Cranial Nerves: Location in the Brainstem 5/6th week, 5 vesicle stage, 8-10mm: In embryos at 5/6 weeks all cranial nerves are recognizable except for olfactory and optic. Sensory nerves have conspiculous ganglia near the brain (most have motor components except for the 8th); the pure motor cranial nerves (III, IV, VI, and XII) have no external ganglia.