308 ELLINOR HELENE BEHRE.. 



From the above table it will be seen that the effect of lowering 

 temperature during the beginning of the period of regulation 

 is of the same sort as the effect of a temperature continued during 

 the whole period. Head-frequency is lowered throughout the 

 series by the lowering of temperature. 



B. The Analysis of Acclimation by the Method of Head-Frequency. 



In the experiments of the preceding section, worms were 

 taken from temperatures at which they had been living so long 

 that there could be no doubt that acclimation is practically 

 completed. In this section worms were used which had been 

 living at a given temperature for shorter periods of time before 

 putting into a new temperature to regulate. In this way an 

 attempt was made to determine for how long a period worms 

 must live in one temperature before acclimation to that tempera- 

 ture occurs to such a degree that it will affect the head-frequency 

 at a new temperature. 



The method consisted in keeping worms at a certain tempera- 

 ture for various lengths of time, from three days to three weeks. 

 Then cutting A, B and C pieces for regulation at another tem- 

 perature and comparing their head-frequency with that of pieces 

 from animals living at the second temperature. A difference in 

 head-frequency between the two lots will indicate that the first 

 temperature has had some effect on the reaction to the second. 



The following table gives a resume of the results of such 

 experiments with acclimation periods of from 3 days to 3 weeks. 



Taking the first series as an illustration, the animals were 

 kept at low temperature for three days and the A, B, C pieces 

 when cut were placed in medium temperature for regulation. 

 The head-frequency of this series was then compared with that of 

 a control series which had been living and underwent regulation 

 at the regulation (in this case medium) temperature. In this 

 first series the head-frequency of the worms kept at low for three 

 days before the pieces were cut shows in the A pieces a decrease, 

 in the B and C pieces an increase in head-frequency as compared 

 with the control. The data for the other series are recorded in 

 the same way. 



The table shows that the effect on head-frequency for shorter 



