An ultrastructural study was performed on trigeminal ganglia removed from foetuses aged 10 to 23 weeks. The first turns of lemmocyte processes around the axons were observed in the trigeminal ganglion in the 12th week of development. During the period between the 15th and the 18th weeks the myelin sheath increases in thickness and becomes a compact, laminated structure. In foetuses of 23 weeks the compact myelin sheath has up to 24 myelin lamellae
