REGENERATION OF HETEROMORPHIC TAILS IN 



POSTERIOR PIECES OF PLANARIA 



SIMPLICISSIMA/ 



BY 



T. H. MORGAN, 

 With 20 Figures. 



The regeneration of a heteromorphic head from the posterior 

 end of short cross-pieces of Planaria maculata. Figs. 1-5, led me 

 to ex2im'ine Planarta stTnplifissima^ in order to see if the same result 

 could be obtained here when short cross-pieces of the worm were 

 made. The regularity with which a heteromorphic head can be 

 obtained in the latter species when the old head is cut off just 

 behind the eyes, Fig. 10, led me to expect that short cross-pieces 

 from the body would behave in the same way as do similar pieces 

 o( Planaria maculata. The results have proven, however, in part 

 otherwise, for while heteromorphic heads do appear on short 

 cross-pieces from the anterior regions of the worm, Fig. 11, none 

 such develop from the posterior end of short cross-pieces from the 

 more posterior regions of Planaria snnplictssitna. On the contrary 

 these pieces regenerate a structure from the anterior cut surface 

 that appears to be a heteromorphic tail, and another tail from the 

 posterior cut surface, Figs. 12, 13. The result is a two-tailed 

 and not a two-headed piece. In order to determine if the 

 new anterior structure is really a tail, and not simply an undevel- 

 oped head, a number of experiments were carried out during the 

 winter and spring of 1903-04. 



Before describing the results certain general considerations 

 must be spoken of that are intimately connected with the question 



'The principal facts recorded in this paper were reported at the Christmas meeting 

 of the American Zoological Society, 1903. 



^This is the same species which, in my earlier papers, has figured as Planaria 

 Itigubris. Stevens has recently determined that this worm is P. sim'plicissima. 



