36 EFFECTS OF (8-RAYS OF DIFFERENT VELOCITIES 



the eggs were begun. In making these measurements one egg from 

 each compartment was measured in turn, then a second from each 

 compartment, then a third, and so on until ten or twelve eggs from 

 each compartment had been measured, by which time the eggs had 

 begun to divide and it was impossible to obtain further exact meas- 

 urements. In this way we were assured that the figures for each 

 compartment were made the same average time after fertilization, a 

 matter of much importance. This procedure had the advantage that 

 all the eggs were radiated at one time after they had been freshly 

 removed from the female worm. Its disadvantage lay in the fact 

 that it was impossible to measure very many eggs from each lot 

 since all the lots had to be measured at once. 



The alternative procedure consisted in placing successive lots of 

 eggs in the same position on the glass plate so that each lot was 

 exposed to rays travelling through the same path. In this case R, 

 the radius of curvature of the effective beam, remained constant. 

 Each lot of eggs was exposed for an equal period of time, 1 or 2 hours, 

 but with a different amount of current flowing through the solenoid, 

 so that the strength of the magnetic field, H, varied. At the ter- 

 mination of the period of radiation each lot of eggs was washed into 

 a watch-glass of sea water, fertilized, and after a uniform period the 

 membranes of twenty-five eggs were measured. This procedure pos- 

 sessed the advantage that a larger number of eggs from each lot could 

 be measured, but it was so time-consuming that only a limited number 

 of homogeneous beams could be measured in each experiment. More- 

 over, as the experiment proceeded measurements were necessarily 

 made with eggs which had been removed from the female worm for 

 longer and longer periods. We have some reason to suspect that these 

 eggs become shghtly less sensitive to radiation after they have stood 

 in sea water for a long period. 



The figures obtained from such experiments as these give a measure 

 of the amount of physiological effect from each beam of homogeneous 

 radiation. We have shown, however, that the amount of physio- 

 logical eft'ect, as indicated by the volume of the fertihzation membrane 

 of Nereis eggs, is not a direct measure of the intensity to which they 

 have been exposed. What we desire is a measure of the physio- 

 logically eft'ective radiation in each group of homogeneous rays which 



