202 ALEXANDER HOLLAENDER 



cannot be interrupted. The fields in which we are most interested — 

 cytological and genetic effects — readily lend themselves to quantitative 

 analysis. This type of damage may have very serious long-term effects 

 and is of greatest importance to the sensitivity of man to radiation 

 damage. 



The first problem is the effect of very small quantities of radiation 

 on the rate of mitosis. The immediate effect of less than 30 r is a slowing 

 down of the mitotic rate. I refer to the work of Mary Esther Gaulden of 

 Oak Ridge National Laboratory and J. Gordon Carlson of The Uni- 

 versity of Tennessee. Since cytological changes produced by radiation 

 can now be observed readily in living cells, a method has been developed 

 to follow the rate of mitosis in the grasshopper neuroblast under con- 

 stant, carefully controlled conditions. In this case the rate of mitosis is 

 very reproducible and can very well be used as a criterion of radiation 

 damage. The grasshopper neuroblast is suspended in a nutrient medium 

 in a hanging drop slide and observed under constant temperature con- 

 ditions under the microscope. The rate of damage is determined by 

 observation about every 20 min. For details I refer you to the ajjj^ro- 

 ])riate pubhcations for I will discuss here only the initial effects of 

 radiation. A small amount of radiation — less than 25 r and even an 

 effect of as little as 1 to 3 r can be observed — I'esults in a slowing down 

 of the rate of mitosis. This slow-down is followed by a period of com- 

 pensating speed-up after a period of inhibition. 



However, we have found that we can reverse the entire effect by 

 treatment with hypertonic salt solution. Immediately after irradiation 

 — less than 60 sec later — we treat the cells with a slightly (l-2x 

 the isotonic salt concentration) hypertonic culture mediinn. The hyper- 

 tonic salt solution itself has the effect on normal cells of doubling the 

 rate of mitosis. In cells irradiated with 3 r of X-rays the solution ob- 

 scures entirely the slowing down effect of the radiation. Irradiation 

 only affects mitotic rate of middle and late prophase cells. Inasmuch as 

 Gaulden (1950) has observed that the grasshopper neuroblast synthe- 

 sizes deoxyribonucleic acid (DNA) only during the period from middle 

 telophase to very early prophase, it is obvious that the slow-down 

 effect must lie outside the area of nucleic acid synthesis. It appears 

 that the initial effect of radiation in this case is a purely physical and 

 not a chemical one. It should be pointed out that at these low levels of 

 radiation the slowing of the mitotic rate is not influenced by oxygen. 



The next project for discussion is the induction of chromosomal 

 aberrations in Tradescantio pollen, a synergistic effect between very 

 small amounts of combined X-ray and ultra-violet (vSee Table I). This is 

 the work of J. S. Kirby-Smith, Benedetto Nicoletti, and M. L. Gwyn 



