Biological Effects of Ionising Radiations 133 



ments with chick-embryo fibroblasts, the immediate effect of 

 neutrons on the mitotic activity of the developing rat retina was 

 approximately the same as that of an equal dose of X-radiation. 

 There are certain differences in the subsequent return of mitotic 

 activity, but these may be bound up with the markedly greater 

 efficiency of neutrons in causing cell degeneration. 



Not only was much more cell degeneration produced in the 

 rat retina by neutrons than by an equal dose of gamma radia- 

 tion, but the degenerate cells appeared much earlier. This may 

 indicate that cell degeneration follows a different course accord- 

 ing to the radiation which causes the primary injury. 



The effects of various radiations have been compared in regard 

 to their ability to injure tumor tissue by irradiation in vitro 

 iti such a way that it does not "take" when inoculated into 

 test animals. It appears to be established, particularly by the 

 careful experiments of Sugiura (1939), working with mouse 

 tumors, that X-radiation is about 50% more effective than 

 gamma radiation. The experiments were extended ^ to neutron 

 radiation of ion density about 300 ions per [i. The relative 

 efficiencies of neutrons and X-rays, as tested on a lympho- 

 sarcoma, a lymphoma, and a carcinoma of the mouse, were 3, 

 2.3, and 2.4 respectively. When these data are taken in con- 

 junction with Sugiura's, we find that the ion density curve 

 (figure 3) follows closely the course of the curve for the lethal 

 effect on root tips. Experiments at higher ion density are much 

 needed. 



Gray, Mottram, and Read (unpublished) carried out in vivo 

 irradiations of inoculated mouse tumors, using neutron and 

 gamma radiation. The neutron radiation appeared to be some 

 15 times as efficient as gamma radiation. In comparing this result 

 with the in vitro studies already fnentioned, we have to note first 

 that the neutron ion densities were much higher in the in vivo 

 experiments (A =: 1,100 ions per \i) , and, secondly, that the 

 influence of ion density and duration of exposure may be inter- 

 connected. The gamma ray and neutron exposures were of 

 equal duration (3 hours), but the time may have been such that 

 the effect of the gamma radiation, but not of the neutron radia- 



