474 CHARLES RUSSELL BARDEEN 



ture at this time varied from 40 degrees - 57 degrees and averaged 

 below 50 degrees ; this proved to put a severe strain on the eggs 

 kept outside 42 hours although of these about half of those eggs 

 in the sixteenth to thirty-two cell stage ('C') developed further 

 than anj^ of those of the same lot exposed at room temperature, 

 A large part of the eggs kept outside for 18 hours after expo- 

 sure at 50° developed better than any of those exposed at room 

 temperature. 



In Experiment 11 eggs from two batches, one fertilized natur- 

 ally, one artificially, were exposed for 15 and for 30 minutes,, 

 some at 77 degrees, some at 67 degrees, and some at 50 degrees- 

 57 degrees. The eggs exposed at 77 degrees were subsequently 

 kept at that temperature on the top of an incubator. The other 

 lots were kept at room temperature after exposure. A greater 

 per cent of the eggs exposed at 77 degrees than those exposed at 

 66 degrees showed injury early in development but, on the other 

 hand, a greater number recovered. A much greater percentage 

 of the eggs exposed at 50 degrees than those of the two later men- 

 tioned, developed normally showing that a moderately low tem- 

 perature partially protects the eggs against the action of the rays. 



4. Advanced cleavage and gastrulation stages up to the closure 

 of the blastopore {twelve to thirty-six hours after fertilization). 



During this period there is at first a rapid and then a much 

 more gradual decline in susceptibility. 



In table 6 are summarized the results of experiments with five 

 different batches of fertilized eggs. In Experiments A, A-1 and 

 B, forty-five minute exposures were given. In Experiment 

 A-14^, exposure from fourteen and a half to fifteen and a quarter 

 hours after fertilization, the susceptibility was decidedly less 

 than in Experiment A-11, table 5. In a third of the eggs in the 

 latter gastrulation was incomplete or very abnormal and only a 

 fourth of the eggs developed into larvse with distinct heads and 

 tails. None of these larvse developed into tadpoles. In Exper- 

 iment A-14^ the blastopore was closed in all specimens and all 

 reached the larval stage. Over half of the larva? became free 



