In several animals regeneration takes place more readily from one end than from the other of the same cut, and this difference seems to be connected with the kind of new part that is to be regenerated, and not with the actual power of regeneration of the region itself. For instance, if a short piece is cut from the anterior end of an earthworm, a new anterior end is quickly regenerated from the anterior cut-surface of the posterior piece, but no regeneration takes place, or only after a long time, from the posterior cut-surface of the anterior piece. These relations are reversed if the posterior end of a worm is cut off. There regenerates very quickly a new posterior end from the posterior cut-surface of the anterior piece, but no regeneration takes place, or only after a long time, from the anterior cut-surface of the posterior piece. The new structures that develop after a long time from the posterior surface of a short anterior piece, and from [Illustration: FIG. 16.--_A._ Head of _Planaria lugubris_ with line indicating level at which _A¹_ was cut off. _A¹._ Head of last regenerating a new head at its posterior end. _B._ Piece of _P. maculata_ regenerating head at each end. _C._ Posterior end of _Allolobophora fœtida_ regenerating a new tail at its anterior end. _C¹._ Enlarged anterior end of last with new tail. _C²._ Tip of new tail. _D._ Anterior end of one individual of _A. fœtida_, grafted to anterior end of another worm, leaving posterior end of piece exposed. This has begun to regenerate. _E._ After Hazen. Similar experiment in which a _new head_ regenerated at posterior end of grafted piece. _F._ Two longer pieces of _A. fœtida_ united by anterior ends. One end was subsequently cut off and a new tail regenerated. _G._ End of a developing piece of _Tubularia mesembryanthemum_ that had been cut off; it has regenerated, at its proximal end, another proboscis.] the anterior surface of a short posterior piece, correspond to a different part of the worm from that which would be expected to develop, if the polarity of the piece is taken into account. Another reversed head develops on the posterior cut-surface of the anterior piece, and another tail on the anterior end of the posterior piece. The polarity of the new part is in this case reversed, as compared with that of the piece from which it arises. In the earthworm there is a marked delay in the regeneration of these heteromorphic parts. Even in tubularia in which heteromorphosis takes place, there is usually a delay of twenty-four hours in the formation of the reversed head. In _Planaria lugubris_, in which a reversed head develops, if a piece is cut from the anterior end just behind the eyes, the delay in the formation of the reversed head is very slight, if indeed there is any delay at all.