SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-BAYS 489 



For the percentage of the various defects at each interval con- 

 sult tables 3, 4, 5 and 6. The vertical columns indicate intervals 

 of one hour after the beginning of fertilization. The base line 

 indicates normal development and the height above the base line 

 the relative extent of deviation from the normal. Each period 

 of exposure is marked by a heavy line. The successive periods 

 of exposure of each batch of eggs are connected with one another 

 by dotted lines. The relative effects at the end of any given per- 

 iod of exposure are indicated by the height of the right hand end 

 of the heavy line representing that period. 



Thus, in Lot A, the effects at the end of the first forty-five 

 minutes were distinctly more marked than at the end of the second 

 forty-five minutes, while at the end of the third forty-five minutes 

 they were again greatly increased. About this time the first 

 cleavage plane became distinct and in the succeeding period the 

 effects of exposure slightly decreased only to rise again as the 

 second cleavage approached. Then there was once more a slight 

 decline in the severity of effects followed by a rise to a maximum 

 in the seventh hour (sixteen cell stage for most eggs) and then a 

 somewhat rapid decline to the sixteenth hour and a more gradual 

 decline to the twenty-fifth hour (period of gastrulation for most 

 eggs) followed by apparent insusceptibility to an exposure of 

 forty-five minutes. 



In Lot C, with thirty minutes exposures, the effects were 

 greater in the second half hour than in the first, rapidly declined 

 in the third and still more rapidly rose in the fourth half hour. 

 A half-hour exposure in the sixth hour led to effects nearly as 

 severe as the forty-five minute exposures of Lot A. In the four- 

 teenth hour a half hour exposure gave relatively slight effects, 

 in the eighteenth and twenty-fourth hours, somewhat more 

 marked effects and after this no noticeable effects.^ 



* These results should be compared with those obtained by J. F. McClendon 

 (Arch. f. Zellforschung Bd. v, s. 385-393) in centrifugalizing frogs' eggs during 

 fertilization and cleavage. He finds an increasing susceptibility preceding the 

 appearance of the first cleavage plane, then a decrease, followed by two periods 

 of increased susceptibility preceding the appearance of the second cleavage 

 plane, a decrease during this period and finally an increased susceptibility dur- 

 ing the appearance of the third cleavage plane. 



