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 
‘100 
80 
40 
Fig. 6.— Curves showing retardation of hydranth regeneration from a 27-hour 
exposure to the beta rays of radium. 
The units of the ordinate indicate the per cent of development. 
The units of the abscissa indicate hours from time of cutting. 
The curve with broken lines is based on control pieces. 
The curve with the unbroken line 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 
