Regeneration of Heteromorphic Tails. 387 



the anterior and a tail at the posterior end. The new pharynx 

 always lies at the anterior end of such pieces, i. e., at the edge of the 

 old tissue, and therefore, as it were, in the posterior part of the 

 new head that has developed. Fig. 15. It is this relation of the 

 new pharynx to the old part that first demands especial considera- 

 tion, for, at first sight it is not clear why the pharynx in these pos- 

 terior cross-pieces should shift to the anterior end, and not lie, as 

 in the more anterior pieces, at the posterior edge. If it did so it is 

 obvious that it would appear in a region posterior to that in which 

 the normal pharynx lies in the old worm, and it seems that this 

 cannot take place. The posterior cut surface can form only that 

 part of the tail that lies behind it in the old worm. The anterior 

 cut surface can also produce all that lies in front of it in the old 

 worm, including the pharynx, although the proportionate distances 

 apart of the new structures may be at first very different from 

 those in the adult or embryonic worm. We touch here on one of 

 the fundamental questions of polarity to which I shall hope to 

 return at another time. 



If a heteromorphic tail were to develop on the anterior cut 

 surface of a short posterior cross-piece, what should we anticipate 

 in regard to its relation to a pharynx .? Should we expect to find 

 a pharynx in the new tail turned in the opposite direction, /. e., 

 pointing towards the tip of the tail .? But if the new structure at 

 the anterior end is a heteromorphic tail why should it develop 

 a pharynx at all, since this never develops at the posterior 

 end of cross-pieces from this region .? Should we not rather 

 expect a heteromorphic tail to behave in this respect in the same 

 way as the orthomorphic structure .^ This appears to me to be 

 the correct point of view and the results of experiment seem to 

 bear out this anticipation. 



Let us apply the same point of view to the regeneration of a 

 pharynx in heteromorphic heads from cross-cut pieces of the ante- 

 rior regions of the worm. It has been pointed out above that the 

 new pharynx appears at the posterior end in case a tail develops 

 at this end. Suppose, however, a heteromorphic head instead of 

 an orthomorphic tail develops at the posterior end. It is clear, 

 from our point of view, that no pharynx should develop, and I 



