Figure 11_f_ is about one tenth the length of the embryo posterior to Figure 11_e_. The chief differences here noticed are in the enteric and cœlomic cavities. The former is no longer enclosed, a dorsal fold in the entoderm being all that remains of the cavity that was seen in the more anterior figures, while the latter is here reduced to a narrow cleft between the somatic and splanchnic mesoblast. A thickening of the mesoblast on either side of the notochord, especially on the left, represents a mesoblastic somite. The medullary canal (_mc_) is more open than in the more anterior sections. For about one third of the length of the embryo posterior to Figure 11_f_ there is a gradual flattening, in a dorso-ventral direction, with loss of the amnion, until the condition represented in Figure 11_g_ is reached. The most striking feature of this region is the great thickness of the ectoderm (_ec_), which is still made up of scattered, irregular cells. In the middle line, directly over the medullary canal (here a nearly cylindrical tube), is a sort of break in the ectoderm, as though there had not been a complete fusion of the epidermal layer when the nervous layer came together on the closure of the medullary groove. This break in the ectoderm may be followed back to the region of the primitive streak, and will be mentioned again. As has been noted, the medullary canal (_mc_) is nearly circular in cross-section, and is closely underlaid by the notochord (_nt_), which is several times the diameter that it was in more anterior sections. The mesoblast (_mes_) is a comparatively thin layer, intermediate in thickness between the ectoderm and entoderm. It shows laterally a slight separation to form the body cavity. Figure 11_h_ is about ten sections posterior to Figure 11_g_, and differs from it chiefly in that the notochord (_nt_) is continuous with the lower side of the medullary canal (_mc_), though still distinct from the underlying entoderm (_en_). Figure 11_i_, four sections farther from the head, shows the same greatly thickened ectoderm (_ec_) with the same break (_ec′_) in the middle line. The section is posterior to the notochord and passes through the anterior edge of the blastopore or, as it may now perhaps better be called, the neurenteric canal. The cells of the medullary wall are continuous with those of the entoderm. The mesoderm (_mes_) is still distinct from the other germ layers. Figure 11_j_ is the next section posterior to the one just described and differs from it only in showing the actual opening of the neurenteric canal (_nc_) into the medullary canal (_mc_). The medullary canal extends, with gradually diminishing caliber, for about fifteen sections posterior to the point at which the neurenteric canal empties into it. The mesoblast (_mes_) is so closely attached to the lower wall of the neurenteric canal that it seems to be actually continuous with it.