Figure 9_g_, the twenty-fifth section posterior to that represented in Figure 9_f_, shows a marked change in the form of the embryo. While of about the same lateral dimensions, the dorso-ventral diameter of the embryo in this region is less than one half what it was in the head region. The epidermal ectoderm (_ep_) is now nearly horizontal in position and is not so abruptly separated laterally from the thin lateral sheets of ectoblast. The medullary groove (_mg_) is here a very narrow vertical slit. At this stage the fusion of the medullary folds has taken place over the anterior third of the embryo. For a short distance, represented in about thirty-five sections, the canal is open as in the figure under discussion; for the next one hundred sections (about one third the length of the embryo) in the region of the mesoblastic somites the canal is again closed, while throughout the last one third of its length the canal is widely open dorsally. The enteron is here entirely open ventrally, the entoderm being almost flat and horizontal. The notochord (_nt_) is distinctly outlined and is somewhat flattened in a dorso-ventral direction. The body cavity (_bc_) is well marked, but is separated by a considerable mass of uncleft mesoblast from the notochord and the walls of the medullary groove. A space of about one hundred sections, or one third the length of the embryo, intervenes between Figures 9_g_ and 9_i_. This is the region of the mesoblastic somites, and in this region, as has been above stated, the medullary canal is completely enclosed. It is evident then that the entire anterior two thirds of the medullary canal is enclosed except for the short region represented in Figure 8_g_. Whether or not this short open region between the two longer enclosed regions is a normal condition the material at hand does not show. Figure 9_h_ represents a typical section in the region of the mesoblastic somites just described. It shows the enclosed medullary canal (_mc_), the body cavity (_bc_) on the right, and a mesoblastic somite with its small cavity (_myc_) on the left. The entire section is smaller than the sections anterior or posterior to this region, and seems to be compressed in a dorso-ventral direction, this compression being especially marked in the case of the notochord. Figure 9_i_ is through a region nearly one hundred sections posterior to the preceding, and cuts the embryo, therefore, through the posterior one fourth of its length. The chief difference between this and the preceding section is in the medullary canal, which is here open and is in the form of a wide groove with an irregular, rounded bottom and vertical sides. The size of the section is considerably greater than in the preceding, the increase being especially noticeable in the notochord (_nt_), which is cut near its posterior end. There is little or no sign of mesoblastic cleavage.