Figure 7_b_ is several sections posterior to the preceding; it passes through the head-fold of the embryo, but is just back of the head-fold of the amnion. The deep depression of the ectoderm (_ec_) and entoderm (_en_) caused by the head-fold is plainly seen. In this depression lie the ends of the medullary folds, distinct from each other both dorsally and ventrally. Each medullary fold is made up of two parts--a medial, more dense nervous layer (_nl_), and a distal, less dense epidermal layer (_ep_). The section corresponding to this one will be more fully described in connection with the following stage. Figure 7_c_ is some distance posterior to the preceding, though the exact distance could not be determined because of a break in the series at this point. The section passes through the posterior limit of the head-fold. The medullary groove (_mg_) is very deep and comparatively wide; around its sides the germ layers are so closely associated that they may scarcely be distinguished. Ventral to the medullary groove the foregut (_fg_) is seen as a crescentic slit. Figure 7_d_ is about a dozen sections posterior to the one just described and is about one seventh the length of the embryo from the anterior end. The embryo is much more compressed, in a dorso-ventral direction, and the medullary groove (_mg_) is correspondingly more shallow. Where the ectoderm (_ec_) curves over to form the medullary folds it becomes much more compact and somewhat thicker. The notochord (_nt_) is large and distinct, but is still fused with the entoderm (_en_). The mesoderm (_mes_) forms a well-defined layer, entirely distinct from both the notochord and the entoderm. From this region, as we pass caudad, the size of the embryo in cross-section gradually decreases and the medullary groove becomes more shallow. Figure 7_e_ is about one third of the length of the embryo from the posterior end, and is only a few sections from the caudal end of the medullary groove. The ectoderm (_ec_) is much thinner than in the preceding figure and the medullary groove (_mg_) is much more shallow. The notochord (_nt_) is of about the same diameter as before, but is here quite distinct from the entoderm (_en_) as well as from the mesoderm (_mes_). Figure 7_f_ is seven sections posterior to Figure 7_e_. The medullary groove has disappeared and the medullary folds have flattened to form what might be called a medullary plate (at the end of the reference line _ec_), which continues to the anterior border of the blastopore. The notochord (_nt_) is larger in cross-section than in the more anterior regions; it is still distinct from the entoderm. Figure 7_g_ passes through the blastopore and shows essentially the same structure as was described in connection with the corresponding section of Stage I (Fig. 2_f_).