ACCLIMATION IN PLANARIA DOROTOCEPHALA. 305 



Let us consider briefly the significance of the data in the light 

 of the interpretation of head-frequency suggested above. It 

 will be seen that at a lowered temperature the head frequency 

 throughout is lower than it would have been at the original 

 temperature; at raised temperature it is higher than it would 

 have been at the original temperature.' Alteration of tempera- 

 ture then must affect x more than y; for if this were not the case 

 we should expect head-frequency to remain unaltered. Appar- 

 ently, then, the x cells are more susceptible to the direct effect 

 of temperature changes than those of y. 



This greater susceptibility of the x cells to temperature change 

 is also indicated by the fact that at 4 C. the x region is incapable 

 in almost 100 per cent, of the cases of giving rise to any head at 

 all; although if the changes in temperature have not been too 

 rapid the pieces may still remain alive at this temperature. 

 Pieces kept for some five months at this temperature remained 

 headless; but when brought into medium temperature gave rise 

 to a high percentage of heads, many of which were normal. At 

 this extremely low temperature life was maintained, but little 

 or no growth of the physiologically younger cells of x occurred. 



2. During the First Few Hours of Regulation. Another series 

 of experiments was conducted to see what would be the effect 

 of temperatures applied for shorter periods of time than the 

 whole time of regulation. These experiments were in a sense 

 preliminary to acclimation experiments proper. They lie in 

 intermediate position between those testing the direct effect of 

 temperature during regulation and those testing the power of 

 adjustment of the organism to temporary changes, and the extent 

 to which such changes permanently alter the metabolic rate. 



It has been found experimentally (Child, '146) that whether 

 a head shall be formed or not is determined within 3-6 hours 

 after sectioning. 1 These experiments were repeated by me with 

 like results (see Table IX. below). 



We know that long pieces from standardized stock, such as 

 ax (Fig. 6), produce practically 100 per cent, normal heads. 

 We also know that pieces such as a-b (Fig. 6) whose anterior 



1 Similar data have been worked out for Lumbriculus inconstans by Hyman 

 ('160.1. 



