Figure 16_b_ is in reality more anterior in position, considering the entire embryo, than the preceding; but the region of the embryo represented is more posterior, so that it is described at this point. The greatly elongated outline of the brain is due to its being cut through the region of flexure, so that the forebrain (_fb_), or, perhaps, midbrain, is shown at one end, and the hindbrain (_hb_) at the other. The walls of these cavities are somewhat wrinkled and irregular and their constituent cells are beginning to show slight differentiation, though this is not shown in the figure. On the left side are seen a couple of darkly stained masses; one is the origin of a cranial nerve (_cn_); and the other is one of the auditory vesicles (_o_), which is still more irregular in outline than it was in the preceding stage. The only blood-vessels to be seen are a few very small ones that lie close to the wall of the brain. The ectoderm is quite thin at all points. Figure 16_c_, the largest section of this series, passes through the forebrain in the region of the eyes and through the gill clefts. The forebrain (_fb_) exhibits on the left a marked thickening of its wall (_ch_), the edge of the cerebral hemisphere of that side, which is just beginning to develop; on its right side the lower part of the forebrain is connected by a well marked optic stalk (_os_) with the optic cup (_oc_), in whose opening lies the lens vesicle (_lv_), now reduced to a crescentic slit by the thickening of its posterior wall. The mesoblast is more dense in those parts of the section adjacent to the pharynx than in the more distant regions, and the ectoderm thickens in a marked way as it approaches the borders of the pharynx and gill clefts. Only a few small blood-vessels (_bv_) are to be seen in the region of the forebrain. Parts of three pairs of clefts (_g_) are shown in the figure: one pair opens widely on either side, so that there is a large area of the section that is distinct from the two still larger portions and contains a small, thick-walled cavity (_g_) on the right side; this cavity is a gill cleft that is cut through neither its outer nor its pharyngeal opening. No structures other than this small portion of a gill cleft and a few blood-vessels are to be seen in this middle region of the section. In the more posterior part of the section, in which the notochord (_nt_) is located, a pair of curved clefts may be seen, opening entirely through the wall on the left, but closed on the right (_g_). One distinct pair of aortic arches is shown (_ar_), and also the dorsal aortæ (_ao_), which are of very unequal size. The spinal cord (_sc_) and muscle plates need no special description.