then (since 1886) occupied with the embryology of the sponges, corals, medusæ, and siphonophoræ, and, as I found the same formation of two primary germ layers everywhere in these lowest classes of multicellular animals, I came to the conclusion that this important embryonic feature is common to the entire animal world. The circumstance that in the sponges and the cnidaria (polyps, medusæ, etc.) the body consists for a long time, sometimes throughout life, merely of two simple layers of cells, seemed to me especially significant. Huxley had already (1849) compared these, in the case of the medusæ, with the two primary germinal layers of the vertebrates. On the ground of these observations and comparisons I then, in 1872, in my _Philosophy of the Calcispongiae_, published the "theory of the gastræa," of which the following are the essential points: I. The whole animal world falls into two essentially different groups, the unicellular primitive animals (Protozoa) and the multicellular animals with complex tissues (Metazoa). The entire organism of the protozoon (the rhizopods of the infusoria) remains throughout life a single simple cell (or occasionally a loose colony of cells without the formation of tissue, a _coenobium_). The organism of the metazoon, on the contrary, is only unicellular at the commencement, and is subsequently built up of a number of cells which form tissues. II. Hence the method of reproduction and development is very different in each of these great categories of animals. The protozoa usually multiply by _non-sexual_ means, by fission, gemmation, or spores; they have no real ova and no sperm. The metazoa, on the contrary, are divided into male and female sexes, and generally propagate sexually, by means of true ova, which are fertilized by the male sperm. III. Hence, further, true germinal layers, and the tissues which are formed from them, are found only in the metazoa; they are entirely wanting in the protozoa. IV. In all the metazoa only two primary layers appear at first, and these have always the same essential significance; from the _outer_ layer the external skin and the nervous system are developed; from the _inner_ layer are formed the alimentary canal and all the other organs. V. I called the germ, which always arises first from the impregnated ovum, and which consists of these two primary layers, the "gut-larva," or the _gastrula_; its cup-shaped body with the two layers encloses originally a simple digestive cavity, the primitive gut (the _progaster_ or _archenteron_), and its simple opening is the primitive mouth (the _prostoma_ or _blastoporus_). These are the earliest organs of the multicellular body, and the two cell layers of its enclosing wall, simple epithelia, are its earliest tissues; all the other organs and tissues are a later and secondary growth from these.