It is with the fate of the other portion coming from the first division of the germplasm that we are concerned here. It is set apart to form the nuclear matter, and so to control the building up of the actual individual. Weismann supposes that the subsequent divisions it undergoes are what I call in this translation differentiating divisions (_Erbungleiche Theilung_). According to his theory, in each of these divisions the microcosms of the germplasm are not doubled, but are slowly disintegrated, the division differentiating among the determinants, and marshalling one set into one portion, the other set into the other portion. The differentiating process occurs in an order determined by the historic architecture of the microcosms, so that the proper determinants are liberated at the proper time for the modelling of the tissues and organs. Ultimately, when the whole body is formed, the cells contain only their own kind of determinants. It follows, of course, from this that the cells of the tissues cannot give rise to structures containing less disintegrated nuclear material than their own nuclear material, and least of all to reproductive cells, which must contain the undisintegrated microcosms of the germplasm. As special adaptations for the formation of buds and for the reconstruction of lost parts, cells may be provided with latent groups of determinants to become active only on emergency. But with these exceptions, the nuclear matter of the cells of the body contains only what is called _idioplasm_, a differentiated portion of the germplasm peculiar to cells of their own order, and it can give rise only to idioplasm of the same or of a lower order. And here we come round again to the original observations from which Weismann set out. For he found that among the Hydromedusæ, although the sexual cells seemed to arise in very different topographical positions, there had always been a migration to these localities of a material which he would now call the germplasm. And here also, that the point may be made plain, there may be mentioned the observations of surgeons and physicians, who insist that the growths of disease always conform strictly, in their cellular nature, to the tissues from which they arose, and that in the healing of wounds like only grows from cellular like.