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Methacrylate-embedded E16 embryo (lateral sagittal)

Figure 50.  This sagittal slice is from the same specimen shown in Figure 49, only it cuts through the lateral olfactory bulb and grazes lateral parts of the septal and posteromedial basal ganglionic NEPs.  The medial part of the lateral basal ganglionic eminence NEP is an “island” over the other basal telencephalic structures projecting into the lateral ventricle.  The parenchyma of the expanding olfactory bulb is beginning to show some laminar definition: First, the densely packed NEP; second, cells move out and begin to differentiate; third, larger cells in a variety of orientations—possibly the future internal plexiform layer of both the MOB and AOB.  A cluster of rounded cells in the dorsal bulb may be the accumulating and settling AOB output neurons (surrounded with a dashed line).  Other cells in the MOB have dense accumulations of cytoplasm that supply growing processes.  Farther back, where the olfactory nerve contact zone melds with the brain surface, there are some spindle-shaped cells that are probably mitral cells heading forward into the MOB plexiform layer.  The olfactory nerve itself is full of fibers and interstitial cells (mostly glia).  The brain surface is completely obliterated and cells in the brain and in the peripheral nerve can easily exchange places.  The olfactory nerve forms an unbroken cap around the growing bulb, but individual nerve bundles are nearer to and exit and olfactory epithelium.  The olfactory epithelium contains large superficial cells with dense cytoplasm—probably the first-order olfactory neurons, whose axons form the olfactory nerve.  The olfactory sensory neurons are interspersed with many mitotic figures—that is the superficial band of cell proliferation seen in the short-survival autoradiography slices (Figs. 46-48).