I'UOTUZOA AND INVEKTEHRATE EGGS 30.3 



to (*hanji,v with dose, the second process becoming relatively more impor- 

 tant as the (lose increases. However, even at quite low doses, elTects 

 lasting through about six divisions can be recognized. Ivccovery from 

 lower and higher doses appears to require about the same number of 

 divisions. 



The effect of X rays on cell division in the ciliates has been investigated 

 to only a small extent, mainly because the doses necessary to produce an 

 appreciable delay are of the order of a hundred thousand roentgen units. 

 Perhaps it is this feature more than any other which emphasizes the great 

 radioresistance of protozoa, for most other cells are retarded by much 

 smaller doses. Back (1939) reports that the X-ray dose for death within 

 2 hr in P. caudatum lies between 400,000 and 600,000 r. A dose between 

 two-thirds and five-sixths the lethal dose leads to a permanent cessation 

 of division. About half the lethal dose leads to a retardation of the first 

 division of some 36 to 48 hr after which the normal rate is restored, appar- 

 ently with no effect on divisions later than the first. Giese and Heath 

 (1948) report an effect only on the first division at lower doses but effects 

 on later divisions at higher doses. Powers and Shefner (1950) report that 

 650,000 r reduces by half the rate at which irradiated P. aurclia reaches 

 the first division, but effects on later divisions are not mentioned. Kim- 

 ball et al. (1952) found an effect of X rays on divisions later than the first 

 in P. aurelia, but the effect was much smaller, relative to delay in the first 

 division, than that for ultraviolet. 



Thus the results ior Paramecium and the sea urchin seem quite different. 

 A guess might be made that cleavage delay in the sea urchin corresponds 

 more nearly to the delay in the first division in Paramecium, while the 

 other delays in this organism have no counterpart in the sea urchin. 

 However, it is not quite certain that the latter is true. In Paramecium, 

 the four products of the first two divisions of one treated animal may have 

 cessation periods of rather different duration (Kimball et al, 1952). In 

 the sea urchin, a similar occurrence would lead to abnormal cleavage, and 

 perhaps development would finally stop. A number of workers have 

 reported abnormal cleavages following irradiation of invertebrate eggs 

 (see Giese, 1949, for review). 



Mention may be made here of the investigations of Robertson (1935a, b) 

 with the flagellate Bodo caudatus. She found that continuous exposure to 

 7 rays from radium led, at first, to a decrease in rate of cell division, but 

 later a partial recovery toward the normal rate occurred even though the 

 irradiation continued. Meanwhile, the flagellates became larger than 

 normal in size. Following cessation of irradiation, they multiplied more 

 rapidly for a time before the normal rate was restored. The partial 

 resistance to radiation did not persist. 



Sensitivity. There has been a rather miscellaneous group of investiga- 

 tions on the effects of various factors on retardation of cell division by 



