ENVIRONMENT AND REGENERATION 499 



Of these nos. 1 and 3 remained at the top and underwent regener- 

 ation. No. 2, which did not show this response, died without 

 regeneration. Nos. 4, 6, and 7, which remained at the bottom, 

 underwent progressively better regeneration. The results for 

 the small tadpoles, 34 to 37 mm., are quite typical of what occurred 

 each time an attempt was made to use small tadpoles in oxygen 

 experiments. Nos. 1, 2, and 3 remained at the top of the water. 

 Nos. 2 and 3 lived twelve days and underwent slight regeneration. 

 Nos. 4, 5, 6, and 7 stayed at the bottom, and of these only no. 7 

 survived to undergo any regeneration, but its regeneration was 

 apparently normal. 



Here again, as was shown for the acids and bases, we find 

 the susceptibility of the tadpole as a whole as compared to the 

 susceptibility of the regenerating part higher in smaller than in 

 larger tadpoles. 



All subsequent experiments were carried on in water kept 

 basic to rosalic acid, and from acid to sUghtly basic to phenol- 

 phthalein. The temperature was also corrected by the introduc- 

 tion of the air coil which warmed the water to room temperature 

 before it entered the experimental jars. All of the tadpoles 

 chosen were large and showed rudimentary hind legs, as it was 

 found that tadpoles at this stage remain at the bottom of the 

 jars better than those in earlier stages. The oldest tadpoles 

 used had a total length of 75 to 85 mm., and the hind legs were 

 two-jointed and digited, and stood out from the body so as to 

 be visible from the dorsal aspect. The results of this series 

 (table 7, fig. 21) again show a close correspondence between the 

 amount of oxygen and the rate of regeneration. 



Figure 23 is a curve drawn from these same data representing 

 the length of time necessary to regenerate 20 per cent of the 

 amount removed in increasing concentrations of oxygen. A 

 comparison of this curve with the curves of Krogh ('14 a, b) for 

 increase in the rate of development at increased temperatures 

 and the toxicity curves of Powers ('18 b) for the decrease in 

 length of life with increase in concentration of toxic substances 

 shows that they are only in part similar. If the author's curves 

 for retardation of regeneration in bases and acids and for increased 



