We saw that the power of regenerating a part can be localized, and that it does not belong to all the cells of the body, but only to some of them, and we have to ask how and by what steps it has been imparted to these. The faculty depends on the possession of a regeneration-primordium (_Anlage_), and this again, in our mode of expression, consists of a definite complex of determinants, and as determinants are the products of an evolution, and thus are vital units which have arisen historically, they can nowhere suddenly originate anew in a species, but must be derived directly or indirectly from the sole basis which, in each species, forms the starting-point of the individual--that is to say, in the Metazoa, from the germ-plasm of the ovum. From it the determinant-complex of every regeneration-rudiment mast in the ultimate instance be derived. We may think of the matter thus: all the determinants of the germ-plasm vary, grow slowly or quickly, and in certain circumstances may be doubled. In this way there arise what we may call 'supernumerary' determinants, which are not required in the primary building up of the body from the ovum, and which may remain in an inactive state in the nuclei of certain cells, ready to become active under certain circumstances and to produce anew the part which they control. Such regeneration-idioplasm will at first come to lie in the younger cells of the determinate organ, but it is conceivable that under the influence of selection it may be gradually shifted to other cells of a later developmental origin, or, conversely, to others in a less external position, so that, for instance, the regeneration-rudiment for the finger of a newt may be contained not merely in the cells of the hand, but in those of the fore-arm or even of the upper arm. But all such segregation of determinant-groups cannot have taken place, as we might perhaps be inclined to think, at the periphery in the organ itself during its development; it must take place in the germ-plasm of the ovum, for otherwise it could not be transmissible, and could not be directed and modified by the processes of selection, as is actually the case, as I shall show in more detail later on.