The first Scholion is directed against the theory of preformation, and succeeds in refuting it on the ground of simple observation. The theme of the second Scholion is that the essential nature (_die Wesenheit_) of the animal determines its differentiation, that no stage of development is solely determined by the antecedent stage, but that throughout all stages the _Wesenheit_ or idea of the definitive whole exercises guidance. This guidance is shown most clearly in the regulatory processes of the germ, whereby the large individual variations commonly presented by the early embryo are compensated for or neutralised in the course of further development. Baer in this shows himself a vitalist. It is, however, the third and subsequent Scholia which must here particularly occupy our attention, for it is in these that von Baer comes to grips with morphological problems. Already in the second Scholion he had definitely enunciated the law which runs as a theme throughout the volume, the observational and the theoretical part alike, the law that development is essentially a process of differentiation by which the germ becomes ever more and more individualised. "The essential result of development," he writes, "when we consider it as a whole, is the increasing independence (_Selbstaendigkeit_) of the developing animal" (p. 148). In the third Scholion he elaborates this thought and shows that differentiation takes place in triple wise. The three processes of differentiation are "primary differentiation" or layer-formation, "histological differentiation" within the layers, and the "morphological differentiation" of primitive organs. The first of these differentiations in time is the formation of the germ-layers, which takes place by a splitting or separation of the blastoderm into a series of superimposed lamellae. Baer's account of the process in the chick is as follows:--