In a section (A) through the trunk of the embryo a short way in front of the primitive streak, there is a medullary plate with a shallow groove (_mg_), well-developed mesoblastic plates (_mep_), already divided into somatic and splanchnic layers, and a completely formed notochord independent of the hypoblast (_hy_). In the next section (B), taken just in front of the primitive streak, the notochord is attached to the hypoblast, and the medullary groove is deeper; while in the section following (C), which passes through the front border of the primitive streak, the notochord and hypoblast have become fused with the epiblast. The section behind (D) shews the neurenteric passage leading through the floor of the medullary groove and through the hypoblast (_ne_). On the right side the mesoblastic plate has become continuous with the walls of the passage. The last section (E) passes through the front part of the primitive streak behind the passage. The mesoblast, epiblast, and to some extent the hypoblast, are now fused together in the axial line, and in the middle of the fused mass is seen a narrow diverticulum (_ne_) which is probably equivalent to the posterior diverticulum of the neural canal in Birds (_vide_ p. 164). The general features of the stage will best be understood by an examination of the diagrammatic longitudinal section represented in fig. 127. In front is shewn the amnion (_am_), growing over the head of the embryo. The notochord (_ch_) is seen as an independent cord for the greater part of the length of the embryo, but falls into the hypoblast shortly in front of the neurenteric passage. The neurenteric passage is shewn at _ne_, and behind it is the front part of the primitive streak. It is interesting to notice the remarkable relations of the notochord to the walls of the neurenteric passage. More or less similar relations are also well marked in the case of the goose and the fowl, and support the conclusion, deducible from the lower forms of Vertebrata, that the notochord is essentially hypoblastic. The passage at the front end of the primitive streak forms the posterior boundary of the medullary plate, though the medullary groove is not at first continued back to it. The anterior wall of this passage connects together the medullary plate and the notochordal ridge of the hypoblast. In the stage represented in fig. 126 and 129 the medullary groove has become continued back to the opening of the passage, which thus becomes enclosed in the medullary folds, and forms a true neurenteric passage[71].