congdon: effects of radium on living substance. — I. 355 



of regeneration accomplished at any particular time. Thirty to 

 fifty pieces, for the control as well as for the exposed set, were used 

 in each experiment. At various times during the three or four days 

 occupied in regeneration, the stage of regeneration of each piece 

 was noted and the averages calculated for the exposed and for the 

 control set. 



The most satisfactory method of comparing the regeneration of the 

 normal and the exposed sets was to plot the average development as a 

 curve, using as ordinate degree of development, and. as abscissa the 

 number of hours elapsing between cutting the pieces and making the 

 observation. When one curve was superposed on the other, the area 



3G 



48 



60 



Fig. 6. — Curves showing retardation of hydranth regeneration from a 27-hour 

 exposure to the beta rays of radium. 



Tlie imits of the ordinate indicate the per cent of development. 



Tlie imits of the abscissa indicate hours from time of cutting. 



The curve with broken lines is based on control pieces. 



The curve with the unbroken Une is based on pieces exposed to radium. 



The bracket indicates the period of exposure. 



between the curves showed the product of the degree of difference 

 between the development of the two sets into the time during which 

 the difference occurred (see Figs. 5 and 6). 



The area lying between the axis of abscissas and the part of the 

 control curve extending up to the time of maximum regeneration 



