•292 RADIATION HIOLor.Y 



Kiiplotcs, the possibility of an effect of the medium during irradiation must 

 l)e considered. Packard (1924) reported what appears to have been a 

 major difference in sensitivity to radium (principal effect said to have 

 been due to slow /3 particles) between two species of ciliates. The lethal 

 dose for Paramecium was 3 hr exposure to the source while that for the 

 hypotrichous ciliate Styloni/chia was 15 hr exposure. Since Paramecium. 

 is very resistant to ionizing radiation, this result is (}uite surprising. 

 Packard attributes the difference to the lesser permeability of Stijlonijchia. 

 Unfortunately, there has been no other work with Stylonychia; however, 

 the related hypotrich, Euplotes, is killed at approximately the same dose 

 as Paramecium (Brown et ai, 1933). More recently, Wichterman 

 (1948a, b) has reported results which suggest a small difference in sus- 

 ceptibility to X rays between Paramecium hursaria and P. calhinsi. He 

 has slated that some P. hursaria survived doses between 400, 000 and 

 (iOO.OOO r, whereas all P. calkinsi were killed by 400,000 r. 



More striking differences have been reported in the lethal doses of ultra- 

 violet. Rather extensive comparisons between different species and 

 strains of protozoa, primarily ciliates, are to be found in the works of 

 Giese and Leighton (1953b) for long wave lengths, of Giese (1938b) for 

 shorter wave lengths, and of Harvey (1942) for intense flashes of ultra- 

 violet. Giese (1946b) has made similar comparisons for the eggs and 

 sperm of a number of marine invertebrates. Shalimov (1935) reported 

 that eggs of Ascaris equorum and Enterobius vermicularis were killed by 

 ultraviolet in 5 min, whereas those of Sirongylus equinus were killed in 

 only 3 min. Wright and McAlister (1934) found Toxascaris eggs to be 

 more readily affected by monochromatic light than were the eggs of 

 Toxocara. They suggest that this may be due to differences in the 

 absorption of the shell. This emphasizes the difficulties which are inher- 

 ent in interpreting differences in sensitivity to ultraviolet. Differences in 

 absorption in the outer layers of cytoplasm or egg shells and differences 

 in the action spectrum for superficially similar effects may be involved, as 

 well as more su})tle differences in the biological organization of the organ- 

 isms being compared. 



Recovery. The possibility that recovery may occur from changes which 

 ordinarily lead to death has been investigated by use of fractionated doses. 

 The method assumes that the effect is not due to a single "hit." If this 

 is true and recovery does occur, then fractionated doses with sufficiently 

 long rest periods in between the fractions should have less effect than the 

 same total dose given as a single exposure in a brief period of time. Such 

 a decreased effectiveness of fractionated doses of X rays has been reported 

 by Crowther (1926) for the ciliate Colpidium colpoda and by Back (1939) 

 for Paramecium caudatum. Growther (1926) found that one dose given 

 in about 20 min produced death, whereas a dose one and a half times 

 greater was required when given as three exposures at 2-hr intervals. It 



