EFFECT OF X-RADIATIOX OX CUMINGIA EGGS. 213 



appear that greater exposure caused greater stimulation at first 

 than the shorter, agreeing in this particular with the marine 

 eggs which Packard ('16) studied, rather than with the fresh- 

 water Planorbis and Physa. 



From the 43-minute stage on the radiated eggs become slower 

 in divisions, the percentage of free swimming larvae in A3 being 

 only a little over half the per cent, in the control, 325 minutes 

 after fertilization. Only one or two per cent, of the Ai5 set 

 have reached the free-swimming larval stage. However, from 

 Table I. we see that one hour later or 385 minutes after fertiliza- 

 tion it showed approximately the same percentage as the A3, 

 385 minutes after fertilization. This shows Ai5 at 325 minutes 

 after fertilization is 20 per cent, behind A3 and approximately 

 35 per cent, behind the control in respect to time. 



In figuring percentages of gain or loss during development and 

 cleavage, we, following Packard, divide the difference in time 

 required for the two sets to reach a certain stage by the time 

 'required by the control to reach that stage. Thus in the A 

 series the control passed the 10 per cent, free-swimming larval 

 stage several minutes ahead of A3. Now Ai5 entered that stage 

 just 60 minutes later which is 20 per cent, retardation when com- 

 pared with A3. 



It was deemed inadvisable to carry the experiment farther 

 for in both the control and the radiated lots a marked disintegra- 

 tion set in. It has been shown that disintegration may follow 

 exposure to either radium (Packard) or X-rays (Richards, '15), 

 but our radiation is hardly to be held responsible for the results 

 of this experiment, since both control and experiment show it 

 equally. 



In Table I. reference is made to the percentage of the entire 

 number of eggs which developed in the experiments. At the 

 height of the breeding season a very large percentage of the 

 number of Cumingia eggs fertilized are found to develop, usually 

 over 95 per cent., it is stated. The precaution must be taken 

 in that case to use dilute sperm in order to prevent polyspermy. 

 The eggs used in this experiment, however, were obtained late 

 in the summer and some polyspermy was permitted, for it was 

 found, as will appear later, that the number of sperm entering 



