307 
CONGDON: EFFECTS OF RADIUM ON LIVING SUBSTANCE.— I. 
duced the retardations shown in the preceding table. Upon the whole 
the exposures producing acceleration were the shorter. This is an 
illustration of the well-known condition, shown also for growth of 
Drosophila eggs in the first part of this paper, that many stimuli which 
retard or stop growth if of high 
intensity will accelerate if they be 
weak enough. The maximum ac- 
celeration is not as great as the 
maximum retardation because, in 
the nature of things, it must be 
more limited. 
The table of retardations (Fig. 7) 
shows that on the whole the amount 
of retardation varies directly with 
the length of exposure; but in the 
acceleration table the position of 
the circles is too irregular to prove 
a correlation between these two 
factors. 
There is much to indicate that, 
6 12 18 24 30 
Fig. 8.— Correlation table showing 
the relation between length of exposure 
and per cent of acceleration. 
The units of the ordinate indicate the 
hours of exposure. 
if time be given for the cut pieces 
to partially regenerate before ex- 
posure, the effect of the beta radia- 
tions is decreased. This would 
explain the aberrancy of the two 
fourteen per cent retardations from 
the twenty-four and twenty-seven 
The units of the abscissa indicate the 
per cent of development. 
The numbers in parentheses indicate 
relative development before beginning 
of exposure. 
Dots stand for retardation found in 
the course of an experiment which also 
showed, at a different time, the accelera- 
tion indicated by the circle connected 
: the dot by the broken line. 
hour exposures. It wouldalso give ‘ *® C08 ¥ * 
significance to the fact that the five 
greatest retardations resulted from exposures all but one of which 
began at once after cutting, while for the accelerations there was an 
average development of twenty-nine per cent before the exposures 
were begun. A careful examination of the two tables makes it clear, 
that differing amounts of regeneration before exposure has played only 
a minor part in the various retardations and accelerations, yet it is 
true that the slope of the line of correlation in the retardation table 
would be less steep were a correction made for this factor. The 
moderate slope of the curve shows that the retarding effect does not 
increase so rapidly as the length of exposure, though within the limits 
of these exposures, it does continue to increase. In other words the 
sensitiveness of the hydroid decreases as the length of exposure in- 
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