460 CHARLES RUSSELL BARDEEN 



to, although perhaps less marked than the exposure of the sperm 

 for from one-quarter to one-half hour before fertilization. Ex- 

 periments 2,-3, table 2, likewise indicate that exposure of the 

 ovum within the female for thirty to forty-five minutes gives rise 

 to less severe effects than exposure of ova for half an hour dur- 

 ing fertilization. 



Experiment C^ indicates that during the second half-hour 

 after the beginning of fertilization the fertilized ova are somewhat 

 more susceptible than during the first half hour. Compare 

 Experiments CO and C^, table 3; in the former experiment 20 

 per cent, in the latter but 8 per cent of the specimens appeared 

 normal at the end of two weeks. Experiment Dj indicates a 

 possible greater susceptibility of the eggs during the middle two- 

 fourths than during the first or last half of the first hour after 

 the beginning of fertilization, but the differences may be due to 

 individual peculiarities in the different batches of eggs used. In 

 Experiment D| but 1 . 9 per cent of the eggs were normal two 

 weeks after fertilization. 



Summary. From the above experiments we conclude that the 

 ova immediately after fertilization are somewhat less susceptible 

 than the spermatozoa or unfertilized mature ova to short or weak 

 exposures but are more susceptible to long exposures. During 

 the conjugation of the pronuclei the susceptibility is greater than 

 during the preceding period. There probably exist in the proto- 

 plasm of the egg substances capable of protecting the pronuclei 

 against moderate injury or of restoring them after such injury, but 

 these substances, if such exist, cannot overcome the effects of 

 exposures to powerful X rays for three-quarters of an hour and 

 they become less potent during the fusion of the pronuclei than 

 in the period preceding. The fusion of the pronuclei seems to 

 start a protoplasmic reorganization during w^hich the suscepti- 

 bility of the cell is increased. 



The effects of the X rays during fertilization become manifest 

 after half-hour exposures, chieflj^ in the later larval periods, after 

 the rudiments of the head and tail appear and while the larva is 

 becoming transformed into a free swimming tadpole. During 

 the tadpole stage many specimens which at first appear normal 



