The beginning of the foregut is seen in Figure 3_n_ (_fg_), which is the more nearly median of the two sections, Figure 3_o_ being a short distance to the side of the middle line. In Figure 3_o_ the thin entoderm (_en_) is separated from the much thicker ectoderm (_ec_) by a considerable layer of rather loose mesoderm (_mes_). In Figure 3_n_, which is almost exactly median in position, there is, of course, no mesoderm to be seen in front of the blastopore, and the entoderm shows a considerable increase in thickness, due to the formation of the notochord (_nt_). The blastopore (_blp_) is the most striking feature of the figure, and is remarkable for its great width in an antero-posterior direction. Its anterior and posterior borders are outlined by sharply defined layers of ectoderm and entoderm. Posterior to the blastopore the lower side of the ectoderm is continuous with a considerable mass of cells, the primitive streak (_ps_). STAGE III FIGURES 4, 4_a_, 5, 5_a_ AND 6-6_i_ (PLATES X., XI.) “Figures 4 and 4_a_ are of an embryo removed, on June 18th, from an egg which had been taken out of an alligator two days before. Figure 4, a dorsal view, is of special interest in that it shows a secondary fold taking place in the head-fold. This grows posteriorly along the median dorsal line, forming a V-shaped process with the apex pointing backward toward the blastopore. There is quite a deep groove between the arms of the V. The head-fold on the ventral side, as seen in Figure 4_a_, made from the same embryo as Figure 4, grows most rapidly on the mid-line, and also becomes thicker at that place. The medullary folds now begin to form on either side of the medullary groove, ending posteriorly on either side of the blastopore and anteriorly on either side of the point of the V-shaped process in the middle of the head-fold. This is seen in Figure 5, which is a dorsal view of an embryo from an egg three days after it was taken out of an alligator. A ventral view of the same embryo (Fig. 5_a_) represents the thickened process on the mid-line at its greatest development. For some reason the notochord did not show in this embryo, possibly owing to particles of the yolk material adhering about the mid-line. “In an embryo a day or two older, the V-shaped fold of the head-fold is seen to have broken through at the apex, and each of the arms thus separated from one another unites with the medullary fold of its respective side. This can be seen in Figure 6, which is a dorsal view of part of an embryo a day or two older than the one represented by Figures 5 and 5_a_.