I have already pointed out the importance of the rôle played by liberating stimuli in regeneration, and not only of extra-organismal stimuli, such as gravity, but above all of intra-organismal stimuli that is, the influences exerted in a mysterious manner by other parts of the animal on the parts which are in process of regeneration. It is a great merit of the modern tendency in evolution theory that it has demonstrated the importance of such internal influences. Although we are still far from being able to define the manner in which these influences operate, we may say so much, that it depends essentially on the nature and extent of the loss which parts are reproduced by the regenerating cells, and, also, on the position and direction of the injured surface from which the regeneration starts. The influences, still quite beyond our comprehension, which are exerted on the regenerating part by the uninjured parts constitute the liberating stimuli, which evoke the activity of one or other of the determinants contained in the regeneration-idioplasm. [Illustration: FIG. 100. Regeneration of Planarians. _A_, an animal divided into three parts by two oblique cuts. _B_, the fragments(_a_, _b_, _c_) in process of regeneration. _C_, an animal with various oblique incisions in the margin of the body, which have induced the new formation of heads (_k_), of tails (_s_), and pharynx (_ph_). _A_ and _B_ after Morgan; _C_ after Walter Voigt.] Walter Voigt has shown, by a series of most interesting experiments, that it is possible not only to cause the development of a new head in Planarians by cutting them, in which case a tail may grow from the anterior portion and a head from the posterior portion, but it is also possible in an intact animal, that is, one with both head and tail, to cause the production of a second head, or a second tail, or both at once, at any part of the body margin at will, according to the direction of the cut. If the margin of the body be cut obliquely forwards (Fig. 100, _A_) a supernumerary tail arises (_C_, _s_), if it be cut obliquely backwards a supernumerary head arises (_C_, _k_), and in this way several heads and several tails may be produced in the same animal. It is obvious, then, that the interaction, in the first place, of the cells of the cut surface, but probably also of the deeper-lying cells, decides which determinants are to come into action, those of the head or those of the tail, but both must be present at every part of the cut. How far below the cut surface the cells take part in this determination we cannot make out, but that it cannot be due to the co-operation of all parts is clear in this case at least, since the animal still possesses its original head and tail. The extra heads and tails thus produced prove, at any rate, that there can be no question here of the expression of an adaptive principle, a _spiritus rector_, or a vital force, which always creates what is good, but that