[13] *Gesammelte Abhandlungen*, Leipzig, 1895. Most important theoretical papers:--*Zeitschr. Biolog.* 21, 1885; *Die Entwickelungsmechanik der Organismen*, Wien, 1890; *Vorträge und Aufsätze über Entwickelungsmechanik*, Heft i., Leipzig, 1905. I cannot let this occasion pass without emphasising in the most decided manner how highly in my opinion Roux’s services to the systematic exploration of morphogenesis must be esteemed. I feel the more obliged to do so, because later on I shall have to contradict not only many of his positive statements but also most of his theoretical views. He himself has lately given up much of what he most strongly advocated only ten years ago. But Roux’s place in the history of biological science can never be altered, let science take what path it will. It is not the place here to develop the logic of experiment; least of all is it necessary in the country of John Stuart Mill. All of you know that experiment, by its method of isolating the single constituents of complicated phenomena, is the principal aid in the discovery of so-called causal relations. Let us try then to see what causal relations Wilhelm Roux established with the aid of morphogenetic experiment. THE WORK OF WILHELM ROUX We know already that an hypothesis about the foundation of individual development was his starting-point. Like Weismann he supposed that there exists a very complicated structure in the germ, and that nuclear division leads to the disintegration of that structure. He next tried to bring forward what might be called a number of indicia supporting his view. A close relation had been found to exist in many cases between the direction of the first cleavage furrows of the germ and the direction of the chief planes of symmetry in the adult: the first cleavage, for instance, very often corresponds to the median plane, or stands at right angles to it. And in other instances, such as have been worked out into the doctrine of so-called “cell-lineages,” typical cleavage cells were found to correspond to typical organs. Was not that a strong support for a theory which regarded cellular division as the principal means of differentiation? It is true, the close relations between cleavage and symmetry did not exist in every case, but then there had always happened some specific experimental disturbances, *e.g.* influences of an abnormal direction of gravity on account of a turning over of the egg, and it was easy to reconcile such cases with the generally accepted theory on the assumption of what was called “anachronism” of cleavage.