PROTOZOA AND INVKRTKBRATE EGGS 293 



can be estimated from Crowther's data that the doses used were of the 

 order of 10* r. Back (1939) reported that about half the immediate lethal 

 dose given every day for 3 to 4 successive days produced the same effect 

 as two-thirds the immediate lethal dose given in a single exposure; i.e., 

 the paramecia survived for some time without division, but eventually 

 died. Since the final dose was several times the single exposure dose for 

 immediate death, it can be concluded that fractionated exposures were 

 less effective than single exposures. Quite different results were reported 

 by Berner (1942) for immediate killing of Paramecium caudatum. He 

 found that doses of X rays given in fractions, one fraction every 24 to 48 

 hr, were considerably more effective than doses given at more frequent 

 intervals. He presented evidence that X irradiation decreased the min- 

 eral content as shown by ashed preparations and that recovery from this 

 decrease was just complete in 48 hr. He believed that at this time the 

 animals were more susceptible to X rays because their reserves had been 

 depleted by the recovery process. The results of his investigations with 

 ashed preparations and with death were quite variable, indicating a need 

 for further investigation before these conclusions can be fully established. 

 Nonetheless, the idea that a recovery process may lead to a temporary 

 increase in sensitivity to radiation is an important one. 



Halberstaedter and Back (1942) found that fractionation into several 

 parts with 1 or more days between had no noticeable effect on the action 

 of X rays in Payidorina morum, either on immediate death or death after 

 division. Halberstaedter and Luntz (1929) had previously found a simi- 

 lar lack of effect of fractionation of the dose of radium rays on the related 

 species Eudorina elegans. As just pointed out, death after division in 

 Pandorina might be due to chromosomal aberrations. If a large propor- 

 tion of the total were attributable to one-hit aberrations, the failure to 

 find an effect of fractionation would be expected. However, the lack of 

 an effect of fractionation on immediate death is surprising, especially since 

 doses of 300,000 r and greater are necessary to bring it about. On the 

 other hand, Forssberg (1933) found a marked effect of the intensity of 

 X rays on killing and division delay in the single-cell algae, Chlorclla 

 vulgaris, Scenedesmus hasiliensis, and Mesotaenium caldariorum. The 

 effectiveness increased with intensity, and reached a maximum at about 

 1600 r/min. Halberstaedter and Back (1942) used intensities of 9000 

 r/min and greater. Thus it is possible that this intensity was too high to 

 allow discovery of an effect of fractionation. 



Substances Present in the Medium during Irradiation. There have been 

 a number of reports of the combined action of radiation and substances of 

 one sort or another added to the medium. Dognon and Pift'ault (1931c) 

 reported that the lethal dose of X rays for Paramecium was distinctly 

 decreased in the presence of several dyes and toxic salts, e.g., potassium 

 cyanide or iodide. Preliminary irradiation of the compounds had no 



