Section 11. Simultaneously, on Either Side of the Notochord Appear a

series of solid masses of cells, derived mainly by cell division from the cells of the wall of the archenteron, and filling up and obliterating the segmentation cavity. These masses increase in number by the addition of fresh ones behind, during development, and are visible in the dorsal view as brick-like masses, the mesoblastic somites or proto-vertebrae (Figure 6, i., ii., iii.). In Figure 7, these masses are indicated by dotting. In such a primitive type as amphioxus these mesoblastic -somites- [masses] contain a cavity, destined to be the future body cavity, from the first. In the frog, the cavity is not at first apparent; the mesoblast at first seems quite solid, but subsequently what is called the splitting of the mesoblast occurs, and the body cavity (b.c. in Figure 7) appears. The outer mesoblast, lying immediately under the epiblast, constitutes the substance of the somatopleur, and from it will be formed the dermis, the muscles of the body wall, almost all the cartilage and bone of the skeleton, the substance of the limbs, the kidneys, genital organs, heart and bloodvessels, and, in short, everything between the dermis and the coelom, except the nervous system and nerves, and the notochord. The inner mesoblast, the mass of the splanchnopleur, will form the muscle and connective tissue of the wall of the alimentary canal, and the binding substance of the liver and other glands that open into the canal.