[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.
Scarcely any exterior formative stimuli are responsible for animal organisation; and one would hardly be wrong in saying that this morphogenetic independence in animals is due to their comparatively far-reaching functional independence of those external agents which have any sort of direction. But many organogenetic relations are known to exist between the single parts of animal germs, each of these parts being in some respect external to every other; and, indeed, it might have been expected already *a priori*, that such formative relations between the parts of an animal embryo must exist, after all we have learned about the chief lines of early embryology. If differentiation does not go on after the scheme of Weismann, that is, if it is not carried out by true “evolutio” from within, how could it be effected except from without? Indeed, every embryonic part may in some respect be a possible cause for morphogenetic events, which are to occur on every other part: it is here that the very roots of epigenesis are to be found.