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Regeneration of Heteromorphic Tails. 391 



is removed. The case recalls those in which the worm is split 

 from the posterior end far forward and a head develops at the 

 anterior end of the cut surface on one or on both sides. I have 

 discussed the meaning of this case elsewhere.^ In a strict use of 

 the word heteromorphosis, as I have tried to use it for purposes 

 of greater clearness, neither this case nor that of the split-worm 

 can be looked upon as an example of axial heteromorphosis, since 

 the result depends largely, apparently, on the new part alone 

 without relation to the old, and the head and tail are orthomor- 

 phic from this point of view. 



In some other cases in which the anterior end is very oblique, 

 two structures appear on the anterior edge, as shown in Figs. 19- 

 20. One of these is a head and the other appears from its struc- 

 ture and movements to be a tail. If so, the case is comparable to 

 the last one, and shows the converse condition. Here the tail on 

 the side of the anterior cut surface cannot be looked upon as an 

 example of axial heteromorphosis. It is rather an orthomorphic 

 structure, since it stands in this relation to the remainder of the 

 new material on the anterior cut surface. Both cases, however, 

 present something of a paradoxical relation. 



The results described in the first part of this account recall cer- 

 tain conditions that I have recently described in connection with 

 the regeneration of Dendro caelum lacteum. It had been shown 

 by Lillie that posterior pieces cut off just in front of, or through, 

 or behind the pharynx-chamber do not regenerate an anterior 

 end. A histological examination of the anterior end of such 

 pieces showed me that a certain amount of new tissue is formed 

 at the anterior cut surface, and it was not apparent why the re- 

 generation should not go further and produce a new anterior end. 

 The results with Planaria simplicissima suggest, although they by 

 no means prove, that the anterior part that regenerates in Dendro- 

 ccelum may be a heteromorphic tail. For the present, however, 

 I wish to leave this question open, until further work reveals the 

 nature of the anterior part in this worm. There are some gen- 

 eral considerations in connection with the problem of polarity 



^Regeneration. 1900. 



