Figure 3_i_, still farther toward the blastopore, shows the medullary groove wider and shallower than in the more anterior sections. The mesoderm (_mes_) is here a layer laterally distinct from the entoderm. In the middle line it is still continuous with the entoderm, and at this place it is the more dense mass of cells that may be recognized as the notochord (_nt_). It is evidently difficult to decide whether this group of cells (_nt_), which will later become a distinct body, the notochord, is derived directly from the entoderm or from the mesoderm, which is itself a derivative of the entoderm. There is here absolutely no line of demarcation between the cells of the notochord and those of the mesoderm and entoderm. In Figure 3_j_ the ectoderm (_ec_) is nearly flat, scarcely a sign of the medullary groove appearing. The mesoderm (_mes_) is here a distinct layer, entirely separate from both notochord (_nt_) and entoderm (_en_). The notochord is a clearly defined mass of cells, distinct, as has been said, from the mesoderm, but still closely united with the underlying entoderm, which is much thinner than the ectoderm. This condition of the notochord, which is found throughout about one third of the length of the embryo, would give the impression that the notochord is of a distinctly entodermal origin. In Figure 3_k_ there is no sign of the medullary groove, though the ectoderm (_ec_) is still much thickened in the middle line. The section passes, posterior to the notochord, through the anterior edge of the ventral opening of the blastopore (_blp_). The mesoderm (_mes_) is here again continuous with the entoderm, around the edge of the blastopore, but is distinct from the ectoderm. Figure 3_l_ represents the third section posterior to the preceding. The blastopore, which passes upward and backward through the blastoderm, is seen as an enclosed slit (_blp_). It is surrounded by a distinct layer of compactly arranged cells continuous with the thickened ectoderm (_ec_) above, with the thin entoderm (_en_) below, and laterally with the gradually thinning and scattering mesoderm (_mes_). Figure 3_m_ is the next section posterior to the one just described. It passes through the dorsal opening of the blastopore (_blp_), which appears as a deep, narrow cleft with thick ectodermal borders. The three germ layers are still continuous with each other, though the connection of the entoderm with the other two is slight. The sections posterior to this one will be described in the next stage, where they have essentially the same structure and are better preserved. Figures 3_n_ and 3_o_ are sagittal sections of an embryo of about the stage under discussion. In both figures the head-fold is seen as a deep loop of ectoderm and entoderm, while the head-fold of the amnion is seen at _a_.