPHOLOGY, VIABILITY 769 



little or no delay, if the dose is less than 250 r. Larger doses produce a 

 delay that increases with the dose. This seems to result partly from 

 delay in the breakdown of the nuclear membrane and partly from chromo- 

 some "stickiness," which prolongs the anaphase separation of daughter 

 chromosomes. The more recently the irradiated cell has passed the 

 critical period, the greater is the delay in completing mitosis. If the cell 

 is in prophase at the time of treatment, but has not reached the critical 

 period, the effect on mitosis is determined both by the dose of radiation 

 and by the nearness of the cell to the critical period. After a very small 

 dose, such as 8 or 16 r, most cells in middle or late prophase, 3 which have 

 not reached the critical period, are stopped mitotically for a length of time 

 that increases with increase of dose and decreases with rise of temperature. 

 The cell treated in interphase or early prophase is gradually slowed as it 

 approaches the critical period. Larger doses, 250 r, for example, produce 

 mitotic stoppage of those prophase cells that are nearing the critical 

 period. This is followed by reversion or simulated reversion to a stage in 

 which the chromatin resembles that of interphase (Carlson, 1940, 1941, 

 1950). Prophases are entirely absent from the tissue until recovery has 

 occurred; then these cells, together with those retarded in interphase, 

 again progress mitotically. 



The most extensive series of studies yet made to determine the effect of 

 irradiation on the mitotic rate of cells treated at a known stage of mitosis 

 have been carried out by Henshaw and his co-workers on the marine 

 invertebrate egg. If Arbacia eggs or sperm are X-rayed and used immedi- 

 ately afterward in insemination, the first cleavage division is delayed, and 

 the larger the dose of radiation to which they have been exposed, the 

 greater is the delay (Fig. 11-2) (Henshaw, 1932, 1940a). Irradiation of 

 either sperm or egg is about equally effective in retarding cleavage 

 (Henshaw and Francis, 1936; Henshaw, 1940b; Henshaw and Cohen, 

 1940). When sperm alone are treated, it is found that the amount of 

 delay varies linearly with the logarithm of the dose (Henshaw, 1940a). 

 If the eggs and sperm are both treated, the zygote shows a greater delay 

 in the first cleavage division than if one or the other is treated (Henshaw 

 and Francis, 1936), but the effects are not additive in the sense that the 

 delay in the zygote formed from irradiated egg and irradiated sperm is 

 equal to the sum of the delays produced in zygotes formed from irradiated 



3 Recent unpublished studies in our laboratory by Nancy D. Wolfson indicate that 

 appreciable numbers of neuroblasts treated at late prophase with 32 r of X rays pass 

 the critical period and complete mitosis with little or no delay. Since mitotic activity 

 falls to zero after this dose, however, cells in middle prophase at treatment must suffer 

 complete temporary blockage at some stage between middle prophase and the break- 

 down of the nuclear membrane. After so small a dose perhaps a certain amount of 

 time is necessary for the physical and chemical changes induced to be translated into 

 mitotic blockage. 



