congdon: effects of radium on living substance. 



357 



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 the}' 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, 

 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-se\en 

 hour exposures. It would also give 

 significance to the fact that the ^ve . 



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- 



C 12 18 2i 30 • 



Fig. S. — Correlation table showing 

 the relation between length of exposure 

 and per cent of acceleration. 



The imits of the ordinate indicate ilie 

 hours of exposure. 



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 

 to the dot by the broken line. 



