The older research into the origin of the nutritive apparatus in the metazoa--especially its chief part, the alimentary or gastric canal--had led to the erroneous belief that in several groups of the metazoa it owed its origin to very different growth-processes, and that particularly in the higher vertebrates (the amniotes) it was a comparatively late product of evolution. On the other hand, the comparative study of the embryology of the lower and higher animals led me thirty-four years ago to the opposite conclusion, that a simple gastric sac was the first and oldest organ of all the metazoa, and that all the different forms of it had been developed from this primitive type. I gave this view in my _Biology of the Sponges_ in 1872; and I developed and established it in my _Studies of the Gastræa Theory_ in 1873. In the latter book I also worked out the important conclusions that follow from this monistic reform of the theory of germinal layers for the phylogenetic natural classification of the animal kingdom. I began with the consideration of the simplest sponges (_olynthus_) and cnidaria (_hydra_). The whole body of these lowest and oldest of the cœlenteria is in essence nothing but a round, oval, or cylindrical gastric vesicle, a digestive sac, the thin wall of which consists of two simple layers of cells. The outer layer (the ectoderm or skin-layer) is the covering layer of the external skin (epidermis); it is the instrument of sensation and movement. The inner layer of cells (entoderm or gastric layer) serves for nutrition; it clothes the simple cavity of the sac, which admits the food by its opening and digests it. This opening is the primitive mouth (_prostoma_ or _blastoporus_), the inner cavity itself the primitive gut (_progaster_ or _archenteron_). I proved that there was the same composition in the young embryos or larvæ of many of the lower animals, and showed that the manifold and apparently very different embryonic form of all the higher animals may be reduced to the same common type. To this I gave the name of the "cup-embryo" or gastric larvæ (_gastrula_), and concluded, in virtue of the biogenetic law, that it is the palingenetic reproduction of a corresponding ancestral form (the _gastræa_) maintained until the present by heredity. It was not until much later (1895) that Monticelli discovered a modern gastræad (_pemmatodiscus_) which corresponds completely to this hypothetical ancestor (see the last edition of my _Anthropogeny_, fig. 287). The simplest living forms of the sponges (_olynthus_) and the cnidaria (_hydra_) only differ from this hypothetical primitive form of the gastræa by a few secondary and subsequently acquired features.