SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 481 



come out later when larval formation calls for the differentiation 

 of new types of cells. 



5. Period from the closure of the blastopore to the period 



of hatching 



Exposure to powerful X rays for an hour or less during this 

 period seems to have little effect. In table 7, A, B, D and E 

 each represents a separate batch of eggs. In D the immunity 

 to thirty minute exposures is illustrated ; in B and E the immunity 

 to exposures of from forty-five to sixty minutes. The abnor- 

 malities which a few specimens showed in these experiments are 

 probably to be attributed to other factors than the mere exposure 

 to the rays. 



Exposure for two hours or more during the early part of larval 

 development inhibited normal development in nearly all speci- 

 mens (A-42, 48 and B-64^). In a group from one batch of eggs 

 an exposure of one and one-quarter hours, fifty-two and one- 

 quarter hours after fertilization, at a period when the neural 

 tube was closed, inhibited normal development in 42.9 per cent 

 of the specimens (B-52j.) In a group from another batch, after 

 an exposure of one hour and thirty-five minutes fifty-four hours 

 after fertilization, all but one specimen developed normally (A-54). 



In the latter part of larval differentiation an exposure of two 

 hours had, as a rule, much less marked effect than during the 

 early part (A-65|, B-87^) although in one lot it inhibited growth 

 (B-70|.) An exposure of from two and one-half to three hours 

 sufficed, however, at this period to inhibit normal development 

 (B-64i, A-76i). 



From these experiments we conclude that susceptibility to the 

 X rays is less marked at this period than during the period of the 

 closure of the blastopore and that it grows less as the period of 

 hatching is approached. To prolonged exposures the developing 

 larvae are, however, decidedly susceptible. 



The organs chiefly affected are the central nervous system, ali- 

 mentarj'^ canal and heart. The abnormalities resemble those 

 previously described in connection with the results of exposure 



