Figure 13_f_, about two hundred and fifty sections posterior to the last, passes through the extreme posterior end of the embryo. The section is nearly circular in outline and is somewhat larger than the preceding. The amnion (_a_) completely encircles the embryo. The ectoderm (_ec_) is of fairly even thickness, and the mesoblast which it encloses is of the usual character. The spinal cord (_sc_) is nearly circular in outline, as is its central canal. The digestive tract (_ent_) is larger in section than it was in more anterior regions; it is nearly circular in cross-section and its walls are made up of several layers of cells, so that it resembles to a considerable degree the spinal cord of the same region. In the narrow space between the spinal cord and the hindgut is seen the notochord (_nt_), somewhat flattened and relatively and actually smaller than in the preceding figure. A few scattered blood-vessels may be seen in the mesoblast at various places. A sagittal section of an embryo of this stage, drawn under the same magnification as were the transverse sections, is shown in Figure 13_g_. The embryo being bent laterally could not be cut by any one plane throughout its entire length, so that only the anterior end is represented in the figure. The amnion (_a_) may be clearly seen except at certain places where it is closely adherent to the superficial ectoderm. Under the low magnification used the superficial ectoderm cannot be distinguished from the ectoderm of the nervous system. The plane of the section being in the anterior end almost exactly median, this part of the central nervous system is seen as the usual retort-shaped cavity, while in the region back of the brain, where the neural canal is narrow, the section passes through the wall of the spinal cord (_sc_) and does not show the neural canal at all. The wall of the forebrain (_fb_) is quite thick, especially at the extreme anterior end; the wall of the midbrain (_mb_), where the marked cranial flexure takes place, is somewhat thinner, and it gradually becomes still thinner as it is followed posteriorly over the hindbrain (_hb_). Between the floors of the fore- and hindbrains, in the acute angle caused by the cranial flexure, is the anterior end of the notochord (_nt_), the only part of that structure that lies in the plane of the section. Ventral and posterior to the notochord is a large cavity, the pharynx (_ph_), whose entoblastic lining can scarcely be distinguished under this magnification from the surrounding tissues. The stomodeal opening being as yet unformed, the pharynx is closed anteriorly; posteriorly also, owing to the plane of the section, the pharynx appears to be closed, since its connection with the yolk stalk is not shown. In the floor of the pharynx, almost under the reference line _ph_, a slight depression marks the position of the first gill cleft. In the mesoblast ventral to the pharynx and near the gill cleft