ACCLIMATION IN PLANARIA DOROTOCEPHALA. 



307 



TABLE IX. 



B give only 8 per cent, heads, and in the lots C, D, E, F, cut from 

 the anterior ends of the long pieces at different intervals, it is 

 evident that head-frequency increases with the length of the 

 interval and that 6 hours' connection with the long piece is a 

 sufficient interval to form a head in 92 per cent, of the pieces. 



The demonstration of the occurrence of head determination 

 within so short a period of time furnished good ground for the 

 belief that temperature, acting through similar periods of time, 

 would be able to produce an effect upon head-frequency. A few 

 experiments along this line were undertaken. Worms from me- 

 dium temperature were cut and immediately put for three hours 

 into low temperature, after which they were left to regulate at 

 medium temperature, and their head-frequency compared with 

 that of a medium temperature series not subjected to low tem- 

 perature. The results of such a series of 25 worms are given 

 below in Table X. The death rate is somewhat high but the 

 differences in head-frequency indicate some effect of the tempera- 

 ture change. Although these differences are slight, several 

 repetitions of the experiments showed the same results, which are 

 therefore to be considered typical. 



TABLE X. 



