EXPERIMENTAL MORPHOGENESIS 103 



most of the typical architecture of hydroid colonies certainly 

 is due to internal causes, as is also much of the organisation 

 in plants. 



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 inorphogenetic independence in animals 

 is due to their comparatively far-reaching functional inde- 

 pendence 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 morpho- 

 geuetic events, which are to occur on every other part : it is 

 here that the very roots of epigenesis are to be found. 



Heliotropisin and geotropism are among the well-known 



