It leads from the end of the medullary canal (at the point where its walls are continuous with the cells of the primitive streak) round the end of the notochord, which here becomes continuous with the medullary cord, and so through the hypoblast. The latter layer is still a flat sheet without any lateral infolding; but it gives rise, behind the neurenteric passage, to a blind posteriorly directed diverticulum, placed in the body cavity behind the embryo, and opening at the ventral face of the apparent hind end of the primitive streak. There is very little doubt that this diverticulum is the commencing allantois. At a somewhat later stage the arrangement of these parts has undergone some changes. Their relations are shewn in the sections represented in fig. 128. The foremost section (A) passes through the alimentary opening of the neurenteric passage (_ne_). Above this opening the section passes through the primitive streak (_pr_) close to its junction with the walls of the medullary canal. The hypoblast is folded in laterally, but the gut is still open below. The amnion is completely established. In the next section figured (B), the fourth of my series, the gut is completely closed in; and the mesoblast has united laterally with the axial tissue of the primitive streak. Vessels to supply the allantois are shewn at _v_. The three following sections are not figured, but they present the same features as B, except that the primitive streak gets rapidly smaller, and the lumen of the gut narrower. The section following (C) represents, I believe, only the stalk of the allantoic diverticulum. This diverticulum appears to be formed as usual of hypoblast (_hy_) enveloped by splanchnic mesoblast (_me_), and projects into the section of the body cavity present behind the embryo. Its position in the body cavity is the cause of its somewhat peculiar appearance in the figure. Had the whole section been represented the allantois would have been enclosed in a space between the serous membrane (_se_) and a layer of splanchnic mesoblast below which has also been omitted in fig. B[72]. It still points directly backwards, as it primitively does in the chick, _vide_ fig. 123 A, and Gasser, No. 127, Pl. V. figs. 1 and 2. I do not understand the apparently double character of the lumen of the allantois. In the next section (not figured) the lumen of the allantoic stalk is larger, but still apparently double, while in the last section (D) the lumen is considerably enlarged and single. The neurenteric canal appears to close shortly after the stage last described, though its further history has not been followed in detail. [72] Owing to the difficulty of procuring material I have only been able to prepare the two sets of sections just described, and in the absence of a fuller series there are some points in the interpretation of the sections which must remain doubtful.] _General development of the Embryo._