SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 461 



prove incapable of growth and further differentiation and die off. 

 Experiment CO and C-§ were not carried far enough to determine 

 whether or not any of the eggs exposed for half an hour wdthin 

 the first hour after fertilization were capable of metamorphosis. 

 In Experiment D-j one specimen was followed through meta- 

 morphosis. 



In both Experiments CO and C-^ a considerable number of 

 eggs failed to undergo cleavage but since in the control for these 

 experiments 6 . 9 per cent of the eggs were not fertilized it is prob- 

 able that the ova in the experiments which did not undergo 

 cleavage failed to develop not because of the exposure to the rays 

 but because of lack of fertilization. In Experiments D-j and 

 D-f a considerable number of the eggs became mouldy and 

 decaA'ed before it was determined whether or not they had been 

 fertilized. The percentage of unfertilized eggs given is, therefore, 

 not exact, since under the eggs so classed there may be some which 

 stopped development during cleavage. 



When the exposure is more prolonged and severe the results, 

 as we have seen in Experiment A-0, are manifest just before or 

 early in gastrulation. Most eggs stopped developing at this 

 period but a few went on to form abnormal larvae. In none of 

 the eggs in which cleavage began did development stop until the 

 approach of the gastrulation period. If the cleavage once com- 

 mences at all it is evidently capable of continuing until the crit- 

 ical period of gastrulation is approached. It is, of course, by no 

 means certain, how^ever, that cleavage is perfectly normal during 

 this time. I have, however, been able to detect in sections no 

 very obvious abnormalities, except a tendency for the vegetative 

 pole to divide much more slowly than normal so that the yolk 

 may be but slightly divided into cells while the cells of the animal 

 pole are approximately of normal size. The abnormalities which 

 appear during gastrulation and larval differentiation are essen- 

 tially similar to those previously described as characterizing spec- 

 imens of which one of the parent sex cells had been irradiated 

 previous to fertilization. 



