EFFECT OF X-RAYS ON RATE OF CELL DIVISION. 93 



snail Philine after exposure to radium. He states that in eggs 

 exposed before the first division the segmentation was in no wise 

 delayed by the action of the radium, but that the cleavage as 

 well as gastrula formation was normal. Only later did the effects 

 of radium show themselves. My results do not confirm these 

 observations. 



Congdon studied the effect of the beta rays upon several forms. 

 He found that an exposure of twenty-four hours caused a re- 

 tardation of 31.2 per cent, in the development of the eggs of 

 Drosopliila. The more intense the radiation the greater the 

 retardation. In these experiments the intensity was measured 

 by the distance of the object from the radium. "Secondary beta 

 radiations (slow electrons) produce a much stronger effect than 

 primary radiations (rapid electrons) of like intensity." He 

 experimented upon Tubularia varying the length of the exposures. 

 Both in Drosopliila and in the hydranths, he states, "many 

 stimuli which retard or stop growth if of high intensity will 

 accelerate it if they are weak enough." The retardation varies 

 directly as the length of the exposure. " When the fundaments 

 of regenerating Tubularia hydranths were exposed to beta radi- 

 ations from three hundred milligrams of impure radium one 

 thousandth as strong as the pure bromide for periods up to 

 three days in length, the shorter exposures were found to ac- 

 celerate regeneration and the longer to retard. The degree of 

 retardation increases slowly with lengthening exposure; but the 

 degree of retardation relative to the length of exposure decreased 

 with lengthening exposure." 



Again he found that seeds were most sensitive to radiation 

 when the embryos were turned toward the radium. Here also 

 the slower electrons of the beta radiations were more effective 

 relatively than the more rapid. 



Zuelzer also reports that insects are affected by exposure to 

 radium. 



The vertebrates have served as objects for a large part of the 

 experiments with radium and X-rays. Oilman and Baetjer 

 exposed hen's eggs for ten minutes daily to X-rays. During the 

 first thirty-six hours the development was accelerated. Then 

 there followed a retardation during which the development was 

 greatly altered as well as checked. 



