Something similar happens if the head be cut off a Tubularia-polyp, it forms a new head with proboscis and tentacles. It does so, at least, if the stalk of the polyp be left in the normal position; but if it be stuck into the sand in the reverse position a head arises at the end which is uppermost, where the roots arose previously, and the previous head-end now sends out roots. By suspending a beheaded stalk horizontally in the water a head can be caused to develop at each end of the stalk, so that we must assume that every part of the polyp is, under some circumstances, capable of developing a head, and that it must be 'circumstances'--in this case gravity, contact with earth or with water, and the mutual influence of the parts of the animal upon each other--which decide what is to be produced. Loeb, who was the first to observe this form of regeneration, called it heteromorphosis, to express the fact that particular parts of the animal might be produced at quite different places from those originally intended for them. It would certainly be erroneous to range these cases of heteromorphosis against the determinant theory, but they certainly do not afford any special evidence of its validity as an interpretation, for all that we can say here again is that all, or at least many, cells of the animal must contain the full determinant-complex of the ectoderm, and others those of the endoderm, and that particular groups of determinants become active when they are affected by certain external or internal liberating stimuli. In regard to such animals the theory is hardly more convincing than the rival theory, that the faculty of regeneration is a general property of living substance, which does not attain to equally full expression everywhere, because it is met by ever-increasing difficulties involved in the increasing complexity of structure. The validity of the theory only begins to be seen when we deal with cases where it is demonstrable that every part cannot bring forth every other, where the power of regeneration is limited, and occurs only in definite parts in a definite degree, and can only start from particular parts. Here the assumption of a general and primary regenerative capacity fails. Any one who insists, as O. Hertwig does, that the idioplasm in all cells of the body is the same, can always plead that, in the cases in which regeneration does not occur, the fault lies, not in the regenerative capacity, but in the absence of the adequate liberating stimuli, and at first sight it does seem as if this position were unassailable. We shall find, however, that there are facts which make Hertwig's interpretation quite untenable.