Like the adult, the germ also requires a certain amount of heat, oxygen, and, when it grows up in the sea, salinity in the medium. For the germ, as for the adult, there exists not only a minimum but also a maximum limit of all the necessary factors of the medium; the same factor which at a certain intensity promotes development, disturbs it from a certain other intensity upwards. Within the limits of this minimum and this maximum of every outside agent there generally is an increase in the rate of development corresponding to the increase of intensity of the agent. The acceleration of development by heat has been shown to follow the law of the acceleration of chemical processes by a rise of temperature; that seems to prove that certain chemical processes go on during the course of morphogenesis. Almost all that has been investigated of the part played by the external conditions of development has little bearing on specific morphogenesis proper, and therefore may be left out of account here: we must, however, lay great stress on the general fact that there *is* a very close dependence of morphogenesis on the outside factors, lest we should be accused afterwards of having overlooked it. Of course all “external” means or conditions of morphogenesis can actually relate to morphogenetic processes only by becoming in some way “internal,” but we unfortunately have no knowledge whatever how this happens. We at present are only able to ascertain what must necessarily be accomplished in the medium, in order that normal morphogenesis may go on, and we can only suppose that there exist certain specific internal general states, indispensable for organogenesis but inaccessible to present modes of investigation.[36] [36] Compare the analytical discussions of Klebs, to whom we owe a great series of important discoveries in the field of morphogenetic “means” in botany. (*Willkürliche Entwickelungsänderungen bei Pflanzen*, Jena, 1903; see also *Biol. Centralblatt*, vol. xxiv., 1904, and my reply to Klebs, *ibid.* 23, 1903.)