810 RADIATION BIOLOGY 



can probably kill cells in interphase as well as early prophase, while 7 rays 

 have their main lethal effects on the latter alone. 



Tansley et al. (1948) compared the cell degeneration induced in the 

 germinative zones of the eye and brain of the frog tadpole by a rays with 

 that induced by 7 rays. Small doses of each were found to produce 

 comparable amounts of degeneration, but larger doses of a rays produced 

 a degree of cell degeneration out of all proportion to that following similar 

 doses of 7 rays. They suggest that the greater effectiveness of a rays 

 may be caused either by an effect on cells farther removed from mitosis 

 than those killed by 7 rays or by an abnormally great accumulation of 

 injured cells that break up on attempting mitosis. 



Dosage Rate. In order to test the efficiencies of different dosage rates 

 in causing cell degeneration in the brain and eye of the tadpole, Glucks- 

 mann, Tansley, and Wilson (1945) administered a dose of 336 r of 7 rays 

 at rates of 5.05, 8.37, 12.6, 15.0, and 20.1 r/minute. They found that the 

 dosage rate is positively correlated with the maximal degeneration count 

 and with the length of the interval of time between irradiation and the 

 maximal count, but that the total amount of degeneration is greatest at 

 15.0 r/minute. Lasnitzki (1946) determined the relative efficiencies of 

 dosage rates of 9.7, 29, and 101 r/minute in producing cell degeneration 

 in chick fibroblasts in vitro. After a dose of 100 r the number of degenera- 

 tions expressed as percentage of resting cells was negatively correlated 

 with the dosage rate, but the differences were very slight. After 2500 r, 

 however, there was a positive correlation of dosage rate with degeneration. 

 At the lower dose the degenerations were mainly mitotic ones, but at the 

 higher dose they were mainly resting cell degenerations. Her interpreta- 

 tion of these apparently contradictory dosage-rate efficiencies at small 

 and large doses is based on the relation of degeneration to the lowering of 

 mitotic activity after the different dosage rates. 



Temperature. The experiments of Strangeways and Fell (1927) indi- 

 cate that the amount of cell degeneration that would normally result after 

 X raying, if chick embryos are incubated at 38°C, is greatly reduced if 

 they are kept at or 5°C for 5 to 24 hours after irradiation. This is indi- 

 cated not only by the lowered frequency of degenerating cells seen in 

 fixed and stained preparations after cold treatment, but also in the 

 greater proportion of successful in vitro cultivations obtained from cells 

 that had been cold treated as contrasted with those incubated only at 

 38° C after treatment. 



ULTRAVIOLET RADIATIONS 



Unlike X-ray studies, ultraviolet radiation investigations have of 

 necessity been limited to single, isolated cell types or to cells that are 

 situated or can be grown in cultures on the surface of tissue masses 



