Olfactory bulb and tract
The olfactory bulb and tract belong to the rostral telencephalon. From the 4th week, stage 11 the local mesenchyma and the ventral side of the prosencephalon (prospective telencephalon) cause the surface ectoderm to construct the nasal placodes through cell proliferation. Certain cells in the nasal placodes differentiate to become the primary sensory cells of the olfactory epithelium. The nasal placodes become flanked laterally by growths of the cephalic mesenchyme, the medial and lateral nasal processes. The placodes sink into the depths so that then the olfactory grooves (stage 15) and, subsequently, the olfactory pits (stage 16) arise.
Towards the end of the 5th week (stage 16) the axons of the primary sensory cells grow out and come into contact with nerve cells at the rostral end of the telencephalon.
Note that in the induction processes a reciprocal happening is involved: the primary cells induced by the telencephalon cause the formation of the olfactory bulb.
In stage 22 the olfactory bulb isolates itself from the rest of the brain and grows in length. It lies on the lamina cribrosa that arises through ossification of the ethmoid bone. In the olfactory bulb the two sensory neurons differentiate that are in a synaptic connection with the primary sensory cells of the olfactory epithelium. The second sensory neurons (mitral cells) dispatch their axons through the olfactory tract to the olfactory cortex fields of the prepyriform area (palaeocortex) and the subcallosal area of the archicortex. It could be that the conscious perception of a smell in the neocortex takes place after a switching over in the thalamus (medial dorsal nucleus).