614 EAYMOND PEARL. 



uway from the stimulus (negative reaction) is due to a 

 lengtliening of the side stimuhated. 



It may possibly be objected to the last experiment that 

 the impulse from a stimulation at, for example, B (Fig. 21) 

 took the path indicated by the dotted line in that figure^ and 

 caused a contraction on the left side of the body, so that 

 really the observed turning was the result of a contraction on 

 the side opposite that stimnlated. To this objection it may 

 be answered that by stimulating different points along the 

 edge B it is possible to cause the point about which the 

 turn occurs as a pivot to be located anywhere along the line x y. 

 It is very evident that contraction of muscles in the region N 

 can have nothing whatever to do with turning of the right 

 piece about the point x. So this objection is without force. 



As the process of regeneration of a cut longitudinal half of 

 the body goes on^ the piece will straighten out from the 

 curved form it takes after the cut is made, and it is conse- 

 quently possible to obtain specimens in which the regenera- 

 tion of the missing half of the body has produced only a very 

 small amount of new tissue, and which are at the same time 

 nearly straight in outline and able to make progressive move- 

 ments. The reactions of such partially regenerated speci- 

 mens are of importance as throwing light on the normal 

 mechanism of the reactions. The reactions of a typical 

 specimen of this soT't may be described in detail. On October 

 10th, 1901, a small piece of the anterior end of a specimen of 

 P. maculata was isolated. The piece was cut as nearly as 

 possible in the form shown in Fig. 22, a. On October IGth 

 the piece had the form shown iu Fig. 22, h. A narrow strip 

 of new tissue had formed down the right side, and the forina- 

 tion of the outline of the head and of the right eye was just 

 beginning. At this time the reactions of the specimen were 

 as follows. Stimuli applied at y caused the head to turn 

 sharply away from the stimulus (typical negative reaction). 

 This reaction was quite like that given by a normal individual 

 stimulated in the same way. Stimulation at x, however, 

 produced no trace whatever of a negative reaction. On 



