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Methacrylate-embedded E15 embryo (lateral sagittal)

Figure 35 is a lateral sagittal slice in the same E15 embryo in Figs 33 and 34.  The olfactory nerve contact zone is not in the slice—but there may be some few fibers at the base of the brain (not outlined).  There are a few olfactory nerve fascicles outside the olfactory epithelium.  The presumptive olfactory NEP is a dip between two humps of NEP—both of which may be the lateral basal telencephalic/basal ganglionic NEPs. That position indicates that the olfactory NEP is part of the NEP in the basal telencephalon and probably not an outward growth of the cortical NEP.  Lots of cells are migrating radially from the NEPs (straight arrows).  There is a prominent stream of cells that appears to migrate forward from the posteromedial ganglionic NEP near the pial surface heading toward the anterior forebrain.  But in the region of the future bulb, the spindle-shaped cells disappear.  Probably the cells turn and migrate out of the plane of the section either medially or laterally.  These are most likely pioneer mitral cells (20% of the population) generated on E13 and E14.  Other groups of cells beneath the ganglionic eminences might be the early generated neurons in layer III of the primary olfactory cortex.  There is the intriguing possibility that the pioneer mitral cells might intermingle with primary olfactory cortex neurons as they are migrating.  The thicker zone of differentiating cells outside the olfactory NEP may be accumulating AOB output neurons—the entire population has been completely generated by this time.