[39] In certain cases part of the specific feature of the process in question may also depend on the “cause” which is localising it, *e.g.* in the galls of plants. This definition of “cause” in morphology may be artificial; in any case it is clear. And at the same time the concepts of the prospective potency and of the “means” of organogenesis now acquire a clear and definite meaning: potency is the real basis of the specific character of every act in morphogenesis, and “means,” including conditions, are the sum of all external and internal general circumstances which must be present in order that morphogenetic processes may go on, without being responsible for their specificity or localisation. It is implied in these definitions of cause and potency, that the former almost always will be of that general type which usually is called a stimulus or “Auslösung,” to use the untranslatable German word. There is no quantitative correspondence between our “cause” and the morphogenetic effect. *Some Instances of Formative and Directive Stimuli* Again it is to Herbst that we owe not only a very thorough logical analysis of what he calls “formative and directive stimuli”[40] but also some important discoveries on this subject. We cannot do more here than barely mention some of the most characteristic facts. [40] Herbst, “Ueber die Bedeutung die Reizphysiologie für die kausale Auffassung von Vorgängen in der tierischen Ontogenese” (*Biol. Centralblatt*, vols. xiv., 1894, and xv., 1895); *Formative Reize in der tierischen Ontogenese*, Leipzig, 1901. These important papers must be studied by every one who wishes to become familiar with the subject. The present state of science is reviewed in my articles in the *Ergebnisse der Anatomie und Entwickelungsgeschichte*, vols. xi. and xiv., 1902 and 1905. Amongst plants it has long been known that the direction of light or of gravity may determine where roots or branches or other morphogenetic formations are to arise; in hydroids also we know that these factors of the medium may be at work[41] as morphogenetic causes, though most of the typical architecture of hydroid colonies certainly is due to internal causes, as is also much of the organisation in plants. [41] Compare the important papers by J. Loeb, *Untersuchungen zur physiologischen Morphologie der Tiere*, Würzburg, 1891-2. Light and gravity are external formative causes; beside that they are merely “localisers.” But there also are some external formative stimuli, on which depends not only the place of the effect, but also part of its specification. The galls of plants are the most typical organogenetic results of such stimuli. The potencies of the plant and the specific kind of the stimulus equally contribute to their specification; for several kinds of galls may originate on one sort of leaves.