Bonnet, who first proposed the hypothesis of specific stuffs, went further and assumed also that they move in definite directions in the body, the head-stuff flowing forward and the tail-stuff flowing backward. It was necessary to assume definite movements of the stuffs in order to account for the development of the head at the anterior end of a piece and of a tail at the posterior end. In cases of heteromorphosis of the sort described above, these stuffs, if they brought about the results, would have to move in _opposite_ directions from those assumed in the hypothesis; or else that part of the hypothesis that postulates the movement of the substances must be dropped, and in its place there must be substituted the idea of the excessive amount of such substances in the ends accounting for the heteromorphosis. An hypothesis that must be changed in this fundamental way to explain both classes of facts cannot be given very serious consideration. Of these possible ways in which it has been attempted to account for the phenomenon of heteromorphosis, the first one suggested seems to me simpler and more probable, but which organs are to be made responsible for the result cannot at present be stated. The fact that both Bardeen and I have obtained heteromorphosis in planarians in other regions than in the head indicates at least that other factors than the presence of head tissues or of head substances may bring about the development, and if it can be discovered what produces the result in regions remote from the head we may be in a position to explain the result in the head region in the same way, although it may be, of course, that the same result may be brought about by different factors, when the internal conditions are somewhat different. [Illustration: FIG. 17.--After Voigt. Planarian with three oblique cuts at side. The most anterior cut (left side), directed forward, produced a tail. The one on the right side, directed backwards, produced a head. The most posterior cut (left side) made a head with pharynx, and also a tail-like outgrowth.] Another phenomenon connected with the polarity of a piece is shown by _Cerianthus membranaceous_. When a triangular piece is cut from the side of the body, a half circle of tentacles appears around the lower edge of the cut, as shown in Fig. 15, _C_. The presence of a free distal edge on the lower side of the opening is a sufficient stimulus to call forth the development of tentacles. A somewhat similar result is obtained when an incision is made in the side of the body of a planarian. A lateral head may grow out from the anterior edge of the cut-surface, as shown in Fig. 17. [Illustration: FIG. 18.--_A._ After Loeb. Anterior end of _Ciona intestinalis_ with oral-siphon partially cut off. Eye-specks regenerate, both on oral and aboral edge. _B._ Same (after T. H. M.), showing similar result on excurrent siphon.]