446 CHARLES RUSSELL BARDEEN 



forty minutes and 4.9 per cent in an experiment in which the 

 exposure was twenty minutes. In four experiments in which 

 the exposures were respectively thirty minutes, thirty-seven 

 minutes, one hour and ten minutes, and two hours, no normal 

 tadpoles were developed. In the other experiments the number 

 of tadpoles finally undergoing normal development was not defin- 

 itely determined. In those experiments in which some eggs 

 developed into apparently normal tadpoles, a few of which meta- 

 morphosed, it is by no means certain that the resulting toads or 

 frogs would have been perfectly normal individuals. In one 

 instance a metamorphosed toad lacked a hind leg. It is possible, 

 if facilities had existed for carrying a large number of tadpoles 

 through metamorphosis and up to the adult stage, many more 

 defects might have been discovered as attributable to the expos- 

 ure of the sperm to. the X rays. 



In the tadpoles which failed to develop normally, abnormalities 

 were especially frequently found in the central nervous system. 

 The tissue of the neural tube seemed to lack capacity for higher 

 differentiation and degenerate cells and protoplasmic masses 

 were discharged into the ventricles of the brain and into the cen- 

 tral canal of the spinal cord. Abnormal dilatation of the body 

 cavity and of the pronephric and Wolffian tubules are other 

 phenomena frequent in these specimens. Occasionally the gut 

 exhibits degeneration or marked abnormality of form. Some- 

 times the nasal fossae and the eyes are defective. The optic 

 stalk may be dilated. Occasionally the defects are unilateral. 



Conclusions 



The results of fertilizing amphibian ova with sperm exposed to 

 the X rays indicate that the alterations produced by the exposure 

 in paternal determinants do not as a rule make themselves mani- 

 fest until larval differentiation or later in development except 

 in those instances in which there is reason to believe that the 

 ova were somewhat defective (over-ripe, etc.,) at the time of fer- 

 tilization. The paternal determinants become relatively more 

 and more important as development proceeds. They may be 



