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The various stages of tectorial membrane development were observed postnatally, using the scanning electron microscope. Hamster cochleas were obtained at 2-day intervals beginning with the day of birth and ending at 22 days after birth. Changes in the tectorial membrane structure were quantified by studying the transformation from a pattern of widely spaced fiber bundles to the continuous sheet-like membrane of the adult. Specifically, this transformation was quantified by summing the widths of the fiber bundles along a 25-micron traverse drawn parallel to the outer edge of the minor tectorial membrane, overlying the middle hair cell row. Such measures were performed at different locations along the cochlear spiral. The results indicate a base-to-apex progression of tectorial membrane development with attainment of mature characteristics between 18 and 22 days after birth. Comparison with previous studies indicate that the maturation of the tectorial membrane coincides with a period of major improvement in evoked potential thresholds in the central auditory system.