SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 457 



3. ACTION OF THE X-RAYS ON FERTILIZED OVA AND ON LARVAE 



Introduction 



My studies of the effects of exposure of developing organisms 

 to the X rays have been most systematically carried out on the 

 ova and larvae of the toad. The results of these experiments 

 have been summarized in the accompanying tables. Numerous, 

 although less systematic, experiments on frogs' eggs indicate 

 a variation in susceptibility in these eggs which corresponds 

 closely with that found in toads' eggs. No attempt has been 

 made to summarize these results in the tables. 



The more important experiments are designated in the accom- 

 panying tables as follows: 



Experiment A. Spring of 1909. Successive groups from a 

 batch of fertilized toad eggs were given forty-five minutes expos- 

 ure at varied intervals from the time of fertilization up to the 

 tadpole stage. The different groups are designated by the num- 

 ber of hours intervening between the beginning of fertilization 

 and the period of exposure, i.e., Experiment A, 14^: batch of eggs 

 exposed from 14^ to 15| hours after the beginning of fertilization. 



Experiment B. Spring of 1909. Successive groups were 

 given forty-five minutes exposure from a period begininng 15f 

 hours after fertilization and extending into the tadpole stages. 

 Group designation as in Experiment A. 



Experiment C. Spring 1908. Successive groups from a batch 

 of toad eggs were given thirty minutes exposures from the time of 

 fertilization up to the larval stage, with an interruption during 

 the later cleavage stages. Group designation as in Experiment A. 



Experiment D. Spring of 1908. Successive groups from a 

 batch of toad eggs were given thirty minutes exposures from the 

 period of fertilization up to the early cleavage stages. Group 

 designation as in Experiment A. 



For comparison some other experiments have been included in 

 the tables but since these were less systematic they need not be 

 described here. 



