Figure 15_c_ is posterior to the region affected by cranial flexure and so shows only one region of the embryo, that of the hindbrain (_hb_), which is here of essentially the same structure as above described. On each side of the hindbrain is a large auditory vesicle (_o_); that on the left is cut through its center and shows the beginning of differentiation, its lower end being thick-walled and rounded, while its upper end is more pointed and has a thin, somewhat wrinkled wall. The notochord (_nt_) is slightly larger than in the more anterior sections. Numerous blood-vessels (_bv_, _ar_) are seen in the mesoblast. The pharynx (_ph_) is here open ventrally and also through the gill cleft of the left side; on the right side the plane of the section did not pass through the external opening of the cleft. The mesoblast of the visceral folds is much more dense than that of the dorsal region of the section. Figure 15_d_, as is evident, is a section through the region of the heart, which appears as three irregular cavities (_ht_) with fairly thick mesoblastic walls (_mes′_) lined with endothelium (_en′_). The body wall, though consisting of but little besides the ectoderm (_ec_), completely surrounds the heart, and the pericardial or body cavity thus formed extends dorsally as a narrow space on either side of the foregut, giving the appearance of a rudimentary mesentery, though no especial development of such a structure would naturally be expected in this region of the embryo. The foregut (_ent_) is a moderately large cavity lined with a very distinct entoderm of even thickness. Dorsal to the foregut are three large blood-vessels, a median, and now single, dorsal aorta (_ao_), and a pair of cardinal veins (_cv_). The notochord (nt) is small and is flattened against the ventral side of the spinal cord (_sc_), which latter structure needs no special mention. The muscle plates (_mp_) are considerably elongated, so that they now extend ventrally to a point slightly below the upper angles of the body cavity.