METHODS OF PHYSIOLOGICAL ANALYSIS 73 



least formally, two aspects of this differential: first, the differential in 

 the threshold of irreversible toxic or lethal effect of the agent or the upper 

 limit of tolerance to it; second, a differential in the rise of this threshold, 

 that is, in acquirement of increased tolerance during continuous exposure 

 to the agent. This is differential acclimation or conditioning in the strict 

 sense. Apparently, there is in such cases a differential in the ability of the 

 protoplasm to dispose of the agent in some way or to render it nontoxic or 

 less toxic. The tolerance differential may be so great that a part of the in- 

 dividual finally dies after days or weeks of exposure to the agent, while the 

 other part continues to live indefinitely; or, with effect of agent near the 

 upper limit of tolerance, the whole individual may finally die, but the 

 progress of death along the axis is in the reverse direction from that with 

 action of the same agent above the limit of tolerance. Dugesia {^Eupla- 

 naria) serves as an excellent example. In concentrations of ethyl alcohol 

 which kill within a few hours, physiologically young animals are more 

 susceptible than old, and the head and posterior zooid region are most sus- 

 ceptible, death progressing posteriorly from the head in the anterior 

 zooid. In a certain range of lower concentration young animals are more 

 tolerant than old; and, even though both young and old are at first anes- 

 thetized more or less completely, the young recover activity before the 

 old with continuous exposure to the alcohol, apparently because of ac- 

 quirement of increased tolerance. Also, if death occurs anywhere, it be- 

 gins at the posterior end of the anterior zooid and progresses anteriorly; it 

 may stop at a certain level, and anterior region and posterior zooid region 

 may continue to live indefinitely in the same concentration of alcohol ; the 

 anterior part may even develop a posterior end, and the posterior zooid a 

 head, in the same concentration which was earlier partially lethal. Differ- 

 ential tolerance and apparently also some degree of differential acclima- 

 tion or conditioning appear in early developmental stages with some 

 agents in secondary differential modifications of form and proportion in 

 directions opposite to those of the primary differential inhibition. In 

 echinoderm development regions most inhibited or killed by more extreme 

 toxic action show relatively more rapid development and larger size than 

 others (chap. vi). These modifications occur even though all parts are 

 more or less inhibited; and they follow a differential inhibition in earlier 

 stages, indicating some degree of differential conditioning. Since the re- 

 gions which are primarily most tolerant show the greatest capacity to ac- 

 quire increased tolerance, the modifications of form and proportion result- 

 ing from differential conditioning are, in general, similar to those of differ- 



