618 RAYMOND PKARL. 



may tako place it would be necessary for the side B to give 

 its proper negative reaction. It cannot do this because it is 

 not directly stimulated, but the new very small side A is 

 stimulated. This side may not have the necessary muscles to 

 give a negative reaction itself — as in the experiment 

 described above, — yet may receive the stimulus and so 

 indirectly prevent B from reacting. Another way of ex- 

 pressing this same fact is by saying that in regenerating 

 longitudinal halves of plauarians the physiological middle 

 line remains at the line of the former cut edge for some time 

 after regeneration has begun. ^ In connection Avith this 

 discussion of the reactious of half-animals it is greatly to be 

 regretted that Willey ('97) did not get any data on the 

 reactions of the remarkable form Heteroplana. In this 

 form we have a natural " half-plauarian," or very nearly that. 

 One side is so greatly atrophied as to be practically absent. 

 It seems to me very probable that this organism would react 

 to stimuli in much the same wny that a longitudinally split 

 specimen of Planaria, which had begun to regenerate, 

 does. 



I do not wish it to be understood from the analysis of the 

 negative reaction which has been given that I intend to 

 maintain that in this reaction the side opposite that stimu- 

 lated never contracts longitudinally. It probably often does 

 this,, especially in cases of very strong stimulation which cause 

 a general excitation and reaction of the whole body. I have 

 merely wished to show that the fundamental basis of the 

 negative reaction is the extension of the side stimulated. It 

 seems to me quite possible that it may be shown by close 

 analysis in other cases that supposedly crossed reflexes arc 

 not fundamentally such at all. 



We may now pass to a brief considei-ation of the mechanism 

 of the positive reaction of the ]ilanarian to mechanical 



' 1 luive records in my notes of ('X])oiiim'ntb wliifli bliow that in tlic case of 

 oblifiue cutstlic physiological iiiiiidlc line remains at the cut edge until after 

 the new head is well formed in the new tissue on the obli(jue edge, l^ack of 

 space forbids detailed description of these experiment b here. 



