We assumed that there is a regeneration-idioplasm, and therefore that there are 'primary constituents' at certain positions in the body, but how does it happen that these are able to build up the lost parts in the proper situation and detail? A theoretical formula might well be thought out, according to which the determinants of successive parts would become active successively, and would thus liberate one another in an appropriate order of sequence, but there would not be much gained by this, especially as what we already know in regard to the regrowth of the legs and toes in Triton does not harmonize with such an assumption. It appears to me more important--though even here we must still be very vague as to details--to recognize that, in all vital units, there are forces at work which we do not yet know clearly, which bind the parts of each unit to one another in a particular order and relation. We were obliged to assume such forces even in regard to the lowest units, the biophors, since otherwise they could not be capable of multiplication by division, on which all organic growth depends, unless we are to assume, as Nägeli did, a continual _generatio æquivoca_ of the specific kinds of biophors (his 'micellæ'). But we shall see later, when we come to speak of spontaneous generation, that we cannot acquiesce in such an assumption. If, then, we cannot conceive of a power of division arising from within and depending solely on growth by means of assimilation, without such attractive and repellent forces or 'vital affinities' the internal parts would necessarily fall into disorder at every division. It seems to me therefore that such 'affinities' must be operative at _all stages_ in the life of the vital units, not only in biophors, but also in the cell, and in the 'person' as well as in determinant and id. It is true that 'persons' no longer generally possess the power of multiplying by division, but in plants and lower animals many do possess it; and the power of giving rise anew to certain parts is obviously a part of that power of doubling the whole by division. The ultimate roots of regeneration, then, must lie in these 'affinities' between the parts, which preside over their arrangement and are able to maintain it and to give rise to it anew. In this respect the organism appears to us like a crystal whose broken points always complete themselves again from the mother-lye after the same system of crystallization, obviously in this case too as a result of certain internal directive forces, polarities, which here again we are unable precisely to define. But the difference between the organism and the crystal does not--as people have been hitherto inclined to believe--lie only in the fact that the crystal requires the mother-lye to complete itself, while the vital unit itself procures the material for its further growth; it lies also in the fact that such regeneration is not possible in every organism and at every place, but