A sagittal section of approximately this stage, shown in figure 1A, represents the foregut, _fg_, as a shallow enclosure of the anterior region of the entoderm, while the wide blastopore, _blp_, connects the region of the hindgut with the exterior. No sign of a tail fold being present, there is, of course, no real hindgut. The entoderm, which has the appearance of being thickened because of the fact that the notochord has not yet completely separated from it, is continuous, through the blastopore, with the ectoderm. Posterior to the blastopore the primitive streak, _ps_, is seen as a collection of scattered cells between the ectoderm and the entoderm, apparently formed by proliferation from the ventral side of the ectoderm. A slightly later stage is shown in figure 2, a dorsal view of an embryo with five pairs of mesoblastic somites. A sagittal section of this stage is shown in figure 2A. The foregut is here more inclosed, and the notochord, _nt_, having separated from the entoderm, _en_, is seen as a distinct layer of cells extending from the foregut to the blastopore. A transverse section through the headfold of this stage is shown in figure 2B. The foregut is seen as a wide cavity, _ent_, depressed dorsally, apparently, by the formation of the medullary groove and the notochord; it is wider laterally than in a dorso-ventral direction, and its walls are made up of about three layers of closely arranged, irregular cells; the wall is somewhat thinner on the dorsal side, just below the notochord. Figure 3 is a dorsal view of the next stage to be described; about fifteen pairs of somites are present.