Regeneration of Heteromorphic Tails. 393 



may regenerate from an anterior cut surface, and a tail from a 

 posterior cut surface throughout a very considerable region of the 

 body. In planarians and in hydra similar facts are known. That 

 the specification of the tissues or parts plays a role even in these 

 cases is probable, as shown by the cases of heteromorphosis that 

 I have described. To many writers it has seemed that the factor 

 of polarity maybe something in the nature of a crystallizing force — 

 to use the nearest analogy at hand — a sort of perfecting or com- 

 pleting principle. Newer results have modified our ideas as to 

 this form of explanation, if such an analogy can be called at all 

 an explanation. The fact, for example, that in the earthworm 

 and in planarians the new head may be very short in comparison 

 to the part that is missing indicates that a completing force cannot 

 be acting from the cut surface forwards, but whatever the nature 

 of the factor it must in large part work from without (surface) 

 inward (/. e., toward the cut end). This point has been already 

 urged by myself, and by Driesch. 



It is very significant, I think, to find that in planarians the 

 shortness of the piece is a factor that enters into the problem as 

 to the character of the new part. I have suggested tentatively 

 that this means that in Planaria maculata the tendency is stronger 

 for the new structure to become a head than a tail, and that 

 when the influence of polarity is removed a head appears on each 

 end of short cross-pieces. In other worms, as in Planaria sim- 

 plicissiTua, the tendency in certain posterior regions to produce a 

 tail is stronger than that to produce a head, and two tails appear 

 when the polarity is reduced or removed. Why should the length 

 of the piece be so important a factor .? Can it be that there is 

 a greater difference, chemical or physical, between the two ends 

 of a longer piece, so that a stronger polarity is present ^ In short 

 pieces, from this point of view, the ends being near together are so 

 much alike that the polarity is correspondingly reduced, and, 

 under these conditions, the specification of the material of the old 

 part is not sufficiently strong to determine the nature of the new 

 part. These and many other equally obscure questions remain 

 for future investigation to explain. 



