390 T. H. Morgan. 



oblique. Of the eight survivors all had pointed anterior and 

 posterior ends. 



The pieces just behind the last set (with oblique anterior ends) 

 gave five two-tailed pieces, and one piece with head a.nd tail. 



The tail-ends of this set had the posterior tip cut off. The three 

 that remained alive developed a pointed anterior end. 



In another series like the last, the first pieces produced six two- 

 tailed forms; the second six two-tailed forms; and the tail-ends five 

 two-tailed forms. 



In several other cases I allowed one end of the piece to close 

 for two or three days before cutting it off. In this way the mor- 

 tality of very short pieces, which is otherwise very great owing to 

 their immediate disintegration, is lessened. Some of these pieces 

 also gave two-tailed forms. 



In only one case did I obtain a piece in which a pharynx was 

 present in each tail, and in each turned outward toward the tip 

 of the tail, as shown in Fig. 12. The exact location of this piece is, 

 I am sorry to say, uncertain. It came, in all probability, from 

 the region just behind, or including some of, the old pharynx 

 region. I am inclined to think that the latter is the more probable 

 location, since the cut may sometimes include somewhat more or 

 less than is intended. The direction taken by the pharynges in 

 these pieces shows beyond a doubt that one of the two tails is a 

 heteromorphic structure, and this lends support to the interpre- 

 tation that I have given to the other cases, in which there is no 

 pharynx in the heteromorphic tail, and where none should be 

 expected to be present on theoretical grounds. 



Two other kinds of apparently heteromorphic structures have 

 been met with in carrying out these experiments. In one case a 

 piece of P. maculata, whose posterior end was cut off very 

 obliquely, regenerated one head on the anterior cut surface and 

 another also on the right side of the posterior cut surface, as shown 

 in Fig. 8. A tail also developed on the posterior cut surface at the 

 left side, which is also the more posterior end of this surface. In 

 this case we must look upon the long edge of new tissue on the 

 posterior surface as producing a head at one end and a tail at the 

 other, very much as occurs when a longitudinal piece of the worm 



