I~4 CHARLES PACKARD. 



Paramceciiim increases progressively as the exposure is more and 

 more prolonged. 



Some of the radiated cells were removed to separate drops of 

 culture medium before treatment with the stain, and their 

 division rate observed. Those which were exposed for one hour 

 or a little less frequently showed a higher rate than the controls. 

 This result has been described by Markowits (i i) who studied the 

 effect of mesothorium rays on Paramceciiim. The acceleration 

 can be observed for five or six generations, after which the cells 

 return to normal and show no evidences of injury. The same 

 phenomenon I have observed in sea urchin eggs when lightly 

 radiated (12). Indeed, a stimulation in the rate of growth is of 

 general occurrence, having been noticed in the case of growing 

 plants, embryos, tissue cultures, and abnormal tissue growths. 



From these facts we may conclude that the slowest beta rays 

 increase the permeability of the surface layer of Paramcecium 

 cells. If the exposure is brief, this change in permeability is 

 accompanied by an acceleration in the rate of cell division. If 

 it is more prolonged, a destructive cytolysis ensues, and the cells 



die. 



DISCUSSION. 



Radiations which are absorbed at the surface of the cell pro- 

 duce definite changes which lead to an increase in permeability, 

 and, if the exposure is sufficiently prolonged, to complete cytol- 

 ysis. For a definite dose, such changes must be the same in 

 extent, regardless of the physiological condition of the cell, for 

 the absorptive power of protoplasm remains constant. Yet it is 

 quite apparent that a given dose of radiation may result in no 

 appreciable injury in certain instances, while in others it is 

 followed by the death of the cells. This is clearly shown in the 

 text-figures. For example, at 22 C. radiated Pammcecia under- 

 go cytolysis, under the conditions described , in five hours. From 

 the beginning of the exposure to the death of the cell there is a 

 steady increase in permeability. An exposure of half this dura- 

 tion is followed by no permanent injury. But when evils are 

 radiated at 30 C. they are cytolyzed after only two and oin- half 

 hours. Cytolysis occurs when the permeability of the cell has 

 been raised above a definite limit. It it is already high, due to 

 high temperature- or to other conditions, the cyt]yiir action of 



