ENVIRONMENT AND REGENERATION 491 



latter is much greater. Osterhout ('14) has shown that the 

 effect of acids upon a living membrane is to cause an initial 

 decrease in permeability followed at once by a rapid increase 

 which continues to the point of death. The decrease in size of 

 tadpoles in the strongly acid solutions may be attributed to the 

 loss of substances from the tissues due to the increased permea- 

 bility of the membranes. If these substances are, as is probably 

 the case, largely in the form of phosphates and amphoteric 

 substances such as amino acids, their effect upon the medium 

 would be to decrease the hydrogen ion concentration, as shown 

 by the indicators, by forming with the acids of the solution, new 

 compounds which ionize differently. This, however, would not 

 reduce the' titratable acidity as shown by the power of the solution 

 to neutralize a strong base such as Ba(0H)2. 



That the effects of acids are due to the free H ions, and not to 

 the titratable acidity, was suggested by the observation, made 

 throughout the experiments, that tadpoles which died usually 

 succumbed within a few hours after the solution was changed, 

 and that many tadpoles which showed symptoms of severe 

 disturbances when put into the fresh solution, gradually became 

 more normal during the twenty-four or forty-eight hours before 

 it was changed again. This v/as not due to acclimation, for 

 with the next changing of the solution they again showed the 

 same pathological symptoms and frequently succumbed. But 

 one experiment was performed in this connection: Ten tadpoles 

 of about 55-mm. length were placed in 1 liter of 0.00075N HCl 

 (75 cc. O.OIN HCl per Uter). The pH of this solution was about 

 3.4. After thirty-six hours six of the tadpoles were dead and the 

 other four were in a dying condition. These tadpoles were 

 removed and two fresh tadpoles were placed in the same solution 

 which now had a pH of about 5.3. At the same time two similar 

 tadpoles were placed in a freshly prepared HCl solution. The 

 next morning the two tadpoles in the fresh solution were both 

 dead. Those in the solution in which others had died were still 

 alive at the end of two weeks, at which time the experiment 

 was discontinued. Titration of a sample of the solution made 

 on the third day showed no decrease in its ability to neutralize 



