796 RADIATION BIOLOGY 



when the metaphase count reached zero, i.e., at 66 minutes after X 

 irradiation, and returned to 38°C after 66 minutes, the reappearance of 

 metaphases was delayed by almost the time of the cold treatment, as 

 compared with cells kept at 38°C throughout. Exposure to 3°C, there- 

 fore, brings almost to a standstill both mitotic progress and recovery. 

 Henshaw (1940c) found that Arbacia eggs, when kept at 0°C from treat- 

 ment to insemination, showed less recovery and, consequently, greater 

 cleavage delay than those kept at 24°C during this period. 



Chemical Agents. Immersion of unfertilized Arbacia eggs in 0.35 M 

 potassium citrate to remove most of the calcium from the cell during 

 X-raying was found by Wilbur and Recknagel (1943) to decrease the 

 cleavage delay normally caused by irradiation. The effect, however, was 

 slight and occurred mainly after large doses (30,400 and 53,200 r). 

 Increased calcium and magnesium content of the sea water during X 

 irradiation had no effect on cleavage delay. 



Using chick fibroblast cultures, Paterson and Thompson (1949) found 

 that urethane, which in a concentration of 0.2 per cent inhibits mitosis, 

 when added to the culture medium in a concentration of 0.1 per cent just 

 before or up to 40 minutes after X-raying, reduced the mitosis-inhibiting 

 effect of the X rays. They draw the conclusion that the capacity of 

 urethane to reduce the X-ray effect when added after irradiation indicates 

 that X-ray action is incomplete at the end of irradiation. The method 

 used in determining the degree of metotic inhibition induced and the 

 data on which the conclusion is based are omitted from this preliminary 

 report, so it is not possible to evaluate their interpretation. 



pH. Zirkle (1936) found that the degree of acidity or alkalinity of 

 the medium in which fern spores were X-rayed affected their sensitivity, 

 as determined by the proportion in which the first cell division had 

 occurred by the tenth day after treatment — it normally occurs on the 

 sixth day. In 0.006 M carbon dioxide in culture medium the radio- 

 sensitivity was greatest, being less in stronger or weaker concentrations of 

 carbon dioxide. Minimum radiosensitivity was exhibited in 0.003 M 

 ammonia; it was greater in either stronger or weaker concentrations of 

 ammonia. 



Oxygen Concentration. A series of studies has recently been made by 

 Gaulden, Nix, and Moshman (1953) on the mitotic effects of exposing the 

 grasshopper egg during X irradiation to different concentrations of 

 oxygen. In concentrations of 0, 2, 5, 10, 21, and 100 per cent oxygen, 

 64 r of X rays reduced the number of prometaphase, metaphase, and 

 anaphase cells to zero about 1 hour after treatment. The greater the 

 per cent of oxygen, however, the longer these stages remained at zero and 

 the longer the time interval between irradiation and the peak of mitotic 

 activity following recovery. Treatment with 8 r of X rays, however, 

 which does not reduce the mitotic activity to zero, resulted in no sig- 



