CELL DIVISION, MORPHOLOGY, VIABILITY 785 



Miwa, Yamashita, and Mori (1939b), on the other hand, obtained 

 evidence that mitotic delay may be induced by treatment of the cytosome. 

 Unfertilized eggs of the sea urchin, Pseudocentrotus, were irradiated with a 

 rays through aluminum filters varying from to 45 n in thickness. 

 Sensitivity of the eggs as measured by delay in the first cleavage division 

 decreased markedly as the increased thickness of the filters used pre- 

 vented the a rays from penetrating to the nucleus, but even with a-ray 

 ranges that fell short of the nucleus, some eggs showed marked delay. 



It has also been demonstrated that the amount of cleavage delay 

 resulting from X-raying Arbacia sperm is not influenced by the concentra- 

 tion of the sperm or the composition of the medium in which they are 

 irradiated (Evans et al, 1942). When Arbacia sperm were mixed with 

 heavily irradiated water containing X-ray-induced hydrogen peroxide, 

 and then used to fertilize untreated eggs, delay of the first cleavage divi- 

 sion resulted (Evans, 1947). The amount of delay was positively cor- 

 related with the dose. Heilbrunn and Young (1935), however, found 

 that, while the concentration of unfertilized Arbacia eggs X-rayed in sea 

 water had no effect on the delay of the first cleavage division, irradiation 

 in the presence of ovarian tissue produced a considerably greater delay 

 than irradiation of the eggs in sea water alone or in concentrated suspen- 

 sion. They suggest that ovarian cells exposed to X rays produce a sub- 

 stance or substances that act on the eggs to enhance the direct effect 

 obtained in their absence. 



Increased absorption of 2537 A ultraviolet radiation in the cytoplasm 

 of irradiated neoplastic cells is interpreted by Mitchell (1942a, b, 1943), 

 to result from radiation-induced inhibition of desoxyribonucleic acid 

 (DNA) formation and the consequent accumulation of pentose nucleo- 

 tides in the cytoplasm. He suggests that this might cause inhibition of 

 mitosis. 



That the cytosome, however, may be responsible for recovery from 

 radiation effects is suggested by the findings of Henshaw (1940a) using 

 X rays, and Miwa et al. (1939a) and Mori, Miwa, and Yamashita (1939) 

 using /3 rays, who state that there is no recovery in sea urchin sperm 

 between treatment and insemination. Since the same investigators have 

 found that sea urchin eggs undergo recovery between treatment and 

 insemination, it seems likely that the cytosome of the egg is responsible 

 for recovery. 



Kind of Radiation. Of special importance in the interpretation of the 

 mechanism of the action by which radiations are able to alter the rate of 

 so complex and yet so precise a living process as cell division are studies of 

 the relative capacities of different kinds of radiations to interfere with 

 mitosis. From the point of view of the biologist, different radiations are 

 of interest because of their different ion distributions or densities. These 

 range from the widely separated ions produced by high energy /3 and y 



