202 CHARLES PACKARD 



OBSERVATIONS 



The experiments related below are typical of a large number, 

 and only those are recorded which have been verified by re- 

 peated trials. 



1. Exposure during the period in which the germ nuclei are 

 approaching and fusing with each other. Five to twenty-five 

 minutes after insemination. 



Radium placed f inch above eggs 



Length of exposure Result 



5 minutes Slight acceleration 



10 minutes 5 per cent acceleration 



15 minutes Slight acceleration 



20 minutes No acceleration 



Radium placed f inch above eggs 



10 minutes Slight acceleration 



15 minutes Slight acceleration 



20 minutes No acceleration 



If the radium is placed 1^ or 2| inches above the eggs there is 

 little evidence of any change. These results were constant and 

 clear cut, even when the amount of change in division rate was 

 small. 



Under these conditions it is clear that unless the exposure 

 is intense there is Uttle or no response. These results were an- 

 ticipated for I have found in previous experiments that freshly 

 fertihzed or unfertilized eggs are not very susceptible even to 

 intense exposures. However, if the radiation is prolonged the 

 whole organization of the egg undergoes a profound change, 

 and subsequent development is very abnormal. Inasmuch as 

 I was not trying to produce abnormalities, I did not use very 

 intense radiations. 



2. Exposure made during the prophases of mitosis. Twenty 

 to thirty-five minutes after insemination. 



Radium placed f inch above eggs 



Length of exposure Result 



5 minutes Slight acceleration 



10 minutes 10 per cent retard 



15 minutes 15 per cent retard 



