ACCLIMATION IN PLANARIA DOROTOCEPHALA. 



301 



upon the head-frequency of regulating Planaria, we must con- 

 sider: first, the significance of regulation experiments with this 

 form; second, the factors concerned in head-frequency; and 

 third, the direct effect of temperature on head-frequency. 



In a study of regulation, whole worms are cut into certain 

 pieces of equal size and allowed to re- 

 main for sufficient length of time to un- 

 dergo growth and reorganization or re- 

 differentiation. Planaria dorotocephala 

 ordinarily reproduces asexually by fis- 

 sion, and worms above a certain size 

 consist of two or more zooids which are 

 distinguishable physiologically but not 

 morphologically (Child, 'nc). Since the 

 head-frequency in the regulation of 

 pieces has been found to vary accord- 

 ing to the position of the piece with 

 respect to the boundaries of the differ- 

 ent zooids, it is desirable for the sake 

 of uniformity to use pieces from a single 

 zooid. The anterior zooid extending 

 from the head to a short distance be- 

 hind the mouth is the longest zooid, and 

 most convenient for the purpose. More- 

 over, since head-frequency varies with 

 length of piece and size of animal (Child/ 

 na, '140, '146, '16), pieces of uniform 

 length from animals of approximately 

 the same size must be used. In the ex- 

 periments reported below, three pieces, 

 representing each one third of the length 

 of the first zooid after removal of the 

 head, are used and are designated A , 

 B and C, in order from anterior to posterior (see Fig. 4). In 

 all the experiments unless otherwise stated, the temperatures 

 used were: "low," 10-12; "medium" 20; "high" 29. Dur- 

 ing the period of regulation the water was changed frequently to 

 guard against the accumulation of toxic substances. Professor 



FIG. 4. 



