The four secondary germinal layers are so distributed in the structure of the body in all the cœlomaria (or all metazoa that have a body-cavity) that the outer two, joined fast together, constitute the body-wall, and the inner two the ventral wall; the two walls are separated by the cavity of the cœlom. Each of the walls is made up of a limiting layer and a middle layer. The two limiting layers chiefly give rise to epithelia, or covering-tissues, and glands and nerves, while the middle layers form the great bulk of the fibrous tissue, muscles, and connective matter. Hence the latter have also been called fibrous or muscular layers. The outer middle layer, which lies on the inner side of the skin-sense-layer, is the skin fibre-layer; the inner middle layer, which attaches from without to the ventral glandular layer, is the ventral fibre layer. The former is usually called briefly the parietal, and the latter the visceral layer or mesoderm. Of the many different names that have been given to the four secondary germinal layers, the following are those most in use to-day:— 1. Skin-sense-layer (outer limiting layer). I. Neural layer (_neuroblast_). The two secondary germinal layers of the body-wall: I. Epithelial. II. Fibrous. 2. Skin-fibre-layer (outer middle layer). II. Parietal layer (_myoblast_). 3. Gut-fibre-layer (inner middle layer). III. Visceral layer (_genoblast_). The two secondary germinal layers of the gut-wall: III. Fibrous. IV. Epithelial. 4. Gut-gland-layer (inner limiting layer). IV. Enteral layer (_enteroblast_) The first scientist to recognise and clearly distinguish the four secondary germinal layers was Baer. It is true that he was not quite clear as to their origin and further significance, and made several mistakes in detail in explaining them. But, on the whole, their great importance did not escape him. However, in later years his view had to be given up in consequence of more accurate observations. Remak then propounded a three-layer theory, which was generally accepted. These theories of cleavage, however, began to give way thirty years ago, when Kowalevsky (1871) showed that in the case of _Sagitta_ (a very clear and typical subject of gastrulation) the two middle germinal layers and the two limiting layers arise not by cleavage, but by folding—by a secondary invagination of the primary inner germ-layer. This invagination or folding proceeds from the primitive mouth, at the two sides of which (right and left) a couple of pouches are formed. As these cœlom-pouches or cœlom-sacs detach themselves from the primitive gut, a double body-cavity is formed (Figs. 74–76). Fig.77. Coelomula of sagitta, in section. Fig. 77—Cœlomula of sagitta, in section. (From _Hertwig._) _D_ dorsal side, _V_ ventral side, _ ik_ inner germinal layer, _mv_ visceral mesoblast, _ lh_ body-cavity, _mp_ parietal mesoblast, _ak_ outer germinal layer.