462 



CHARLES RUSSELL BARDEEN 



2. Period of relative immunity 



After the fusion of the two pronuclei there appears to be a 

 passive period during which the ovum prepares for the period of 

 great activity which characterizes cleavage. During the passive 

 period the ovum becomes less susceptible to the rays than during 

 the preceding period of fertilization or during the subsequent per- 

 iods of cleavage. This is indicated in the three experiments 

 summarized in the following table. 



TABLE 4 

 Irradiation following fusion of pronuclei 



The unfertilized specimens in lot D were not counted in figuring the percentage 

 of eggs variously affected by the rays. 



In Experiment A-f , a string of eggs was exposed for three 

 quarters of an hour from a period beginning three-quarters of an 

 hour after the mixture of the sperm and eggs. The effects of 

 exposure during this period were less severe than those found in 

 Experiment A-0, Table 3. Two per cent of the eggs did not 

 undergo cleavage but it is uncertain whether or not this failure 

 was due to the exposure. In the control practically all the eggs 

 were fertilized. Of the eggs undergoing cleavage 37 per cent 

 showed irregularities early in gastrulation and ceased develop- 



