Thus the 'nuclear substance' of unicellular organisms is for us the first demonstrable organ of regeneration, and first of all for normal regeneration, which takes place at every reproduction, for instance, of an Infusorian. For we have already seen that, in the transverse division of a trumpet animalcule (_Stentor_), the anterior part must develop the posterior half anew, while the posterior half must develop the much more complex anterior half, with mouth region and spiral bands of cilia. But as soon as the arrangement for normal reproduction was elaborated, as soon as the nucleus was present, as a depôt of 'primary constituents,' this implied the possibility of regeneration in exceptional cases, that is, after injury. The mechanism was already there, and it came into operation as soon as a part of the animal was missing. It is in the first nucleus, therefore, that we have to look for the source of all regenerative capacity, both in unicellular and multicellular organisms. But with the origin of the latter a limitation took place, either quite at the beginning or a little later, for each nucleus of the cell-colony no longer contained the whole complex of 'primary constituents' or determinants of the species, but, in many cases, only the reproductive cell possessed them. As soon as this began to develop into a whole by cell-division the determinant-complex was segregated. Thus the first cell-colonies with two kinds of cells arose, as we have seen in the case of _Volvox_--the reproductive cells with a complete equipment for regeneration in their nucleus, and the somatic cells with a limited equipment for regeneration in their nuclei. The somatic cell could no longer give rise anew to the whole organism, but could only reproduce itself or its like. But as many of the lower Metazoa and Metaphyta possess _the power of budding_, that is, are able not only to produce a new individual from definite cells--the reproductive cells--with or without sexual differentiation, but from other cell-groups also, these must contain the whole complex of determinants appertaining to the reconstruction of the organism, and we have to ask how this is reconcilable with the differentiation of a multicellular organism, whose different kinds of cells depend, according to our interpretation, on the fact that they are controlled by different determinants. Obviously, there is only one way out of this difficulty, and it is the one we have already indicated, that although the diffuse regenerative capacity which we have just alluded to occurs in species which exhibit gemmation, this does not exclude the control of a cell by a specific determinant; other determinants may be contained in the cell, in a state, however, in which they do not affect it, that is, in an inactive or latent state.