Development of the embryonic chick's statoacoustic ganglion.

During stage 25 (4 1/2 days of incubation), the embryonic chick's statoacoustic ganglion appeared as a homogeneous cell mass. By stage 29 (6 days)- perikarya could be distinguished from Schwann cells because the latter contained more endoplasmic reticulum and ribosome. Granular Schwann cells processes have formed loose boundaries around groups of neurites. During the next 7 days, the Schwann cells clutched the neurites more tightly and divided them into successively smaller bundles. By the 13th day (stage 39) the first compact myelin (2-6 layers) surrounded the larger dendrites, and by the 14th day (stage 40) compact myelin appeared regularly. Until the 17th day (stage 43), the perikaryal were covered by a few layers of loose myelin. Perikaryal compact myelin appeared a day later (stage 44); the larger perikaryl diameter (15-30x) probably accounts for the lag. At hatching (20-21 days), 12-18 myelin layers surrounded many dendrites, though the extent of myelination varied markedly among dendrites in adjacent bundles. By the 2nd week after hatching, large myelinated perikarya outnumbered the smaller and loosely myelinated ones. Perikaryonal myelination required the cooperation of several Schwann cells. The myelin regularly alternated from compact to loose around each perikaryon.