Figure 8_h_ passes through the posterior part of the head-fold, between the limits of the fold of the ectoderm and that of the entoderm. The medullary groove (_mg_) is here very wide and comparatively shallow; its walls are continued laterally as the gradually thinning ectoderm (_ec_). The enteron (_ent_) is completely enclosed, and forms a large, somewhat compressed, thick-walled cavity. Between the dorsal wall of the enteron and the lower side of the medullary canal lies the notochord (_nt_), a small, cylindrical rod of closely packed cells derived, in this region at least, from the entoderm. In the posterior region of the embryo it is not possible to determine with certainty the origin of the notochord, owing to the close fusion of all three germ layers. Between the wall of the enteron and the lower side of the ectoderm is a considerable mass of mesoderm (_mes_), which here consists of more scattered and angular cells than in the preceding section. Figure 8_i_ shows the appearance of a section through the mesoblastic somites, in one of which a small myocœl (_myc_) is seen. As is seen by the size of the figure, which is drawn under the same magnification as were all the sections of the series, the embryo in this region is much smaller in section than it is toward either end, especially toward the anterior end. The medullary groove (_mg_) is still more shallow than in the more anterior sections, and the ectoderm (_ec_), with which its folds are continuous laterally, is here nearly horizontal. The mesoblast (_mes_) is of a more compact nature than in the preceding section and shows little or no sign of cleavage, although a distinct myocœl may be seen and cleavage is well marked in sections between this one and the preceding. The notochord (_nt_) has about the same appearance as in the preceding section, but is more distinctly separated from the surrounding cells.