Almost independently another German author, Wilhelm Roux,[10] has advocated a theoretical view of morphogenesis which very closely resembles the hypothesis of Weismann. According to Roux a minute ultimate structure is present in the nucleus of the germ and directs development by being divided into its parts during the series of nuclear divisions. [10] *Die Bedeutung der Kernteilungsfiguren*, Leipzig, 1883. But in spite of this similarity of the outset, we enter an altogether different field of biological investigation on mentioning Roux’s name: we are leaving hypothetic construction, at least in its absoluteness, and are entering the realms of scientific experiment in morphology. EXPERIMENTAL MORPHOLOGY I have told you already in the last lecture that, while in the eighteenth century individual morphogenesis had formed the centre of biological interest and been studied experimentally in a thoroughly adequate manner, that interest gradually diminished, until at last the physiology of form as an exact separate science was almost wholly forgotten. At least that was the state of affairs as regards zoological biology; botanists, it must be granted, have never lost the historical continuity to such a degree; botany has never ceased to be regarded as one science and never was broken up into parts as zoology was. Zoological physiology and zoological morphology indeed were for many years in a relationship to one another not very much closer than the relation between philology and chemistry. There were always a few men, of course, who strove against the current. The late Wilhelm His,[11] instance, described the embryology of the chick in an original manner, in order to find out the mechanical relations of embryonic parts, by which passive deformation, as an integrating part of morphogenesis, might be induced. He also most clearly stated the ultimate aim of embryology to be the mathematical derivation of the adult form from the distribution of growth in the germ. To Alexander Goette[12] we owe another set of analytical considerations about ontogeny. Newport, as early as 1850, and in later years Pflüger and Rauber, carried out experiments on the eggs of the frog, which may truly be called anticipatory of what was to follow. But it was Wilhelm Roux,[13] now professor of anatomy at Halle, who entered the field with a thoroughly elaborated programme, who knew not only how to state the problem analytically, but also how to attack it, fully convinced of the importance of what he did. “Entwickelungsmechanik,”--mechanics of development--he called the “new branch of anatomical science” of which he tried to lay the foundations. [11] *Unsere Körperform*, Leipzig, 1875. [12] *Die Entwickelungsgeschichte der Unke*, Leipzig, 1875.