604 RAYMOND PEARL. 



normal animal, this being due to the generally lowered tonus 

 in such a piece. I have not been able to obtain any positive 

 reaction (i. e. turning towards the stimulus) in such a 

 posterior piece after operation. Stimuli which are at all 

 effective produce the negative response. This experiment 

 has been tried many times, but always with the same result ; 

 the positive reaction never appears. If the posterior end of 

 such a posterior cut piece is stimulated the crawling move- 

 ment is produced just as in case of the normal complete 

 animal. As has been noted in connection with the move- 

 ment, there is a general reduction of tonus in the posterior 

 pieces resulting from transverse cuts. This low tonus in- 

 volves not only the motor functions, resulting in slower 

 movement, but also to a less extent the sensory functions. 

 Such a piece is somewhat less sensitive to mechanical stimuli 

 than normally. The cut surface is more sensitive to 

 mechanical stimuli than any other part. 



Now it will be seen from the above description of the 

 reactions of a piece from which the brain has been removed, 

 that the most striking difference in the behaviour of such a 

 piece from that of a normal animal is to be found in the 

 absence of the positive reaction. 



There are three conceivable possibilities as to the cause of 

 the absence of the positive reaction in pieces from which the 

 head has been removed. First, the positive reaction might 

 be due to the stimulation of certain sense organs which are 

 removed by the operation. But this is decisively negatived 

 by the fact that in an entire worm stinnilation of points 

 posterior to the level of the cut removing the anterior end 

 will cause the positive reaction. 



Second, it might be conceived that the reaction is brought 

 about by a special localised muscular mechanism, which is 

 removed or destroyed by the cut. But there is no evidence 

 of the existence of such a mechanism ; and further, it will bo 

 sliown later that the ordinary musculatui-e of the body, which 

 is of course uninjured in llio posterior ])art, is suniciciit to 

 brinu' about the reaction. 



