354 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 
100 
80 
Oo 
40 
20 
pov 12 24 36 48 
Fig. 5.— Curves showing acceleration of hydranth regeneration due to 7 hours’ 
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 with 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 exposure. 
