Analytically we must carefully discriminate between the increase in the size of the cavities of an organism by a passive extension of their surfaces and the proper growth of the individual cells, which again may be due either to mere extension or to real assimilation. Osmotic pressure, of course, plays an important part both in the growth of the body-cavities and in simple cellular extension. We repeat the caution against believing too much to be explained by this phenomenon: it is the organism which by the secretion of osmotic substances in the cavities or the protoplasm of the cells prepares the ground for growth even of this osmotic sort. The real cellular growth which proceeds on the basis of assimilation cannot, of course, be accounted for by osmotic events, not even in its most general type. Ontogenetical growth generally sets in, both in animals and in plants, after the chief lines of organisation are laid out; it is only the formation of the definite histological structures which usually runs parallel to it. *On Cell-division.*--We have already said a good deal about the importance of cell-division in ontogeny: it accompanies very many of the processes of organisation in all living beings. But even then, there are the Protozoa, in the morphogenesis of which it does not occur at all, and there have also become known many cases of morphogenesis in higher animals, mostly of the type of regulation, in which cellular division is almost or wholly wanting. Therefore, cellular division cannot be the true reason of differentiation, but is only a process, which though necessary in some cases, cannot be essential to it. It must be conceded, I believe, that the same conclusion can be drawn from all our experiments on very young stages of the germ. The investigations of the last few years have made it quite clear that even in organisms with a high power of morphogenetic regulation it is always the form of the whole, but not the individual cell, which is subjected to the regulation processes. Starting from certain results obtained by T. H. Morgan, I was able to show that in all the small but whole larvae, reared from isolated blastomeres, the size of the cells remains normal, only their number being reduced; and Boveri has shown most clearly that it is always the size of the nucleus--more correctly, the mass of the chromatin--which determines how large a cell of a certain histological kind is to be. In this view, the cell appears even more as a sort of material used by the organism as supplied, just as workmen can build the most different buildings with stones of a given size. β″. *The External Means of Morphogenesis* We now know what internal means of morphogenesis are, and so we may glance at some of the most important “outer means” or “conditions” of organisation.