The first Changes in the form and arrangement of the cells give rise to three definite plates,. Or germ layers, which are termed from their posi tions the ectoderm (outer skin), mesoderm (middle skin) and entoderm (inner skin). Since the ectoderm covers the body, it is primarily pro tective in function, but it also gives rise to the nervous system, through which sensations are received from the outer world. The entoderm, on the other hand, lines the digestive canal and is from the first nutritive in function. The mesoderm, lying between the other two layers, naturally performs. The functions of circulation, Of muscular movement, and of ex cretion; it also gives rise to the skeletal structures which support the body. While all three germ layers form definite Sheets of cells known as epi thelia, the mesoderm takes also the form of a diffuse network of cells, the mesenchyma.