CONTROL OF MORPHOGENESIS IN PLANARIA. 327 



earlier stages and in still larger pieces, e. g., after removal of only 

 the head region, they are never absent. In general the pieces 

 which show more motor activity when left undisturbed in the 

 dishes and which react more readily to slight stimuli are those 

 which have the greater capacity for producing new heads, i. e., 

 for becoming wholes. From a number of pieces it is possible 

 within a few hours after section to select with a considerable 

 degree of certainty those which will form heads, or which will 

 show the greater degree of head forming capacity, merely by 

 observing the differences in motor activity and reaction to slight 

 stimuli. 



With these and various other facts in mind I carried out ex- 

 periments to test the effect of mechanical stimulation to movement 

 upon the formation of heads. The pieces were cut of such size 

 and such distance from the old head that they showed little or no 

 "spontaneous" movement during the first few days after section. 

 Various methods of stimulation were employed: one of these 

 was that of tilting the broad flat dish in which they were kept 

 so that each piece was out of water for a few seconds. This 

 usually serves to induce locomotion, but it does not continue 

 long after the pieces are again in water. Sometimes the pieces 

 were loosened from the glass by currents from a pipette or were 

 turned upon their dorsal surfaces with needles, after which they 

 usually righted themselves. But the most commonly used and 

 most effective method was that of stroking or gently pushing 

 the pieces with a small soft camel hair brush. Usually a few 

 seconds of such stimulation was followed by relatively long sus- 

 tained locomotion. The stimulation began a few moments 

 after section of the pieces and w r as repeated at least every hour 

 and often every half hour from about 8.30 A.M. till 6 P.M. and 

 again at least twice between 8 and n P.M. each day until the 

 heads in such pieces as produced heads were w r ell developed 

 and the eyes visible. Each time that the pieces of the one set 

 were stimulated the water of the dish containing the control 

 was gently disturbed and stirred in order to avoid as far as 

 possible differences in aeration. Care being taken not to touch 

 the control pieces directly, the movement of the water almost 

 never induced movement of the pieces. The dishes w r ere kept 



