PROTOZOA AND INVERTEBRATE EGGS 297 



RETARDATION OF CELL DIVISION 



Giese (1947a) has reviewed in detail much of the work on the effects of 

 radiation on cell division, and Hevesy (1945) has presented a review from 

 a rather different point of view. Nevertheless, it seems desirable to sum- 

 marize the major work on fission delay in the protozoa and cleavage delay 

 in invertebrate eggs and to expand, somewhat, particular topics upon 

 which the reviewer wishes to express opinions. It has been fully estab- 

 lished by many investigators that ultraviolet and ionizing radiations, in 

 sufficient dosage, retard cell division. In some cases at least, this retarda- 

 tion may last for several divisions; but, unless death intervenes, recovery 

 of the normal rate occurs sooner or later. Perhaps this recovery is one 

 of the most interesting features of the effect. 



There are a few cases in which visible light has been reported to retard 

 division. Most of these are the result of photodynamic action (Blum, 

 1941, may be consulted for a review of this phenomenon). Tennent 

 (1942) mentions delay in cleavage in the sea urchin by visible light in the 

 presence of several photodynamic dyes. Giese (1946a) reports delay in 

 cell division in Paramecium caudatum in the presence of eosin and in the 

 ciliate Blepharisma, w^hich contains a naturally occurring photodynamic 

 pigment. However, Zhalkovsky (1938) claims a reduction of cell division 

 in Paramecium caudatum by visible light in the absence of a photodynamic 

 dye. The delay was said to be more marked in direct than in reflected 

 light. Phelps (1946) reports that the division rate of cultures of the 

 colorless Tetrahymena geleii was lowered by exposure to sunlight. This 

 has since been shown to result from destruction of necessary substances in 

 the medium (Phelps, 1949). Perhaps a somewhat similar interpretation 

 would be possible for Zhalkovsky's results. 



There have been a number of purported cases of acceleration of division 

 by small doses of radiation. Giese (1947a) reviews these cases and comes 

 to the conclusion that most of the evidence is of questionable significance. 

 However, he apparently accepts several reports, mainly from the older 

 literature, of acceleration by ionizing radiation. In all cases, the effects 

 are small, and careful statistical analysis has not been made. Moreover, 

 there would seem to be considerable inherent difficulty in being sure that 

 there are no systematic differences between the controls and the experi- 

 mentals other than in the exposure to radiation. Further investigation 

 seems necessary before accepting stimulation of division by low doses of 

 radiation as a real phenomenon. 



Although division delay is an extremely common result of irradiation, 

 it is not universal. Halberstaedter and Luntz (1929) and Halberstaedter 

 and Back (1942) were unable to find division delay in Eudorina or 

 Pandorina at any sublethal dose of radium rays or X rays. 



Recovery. As far as the reviewer is aware, there is no adequate evidence 



