Figure 9_d_, although only five sections posterior to the preceding, shows a marked change in structure. The medullary canal (_mc_) is here of the typical outline for embryos of this age. A large, compact mass of cells (_ent_) appears at first glance to be the same that was noted in the preceding stage at the tip end of the turned-under medullary canal; it is, however, the extreme anterior wall of the enteron, which is in close contact with the above-mentioned tip of the medullary canal. Between this anterior wall of the enteron, of which wall it is really a part, and the medullary canal is the notochord (_nt_). The space surrounding the notochord and enteron is filled with a fairly compact mass of typical, stellate mesoblast cells. The depression of the ectoderm (_ec_) and entoderm (_en_) of the blastoderm caused by the formation of the head-fold is here less marked, and the dorsal side of the embryo in this region is slightly elevated above the level of the blastoderm. Figure 9_e_ represents a section passing through the posterior edge of the head-fold. The epidermal ectoderm is here continuous with the thin layer of superficial ectoderm (_ec_) of the blastoderm, while the entoderm (_en_) of the blastoderm is still continuous beneath the embryo. The thick ectoderm of the embryo is sharply differentiated from the thin layer of ectoderm that extends laterally over the yolk. The pharynx (_ent_) is a large cavity whose wall is thick except at the dorsal side, where it is thin and somewhat depressed, apparently to make room between it and the medullary canal for the notochord (_nt_). Figure 9_f_ is about twenty sections posterior to the preceding section, and passes through the point of separation of the folds of the entoderm (_en_). From this point the entoderm gradually flattens out, leaving the enteron unenclosed. The medullary canal (_mc_) and notochord (_nt_) are about as in the preceding section, but the ectoderm (_ep_) is somewhat thinner and more flattened. The mesoderm (_mes_) on the right side exhibits a distinct cleavage, the resulting body cavity (_bc_) being a large, triangular space.