468 CHARLES RUSSELL BARDEEN 



The frog's egg during cleavage shows a susceptibiUty at least 

 equal to that of the toad's egg. Frog's eggs exposed for forty 

 minutes immediately preceding the first cleavage division stopped 

 development either just before or at the time of the appearance 

 of the dorsal blastopore lip. Frogs' eggs in the four to eight cell 

 stage exposed for forty minutes all stopped developing before 

 the dorsal blastopore lip became clearly marked. Exposed in 

 the sixteen to sixty-four cell stages they stopped developing as 

 soon as the dorsal blastopore lip became well marked. Exposed 

 in the more advanced cleavage stages the blastopore in many 

 specimens became small but all specimens died before assuming 

 definite larval form. 



Summary and analysis. During the early cleavage period, 

 from the latter part of the second to the twelfth hour after fer- 

 tilization, the ova of the toad are exceedingly susceptible to the 

 X rays. The susceptibility increases from the period of 'rest' 

 following the fusion of the pronuclei, up to the first cleavage 

 division, there is then a slight decrease in susceptibility followed 

 by a second increase up to the second cleavage. After the form- 

 ation of four blastomeres there is again a slight decrease in sus- 

 ceptibility followed by a third increase which lasts during the eight 

 and sixteen cell stages and then steadily declines during the sub- 

 sequent cleavage stages. 



It is thus apparent that preparation for cleavage in the ovum 

 brings about some alteration in the organism which renders it 

 especially susceptible to the rays and that corresponding altera- 

 tions are brought about in the first eight blastomeres. The 

 greater irregularity in cell division which takes place after the 

 first eight to sixteen blastomeres are formed may account for 

 the decrease in susceptibility after this period. During the half 

 or three quarters of an hour during which the organisms are 

 exposed cells about to divide will be in a stage of heightened sus- 

 ceptibility, cells in the resting stage will be in a state of relatively 

 reduced sensibility. As the latter increase in relative number we 

 should expect a decrease in susceptibility in the organism as a 

 whole. It is also not improbable that when relatively few cells 



