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Methacrylate-embedded E22 embryo in the horizontal plane (ventral)

Figure 140.  This ventral horizontal slice from the same specimen in Figures 138 and 139 shows only indistinct 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 in a well-defined layer.  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 must form between birth and Postnatal day 5.