132 Applied Biophysics 



diminished by prolonging the exposure time even up to 24 hours, 

 while with X-rays the proportion increases more rapidly than 

 the first power of the dose, and in the case of the larger doses 

 falls markedly as the exposure time is increased from a few 

 minutes to 4 hours. This interrelation between the influence of 

 ion density and duration of exposure is likely to be found also 

 when the effects of neutrons and X-rays are compared. It is 

 interesting to note that the curve for the temporary inhibition 

 of mitosis in Vicia faba follows an entirely different course, 

 showing that in this material, certain disturbances in the mitotic 

 function must be traced to a different primary injury from that 

 which leads ultimately to the death of the root. 



Animal Embryonic Tissue and Tumor Tissue 



The immediate effects of a variety of radiations, from heavily 

 filtered gamma rays to neutrons, on the mitotic activity of chick- 

 embryo fibroblasts cultured i^t vitro have been the subject of 

 many investigations, starting with those of Strangeways, and 

 continued mainly by Spear and his collaborators.^' ^' ^® Spear 

 and Grimmett ^^ found a marked influence of the hardness of the 

 gamma rays employed which, if real, would indicate an unusually 

 rapid increase of efficiency with ion density in the region of 10 

 ions per micron, since the extreme variation of ion density in 

 their experiments could only have been about 30%. The effi- 

 ciency continues to increase with ion density, but more slowly 

 until the X-ray region is reached (A = 80 ions per micron), 

 after which there is little if any further increase up to 1,000 

 ions per micron. 



In its general features, the course of the curve, therefore, 

 closely resembles that for the inhibition of mitosis in root tips, 

 but no data are available to show whether the curve falls at ion 

 densities above 1,000 ions per u, as is the case with Vicia faba. 



Many experiments by the Strangeways Laboratory team have 

 shown that the effect of radiation on mitosis is essentially the 

 same under m vivo as under in vitro conditions. In particular, 

 Spear and Tansley ^* found that, as in the tissue-culture experi- 



