Figure 15_a_, the most anterior section of the series, passes through the forebrain (_fb_) in the region of the eyes, and through the hindbrain (_hb_) anterior to the auditory vesicles. The forebrain is here a large cavity with a dense wall of a comparatively even thickness. Owing probably to the section not being exactly in the transverse plane, the eyes are cut in different regions, that on the left (_ov_) being cut through its stalk, while that on the right (_oc_) is cut near its middle region and hence does not show any connection with the forebrain. The almost complete obliteration of the cavity of the optic vesicle to form the optic cup by the invagination of the outer wall of the vesicle is shown on the right side of the section (_oc_). The lens vesicle (_lv_) is completely cut off from the superficial ectoderm (_ec_), which is comparatively thin. The hindbrain (_hb_) has the usual shape for that structure. Its ventral wall is dense and thick, while its roof is reduced to the usual thin, wrinkled membrane. Close to the floor of the hindbrain lies the notochord (_nt_), which is large and is distinctly vacuolated. To the right of the hindbrain a large mass of darkly stained cells (_cn_) is one of the cranial nerves, which is connected with the hindbrain a few sections anterior to the one under consideration. The pharynx (_ph_), which is cut near its extreme anterior end, is represented by three irregular cavities near the base of the forebrain. Scattered throughout the mesoblast, which makes up the greater part of the section, are numerous blood-vessels (_bv_).