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.