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Methacrylate-embedded E21 embryo in the horizontal plane (ventral)

Figure 133.  This ventral horizontal slice from the same specimen in Figures 131 and 132 still shows remnants of the fan-like array of migrating mitral cells in the lateral internal plexiform layer (curved arrows); the migratory stream is less prominent than in earlier age groups because many mitral cells have already settled.  Mitral cells are clustered neatly in the layer, most of them radially oriented.  The cell-sparse external plexiform layer has quite a few large tufted cells that appear as superficially-placed mitral cells, while the small cells are probably short-axon neurons or glia.  The whorls of fibers in the olfactory nerve do not show any characteristic oval cell-sparse structures typical of glomeruli.  These form between birth and Postnatal day 5.