Figure 8_e_ represents a section a short distance posterior to the one just described, and passes through the short region where the dorsal edges of the medullary folds have fused with each other. The ventral side of the medullary groove (_mg_) is, as in the preceding section, still unclosed. An epidermal layer of ectoblast (_ep_) is now distinct from the nervous layer (_nl_). Figure 8_f_ is through a region still farther toward the posterior end. Here the medullary groove is _again_ open above, and is still open below. A well-marked space is seen between the epidermal (_ep_) and nervous (_nl_) layers of the ectoderm, but no mesoblast is yet to be seen. Figure 8_g_ passes through the middle part of the head-fold, and shows that the medullary folds in this region are fused below, but are widely separated above, where their margins are markedly bent away from the mid-line. Between the epidermal and nervous layers of the ectoderm a considerable mass of mesoderm cells (_mes_) is seen. The curious appearance of the preceding four figures, as well as the first three figures of the next stage, was at first quite puzzling, until a model of the embryo was made from a series of sections. It was then plain, as would have been the case before, except for the unusual depth dorso-ventrally of the head of the embryo, why the medullary canal should at the extreme anterior end be open both dorsally and ventrally, while a few sections caudad it is open only ventrally, and still farther toward the tail it is again open both above and below. These conditions are produced by the bending under of the anterior region of the medullary folds, probably by the formation of the head-fold. It is apparently a process distinct from the ordinary cranial flexure, which occurs later. The fusion of the medullary folds to form a canal begins, as has been already mentioned, near the anterior end, whence it extends both forward and backward. Hence, if the anterior ends of the medullary folds be bent downward and backward, their unfused dorsal edges will come to face ventrally instead of dorsally and sections through the anterior part of this bent-under region will show the medullary canal open both above and below, as in Figure 8_d_, while sections farther caudad pass through the short region where the folds are fused, and we have the appearance represented in Figure 8_e_. In Figure 8_f_ is shown a section passing posterior to the short, fused region of the folds, and we again have the medullary canal open both above and below. Figure 8_g_ represents a section through the tip of the bent-under region of the medullary folds, which are here fused below and open above.