618 RAYMOND PKARL. 



may take 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, — 3'et 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 planarians 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 reactions 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-planarian," 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 way 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 are 

 not fundamentally such at all. 



We may now pass to a brief consideration of the mechanism 

 of the positive reaction of the planarian to mechanical 



' 1 liavc rc'coids in my notes of cxpcrinientb wliicli bliow tliat in llie case of 

 oblique cuts the physiological middle line remains at the cut edge until after 

 the new head is well formed in (he new tissue on tlie obli(|ue edge. Lack of 

 fcpace forbids detailed desciipUon of these expeiiinenls here. 



