I'llOTOZOA AND INVERTEBRATE EGGS 295 



Taylor et al. (1933) found that an irradiated tap- water extract of com- 

 mercial yeast killed Colpidiinn rampi/lutn when added after irradiation. 

 They were able to demonstrate the presence of hydrogen peroxide in th^ 

 irradiated water in concentrations sufficient to kill, and concluded that 

 this substance probably played a major role in the death of irradiated 

 protozoa, although they added that production of other toxic agents by 

 irradiation of the yeast medium was not improbable. Since that time, 

 several workers have reported that their media were not rendered toxic 

 to unirradiated paramecia by doses of X rays sufficient to kill directly 

 irradiated organisms (Piffault, 1939; Back and Halberstaedter, 1945; 

 Giese and Heath, 1948; Wichterman, 1948a). However, Piffault (1939) 

 reported that medium exposed to a dose about four times as great as that 

 necessary to produce death of irradiated animals was toxic and gave posi- 

 tive tests for peroxide. Giese and Heath (1948) reported that medium 

 irradiated with 1,000,000 r was not toxic, while only 560,000 r, directly to 

 the animals, led to complete death within 75 min. It would seem then 

 that the lethal effect of the radiation cannot be ascribed to stable poison- 

 ous substances produced in the medium. Obviously, an unstable poison 

 of short half* life is not excluded, since an appreciable time had to 

 elapse between the end of irradiation and the addition of cells to the 

 medium. 



Mention may also be made here of a report by Heilbrunn and Young 

 (1935) which states that eggs of sea urchins irradiated in the presence of 

 minced ovarian tissue are more affected by X rays (cleavage delay) than 

 eggs irradiated free of such materials. They believe that the irradiated 

 ovarian material produced poisonous substances. This is in line with 

 Loofbourow's finding (1948) of similar injurious substances from yeast and 

 other organisms. Heilbrunn and Young were unable to demonstrate the 

 production of such substances by organs other than ovaries. 



The problem of lethal substances in the medium has been carefully 

 investigated with sea-urchin sperm by two groups of workers (Evans et al., 

 1942; Evans, 1947; Barron et al., 1949a, b). Evans et al. (1942) showed 

 that the effect of X rays on Arbacia sperm as measured by the percentage 

 of fertilized eggs was markedly influenced by dilution of the sperm and by 

 the addition of various protective substances to the medium in which 

 irradiation was carried out. The more dilute the sperm suspension during 

 irradiation, the more effective was a given dose of X rays. A wide variety 

 of substances, such as egg albumin, gelatin, and egg water protected 

 against X rays if present during the irradiation. No effect of protective 

 substances was found on cleavage delay by irradiated sperm. Tests for 

 hydrogen peroxide suggested that too little was formed to account for the 

 effects. The investigators accepted Fricke's activated water as an expla- 

 nation of the effects. 



On the other hand, Evans (1947) comes to the conclusion that hydrogen 



