SUSCEPTIBILITY OF CELLS TO RADIUM RADIATION-. 169 



the experiment- were in progress the room temperature varied 

 from 30 to 36 C. and the cultures flourished. Single lines of cells 

 showed a steady divi>ion rate of about 2^ divisions per day. In 

 the coolest temperatures the cells remained normal, and regained 

 the usual division rate on being brought back into a warmer place. 

 This increase in -u-ceptibility at high temperatures is not due to 

 any in< Tea-ed a< ti\ itv of the radiations, nor to any change in the 

 power of pron.pl.i-m to absorb the rays. The amount of radiation 

 .il.-orl/ed i- <li t. riiiiiu <! by the atomic constitution of protopla-m 

 and thi- dot-- not vary materially during changes in temperature. 

 < >\\c of the condition- which varies with the temperature i- the 

 permeability of the cell membrane. Hober (10) stair- that the per- 

 meability of plant ceil- is doubled with each increa-e \ io ( "., the 

 (!!!- I" Jit time- as permeable at 30 as they a IT at O C. 



That r<irnnnri i<i are more permeable in warm than in cold solu- 

 tions can be demonstrated by staining them at 30 and 14 C. 

 in m-utral red. In the higher temperature they stain deeply: 

 .it tin- lower, they do not stain at all. Whether a change in per- 

 meability is the only cause of increased sensiti\ene to radia- 

 tions remains to be demonstrated. That it is an important 

 factor i- -hown in the next section. 



I in Ki I.ATIOX OF PERMEABILITY TO Sust i i-i 11:11.11 v. 



Tin- fart that cells are most sensitive to radiation- when their 

 permeability is increased by heat suggests that the two phenom- 

 ena are rcLted. I lere again the Protozoa are admirably adapted 

 te-tiiii; thi- point, for the permeability of the cell membrane 

 can l>e nu-a-ured in the living condition. The method employed 

 in these experiments is that followed by Harvey ( > . Anumberof 

 I\ininiti-< i>! from a pure culture are drawn up into a capillary 

 (lipette which is calibrated so that exactly the same amount of 

 liquid i- taken in each experiment. The cells are stained for ten 

 minute- in a solution of 0.02 per cent, neutral red mixed with 

 [O CC. "f tap water. At the end of this time the vacuoU> at the 

 P->-terior end of the cells are a bright pink. The -urrounding 

 l>rotopla-m is also colored. The neutral red in this dilution is 

 not toxic, although in more concentrated solutions it produces 

 cytolysis. 



The cells are now drawn up in a calibrated pipette and added 



