308 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
creases. This may be simply an expression of Weber’s Law, that 
beyond a certain maximum of intensity any stimulus has a decreasing ~ 
power of stimulation. Or, it may be that the early stages of regenera- 
tion are more sensitive to exposures for the same reasons that-the 
embryo is more sensitive than the adult. 
The view is commonly met in medical writings, that the action of 
radium is proportional to the product of length of exposure into in- 
tensity of radiation. The Figure 7 correlation table shows clearly 
that, as far as the growth of hydroids is concerned, that view is errone- 
ous. 
SUMMARY. 
When the fundaments of regenerating Tubularia hydranths were 
exposed to beta radiations 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 accelerate 
regeneration and the longer to retard. The degree of retardation 
increased slowly with lengthening exposure; but the degree of retarda- 
tion relative to the length of exposure decreased with lengthening 
exposure. 
