SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 437 



believe that this difference must have been due to other influences 

 than those exerted by the X rays. 



The period of exposure of the sperm to the X rays has varied 

 in my experiments from twelve minutes to two hours. The rela- 

 tion between the percentage of ova fertilized and the length of 

 time intervening between the removal of the spermatozoa from the 

 animal and the fertilization of the ova is less obvious than one 

 might expect. Thus in one experiment in which the sperm was 

 exposed an hour and ten minutes apparently every egg out of 

 two hundred and fifty was fertilized while in another experiment 

 after fifteen minutes exposure but 66 per cent of the ova were fer- 

 tilized and in one after forty minutes exposure but 44 per cent were 

 fertilized. Within certain limits power of fertilization decreases 

 with the length of time elapsing between the removal of the sperm- 

 atozoa and the fertilization of the eggs; up to an hour this factor 

 has a less marked influence than the physiological development of 

 the sex cells at the period of fertilization. 



Boveri and others have suggested that the spermatozoon cen- 

 trosome (middle piece) contains those chemical substances which 

 excite cleavage in the ovum. If this be true it is evident that 

 centrosomes are relatively resistant to the X rays. 



2. Period of cleavage 



In all experiments, except in Experiment 2, cleavage in most 

 of the eggs fertilized by exposed sperm seemed to be normal. 

 In several of the experiments it appeared to be slightly more rapid 

 than in the control eggs. In Experiment 2 a considerable number 

 of eggs ceased development in the late cleavage stages, but this 

 was found to occur in but one other experiment, (Expt. 7,) and 

 here in but three out of three hundred and seventy eggs. In 

 Experiment 2 the effects of the exposure of the spermatozoa on 

 gastrulation and the early stages of differentiation of the larva 

 were much greater than in any of the other experiments, even 

 where the spermatozoa were exposed much longer to the X rays 

 (table 1.) It, therefore, seems probable that the ova of this lot 

 must have been slightly abnormal so that although most of them 



