358 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 tal)le 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. 



