Figure 6_g_ is nearly one third the length of the embryo posterior to the preceding and passes through the posterior third of the embryo. The medullary thickening of the ectoderm (_ec_) is marked, but shows no sign of a medullary groove; in fact, the median line is the most elevated region of the ectoderm. The notochord (_nt_) is larger in cross-section than in the more anterior regions. It is still continuous with the entoderm (_en_) and is fairly closely attached to, though apparently not continuous with, the mesoderm (_mes_) on each side. Figure 6_h_ passes through the blastopore (_blp_). The appearance of the section is almost identical with that of Figure 2_f_, already described. Figure 6_i_ is five sections posterior to the preceding and has about the same structure as the corresponding sections in the preceding two stages, where this region of the embryo was injured, and hence not drawn. Continuous with the posterior border of the blastopore (seen in the preceding figure) is the deep furrow, the primitive groove (_pg_). The ectoblast (_ec_) bordering this groove is much thickened and may be called the primitive streak. The lower side of this primitive streak is continuous with the mesoblast (_mes_), while the entoderm (_en_) is here entirely distinct from the mesoderm. It is evident that the mesoderm posterior to the blastopore is proliferated from the lower side of the ectoblast and not from the upper side of the entoblast, as is the case anterior to the blastopore. The primitive groove gradually becomes more and more shallow, as it is followed toward the posterior, until it is no longer discernible; back of this point the primitive streak may be traced for a considerable distance, becoming thinner and thinner until it too disappears, and there remains only the slightly thickened ectoblast underlaid by the thin and irregular layers of mesoblast and entoblast. The primitive streak may be traced for a distance equal to about one third the distance between the head-fold and the blastopore. STAGE IV FIGURES 7_a_-7_h_ (PLATES XI., XII.) No surface view of this stage was seen by the writer, and hence is not figured. The figures were drawn from one of the series of sections obtained through the courtesy of Prof. S. F. Clarke. This series was marked “3 Urwirbeln,” so that the embryo was apparently slightly younger than the youngest stage obtained by myself and represented in Figures 8 and 8_a_. Figure 7_a_ represents a section that passed through the head-fold of the amnion (_a_) just in front of the head-fold of the embryo; the amniotic cavity here appears as a large empty space.