784 RADIATION BIOLOGY 



first cleavage division, development of the treated eggs would usually be 

 behind that of the untreated eggs at the 24-celled stage. No stimulation 

 and probably some depression of the division rate was produced by treat- 

 ment of the resting stage. Packard (1916) reported a 7-ray-induced 

 acceleration of the first cleavage division of Arbacia that amounted to a 

 5-15 per cent increase in the speed of division. Richards and Good 

 (1919) found that a small dose of X rays applied to the fertilized egg of 

 Cumingia stimulated division at first, and then retarded it. Irradiation 

 of sperm or eggs or both before fertilization, however, produced either no 

 effect or a retardation of subsequent cleavage divisions. More recently, 

 Darlington and La Cour (1945) found increases in the frequencies of 

 metaphases and anaphases of Vicia root tips 4 hours at 24°C after treat- 

 ment with 45 r of X rays. Since the numbers of cells in these stages 

 exhibited decreases at other doses (90 and 135 r) at the same temperature 

 and at all three doses at 16 and 30°C, they conclude that there is an 

 optimum temperature and dose at which mitosis may be temporarily 

 stimulated by X rays. The numbers of metaphases and anaphases in 

 the controls for their different experiments differ widely, however, and 

 they give no tests of significance for these numbers. La Cour (1951) also 

 reports an X-ray-induced acceleration of mitosis by 90 r at 20°C but does 

 not include the data on which this conclusion is based. In view of the 

 fact that, except for these studies, no other researchers seem to have 

 found evidence of a stimulating effect of radiation, confirmation by other 

 workers, duplicating as exactly as possible the procedures originally 

 used, would seem worth while. An attempt to confirm the earlier studies 

 was made by Seide (1925), who used a different biological material, how- 

 ever. He treated the Ascaris egg in the pronuclear stage with small doses 

 of X and 7 radiation, but was unable to demonstrate a constant increase 

 in the mitotic rate as determined by the time required for the treated 

 cells, as compared with the controls, to complete the first and the second 

 divisions. 



Evidence relating to the means by which ionizing radiations interfere 

 with mitosis has pointed strongly to a primary effect on the nucleus. 

 The first cleavage division of the Arbacia zygote is delayed to about the 

 same extent, whether the sperm or the egg is irradiated (Henshaw and 

 Francis, 1936; Henshaw, 1940b; Henshaw and Cohen, 1940). Since the 

 sperm consists almost entirely of nuclear material, it is natural to con- 

 clude that the X-ray effect must involve changes induced in the nucleus 

 rather than in the cytoplasm. Further, if nucleated and nonnucleated 

 egg fragments obtained by centrifugation are irradiated and then 

 fertilized with untreated sperm, it is found that a delay is produced in 

 the nucleated fragments comparable to that produced in the whole egg, 

 while no delay is produced in the nonnucleated fragments (Henshaw, 

 1938). 



