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bulletin: museum of comparative zoology. 



hundred milligrams of impure radium one thousandth as strong as the 

 pure bromide was held in a tightly closed cell (Fig. 2) consisting of a 

 glass floor 25 mm. square, sides of sealing-wax two millimeters high, 

 and a roof consisting of a very thin mica window. The cell rested 

 in a Petri dish of salt water. Pieces of the stem of the hydroid six 

 tenths of a millimeter in diameter were laid upon the window. The 

 quality of the radiations was constant for all the experiments, since 

 this method of exposure was always employed. The alpha rays were 

 cut off from the hydroid stems by the mica. The gamma radiations, 

 were so weak as to be negligible. 



The pieces of stem were sixteen millimeters in length, of nearly 

 equal diameter and from like portions of thrifty colonies. With 

 hardly an exception they regenerated one hydranth. The pieces 

 used as control received treatment like the others, except that the cell 

 on which they were placed contained a non-radioactive yellow powder. 



The whole process of regenerating a hydranth was arbitrarily divided 

 into eight stages for the purpose of numerically expressing the amount 



Fig. 5. — Curves showing acceleration of hydranth regeneration due to 7 liours' 

 exposure to the beta rays of radium. 



The units of the ordinate indicate per cent of development. 



The units of the abscissa indicate hours from time of cutting. 



The curve witli the broken lines is based on control pieces. 



The curve with the continuous line is based on pieces exposed to radium. 



The bracket indicates the period of exposiu-e. 



