490 C. M. Child. 



been considered by me in several papers (Child, '02, '03a, '04). 

 It is these same conditions which we have to consider in the pres- 

 ent case, and the present question becomes from this stage on 

 identical with the first, which has already been discussed. 



According to this view then regeneration in the earlier stages is 

 about the same in pieces with and those without ganglia because it 

 depends to a large extent upon local factors connected with the 

 absence of the part removed and the presence of the cut surface 

 while in later stages the special functional conditions connected 

 with the use of the part in a characteristic manner, determined 

 essentially by its position in relation to the whole, constitute the 

 important factors in determining both the amount of regeneration 

 and the structural differentiation. 



The fourth question has special reference to the general form or 

 outline of the regenerated part which is as a rule more slender and 

 tapering in the pieces with ganglia than in those without, the 

 difference being greatest when the cut surface is near the anterior 

 end. 



The answer to this question is simple. I believe that the differ- 

 ence in form is due primarily to the tension in the direction of the 

 longitudinal axis exerted upon these parts in consequence of the 

 use of the posterior end during locomotion as an organ of attach- 

 ment. In Leptoplana both the posterior end and the lateral 

 margins are employed for attachment to the substratum and are 

 in consequence subjected to tension as the animal moves forward 

 holding by one part or another of this region. Brief examination 

 of creeping specimens is sufficient to show that these conditions 

 exist. In pieces containing the ganglia the use of these parts in 

 this manner is much more frequent and the tension is much greater 

 since locomotion is much more rapid than in pieces without gan- 

 glia. Undoubtedly these conditions play a part in the arrange- 

 ment of the physically plastic new material, as was very clearly 

 shown in the preceding paper of this series (Child, '04) and it is 

 not improbable that they also serve as stimuli to growth. The 

 form of the new part in Figs. 11, 15, 16, and especially in Figs. 

 2-5 (summer experiments) is very evidently a form resulting 

 from mechanical tension exerted chiefly at the posterior end. 



