504 MINNA E. JEWELL 



It was further noticed throughout these experiments that the 

 appearance of pigment in the regenerated part, which may be 

 supposed to show a certain stage of differentiation, occurred as 

 early or earUer in the tadpoles in high concentrations of acids 

 or bases as in the controls; at about the same time in the low 

 oxygen water, and somewhat later in the low temperatures. 

 From this it would seem that growth and differentiation (if ap- 

 pearance of pigment be accepted as a criterion of differentiation) 

 in the regenerating parts are differently affected by the environ- 

 mental agents studied, so that a hydrogen ion concentration, an 

 oxygen content of the water, or a temperature of the surrounding 

 medium which retards one process may have little or no effect 

 upon the other. If, then, as is frequently supposed, the presence 

 of differentiated tissue be the inhibitory factor which causes the 

 cessation of growth of the new part, this difference in effect upon 

 growth and differentiation by the environmental factors con- 

 sidered would account for the differences in the ultimate amount 

 regenerated in the various concentrations of hydrogen ions and 

 oxygen or at the various temperatures. 



SUMMARY 



A study has been made of the effects of various conditions of 

 hydrogen ion concentration, deficiency of oxygen and low tem- 

 perature upon regeneration and oxygen metabolism in tadpoles 

 of Rana clamata. 



1, These studies show that the optimum hydrogen ion con- 

 centration for regeneration is neutrality, or near neutrality, 

 probably between pH 6.7 and pH 7.6. As the hydrogen ion 

 concentration varies from the optimum in the direction of either 

 acidity or basicity, both the rate of regeneration and the total 

 amount regenerated decrease, at first gradually and then very 

 rapidly (figs. 1, 2, 5, 6, 9, 10, 13, 15, and 17). 



2. The relative effect upon regeneration of any given hydrogen 

 ion concentration is practically the same for all stages of regener- 

 ation (p. 488; figs. 8, 11, 12, 14, 16, and 18). 



