Fig.36 Gastrula of a lower sponge (olynthus). Fig. 36—Gastrula of a lower sponge (lynthus). _A_ external view, _B_ longitudinal section through the axis, _g_ primitive-gut cavity, a primitive mouth-aperture, _i_ inner cell-layer (entoderm, endoblast, gut-layer), _e_ external cell-layer (outer germinal layer, ectoderm, ectoblast, or skin-layer). The two layers of cells which line the gut-cavity and compose its wall are of extreme importance. These two layers, which are the sole builders of the whole organism, are no other than the two primary germinal layers, or the primitive germ-layers. I have spoken in the introductory section (Chapter III) of their radical importance. The outer stratum is the skin-layer, or _ectoderm_ (Figs. 30–35_e_); the inner stratum is the gut-layer, or _entoderm_ (_i_). The former is often also called the ectoblast, or epiblast, and the latter the endoblast, or hypoblast. _From these two primary germinal layers alone is developed the entire organism of all the metazoa or multicellular animals._ The skin-layer forms the external skin, the gut-layer forms the internal skin or lining of the body. Between these two germinal layers are afterwards developed the middle germinal layer (_mesoderma_) and the body-cavity (_cœloma_) filled with blood or lymph. The two primary germinal layers were first distinguished by Pander in 1817 in the incubated chick. Twenty years later (1849) Huxley pointed out that in many of the lower zoophytes, especially the medusæ, the whole body consists throughout life of these two primary germinal layers. Soon afterwards (1853) Allman introduced the names which have come into general use; he called the outer layer the _ectoderm_ (“outer-skin”), and the inner the _entoderm_ (“inner-skin”). But in 1867 it was shown, particularly by Kowalevsky, from comparative observation, that even in invertebrates, also, of the most different classes—annelids, molluscs, echinoderms, and articulates—the body is developed out of the same two primary layers. Finally, I discovered them (1872) in the lowest tissue-forming animals, the sponges, and proved in my gastræa theory that these two layers must be regarded as identical throughout the animal world, from the sponges and corals to the insects and vertebrates, including man. This fundamental “homology [identity] of the primary germinal layers and the primitive gut” has been confirmed during the last thirty years by the careful research of many able observers, and is now pretty generally admitted for the whole of the metazoa.