Figure 2_d_ represents the condition of the blastoderm throughout about one third of its length, posterior to the preceding section. The somewhat regular folds in the ectoderm (_ec_) are probably not medullary folds, but are such artificial folds as might easily be produced in handling the delicate blastoderm. The thickening of the entoderm, noticed in the preceding figure, is here more sharply defined, and as we pass toward the blastopore becomes separated somewhat from the entoderm proper as a middle layer or mesoderm (Fig. 2_e_, _mes_). It would thus seem, from a study of these sections, that most of the mesoderm is derived from the entoderm. In fact, all of the mesoderm in front of the blastopore seems to have this origin, for it is not until the anterior edge of the blastopore is reached that there is any connection between the ectoderm and entoderm (Fig. 2_e_). Figure 2_e_ is a section through the region just mentioned, where, medially, the ectoderm, mesoderm, and entoderm form a continuous mass of cells. Laterally the mesoderm (_mes_) is a distinct layer of cells of a fairly characteristic mesodermal type. The notochord is not yet discernible, though a slight condensation of cells in the middle line may indicate its position. Figure 2_f_ is one of the four sections that were cut through the blastopore (_blp_), which is a hole of considerable size that opens, as the figure shows, entirely through the blastoderm. Along the walls of the blastopore the ectoderm and entoderm are, of course, continuous with each other and form a sharply defined boundary to the opening. As we pass laterally from the blastopore the cells become less compact, and are continued on each side as the mesodermal layer (_mes_). In this series the sections posterior to the blastopore were somewhat torn, and so were not drawn; but they probably did not differ materially from those of the corresponding region of the immediately following stages, which are shown in Figures 3_m_ and 6_i_ and will be described in their proper order. STAGE II FIGURES 3-3_o_ (PLATE VII., VIII., IX.) The next stage to be described is shown in surface views in Figures 3 and 3_a_. Of this stage Clarke says: