Figure 13_c_ passes through the anterior part of the heart about seventy-five sections posterior to Figure 13_b_. The embryo in this region is narrow but deep (dorso-ventrally), the depth being largely due to the size of the heart. The ectoderm (_ec_) is considerably thickened on each side of the pharynx (_ph_); this thickened area may be traced for some distance both anteriorly and posteriorly from this point; its significance could not be determined. The spinal cord (_sc_) and notochord (_nt_) need no special description; the former is smaller and the latter larger than in the more anterior sections. The two large blood-vessels (_ac_) near the spinal cord and notochord are probably the anterior cardinal veins. The aortæ are cut by the plane of this section just anterior to their point of fusion into a single vessel. A few blood corpuscles are seen in the right aorta. The enteron (_ent_), cut posterior to the region of the gill clefts, is a large elliptical cavity, with its long axis in a transverse position. Its entodermal wall is comparatively thin and smooth, with the cell nuclei arranged chiefly on the outer side, _i. e._, away from the cavity of the enteron. The body cavity (_bc_) is here still unenclosed, and its walls, the somatic stalk, are cut off close to the body of the embryo. The heart (_ht_), the most conspicuous feature of this section, is nearly as large in cross-section as all the rest of the embryo. As seen in such a section it is entirely detached from the body of the embryo, and in this particular case has about the shape of the human stomach. The mesoblastic portion of its wall (_mes′_) is of very irregular thickness; it forms a dense layer entirely around the outside, except for the pointed dorsal region, and is especially thick along the ventral margin, where it is thrown into well marked folds, the heavy muscle columns. Lining the cavity of the heart is the membranous endothelium (_en′_), and between this and the dense outer wall just described is a loose reticular tissue with but few nuclei. As the series is followed toward the tail the sections diminish in size until, at a point about one third the embryo length from the posterior end, they are of scarcely one fourth the area of the sections through the region of the hindbrain.