Cutaneous thoracic nerves from 16-day embryos to 6-month chickens were studied. Special attention was given to the increase in caliber of developing axons, number of lamellae in the myelin sheath and distribution of group sizes. Myelination begins at 14 days of incubation but occurs mainly after hatching: 5% of fibers are myelinated in the 3-day chickens, 20% at 16 days and 63% at 6 months. During development, a rectilinear positive correlation between the axon diameter and the number of myelin lamellae was found at all stages, except in 16-day embryos, and the slope of the calculated regression lines decreased with age, as mean diameter and sheath thickness increased. The rate of daily myelin initiation was maximum between 3 and 16 days after hatching while the speed of myelin formation decreased with development. For nonmyelinated fibers, a regular reduction of the number of fibers per Schwann cell takes place during nerve maturation.