312 ELLINOR HELENE BEHRE. 



occurring gradually and proceeding in the opposite direction 

 from the first; that is, after a change in temperature, the velocity 

 of physiological processes must undergo a gradual progressive 

 change apparently approaching a limit in a constant temperature. 

 Such a change manifestly cannot be an effect of temperature on 

 the velocity of chemical reaction, but must result from an action 

 of the altered velocity and other temperature effects associated 

 with it upon the constitution or condition of the heterogenous 

 protoplasmic system. These changes in the system in turn 

 condition further gradual alterations in the velocity of reactions 

 even though the temperature remains constant. 



The effect of temperature upon head-frequency suggests that 

 the processes in the region x (Fig. 5), the cells of which undergo 

 extreme dedifferentiation and rapid growth and are directly con- 

 cerned in head-formation, are altered to a greater degree by 

 change in temperature than those in the region y. Regulation 

 at a higher temperature than that at which the animal has been 

 living increases, and regulation at a lower temperature decreases 

 head-frequency. Moreover, acclimation to a given temperature 

 alters the reaction, as indicated by head-frequency, to another 

 temperature (Table XI.). This table also shows that animals 

 acclimated to a high temperature show a lower head-frequency, 

 and animals acclimated to a low temperature, a higher head-fre- 

 quency, than those brought into that temperature at the begin- 

 ning of regulation. These effects are in general opposite in 

 direction to the direct effects of temperature change. If head- 

 rate x 



frequency = - - (pp. 302-3), as various lines of experimental 

 rate y 



evidence indicate, it is evident; first, that changes in tempera- 

 ture must directly alter rate x more than rate y; and second, 

 that acclimation to a given temperature preceding regulation 

 produces its effect by altering the metabolic level at which the 

 regulatory processes in the cells of x begin. 



These conclusions are in complete agreement with other data 

 on acclimation to temperature and to other conditions in 

 Planaria. In general, direct susceptibility to change in condi- 

 tions and also capacity for acclimation to conditions which are 

 not too extreme vary with rate of oxidation (Child, '130, ' 



