The time- graded regeneration field in planar ians 



purpose of the experiment is of course to cut across the time-graded field at different 

 levels in the two halves. 



As a result of this treatment, a blastema was formed all over the anterior transverse 

 wound. Three distinct types of regeneration followed. 



If the shift was slight a normal symmetrical head would regenerate, although the 

 eye was a little bigger in the part of the blastema belonging to the half where the cut 

 had hit a higher level; this indicates of course a faster regeneration rate (Figure 15). 



If the shift was severe, head-regeneration only occurred in the half in which the 

 cut had hit a high level of the field; the head in the other half was inhibited (Figures 

 16 and 17). 



A 



Figure 16. Planaria lugubris. 

 Two halves were reunited with 

 a severe shift; the transverse line 

 indicates the cut made after 

 healing. 



Figure 17. Planaria lugubris. 

 Head-regeneration only occurred 

 in the right half after the opera- 

 tion indicated in Figure 16. 



In a few instances, namely when the shift was intermediate between'slight and 

 severe, each half of the common blastema regenerated a complete head (Figures 

 18 and 19) ; this was rather puzzling, but it is in fact as would be expected. 



Let us consider what we have learnt about inhibiting forces emanating from the 

 high-point, and we shall see that the results of these experiments agree with the in- 

 hibition hypothesis. 



If the shift has only been slight, the inhibiting force from the right half of the blas- 

 tema will not have time enough to inhibit eye-formation in the left half before this 

 one has carried its differentiation so far that it has not only made its own left eye but 

 also has set up inhibition towards the right half, in which, accordingly, regeneration 

 of a left eye is inhibited. 



129 



