Although capping structures are present at the tips of microtubules in both growing cilia and mature cilia, previous work has not determined the time of cap formation. The results reported here reveal that the large caps of mature palate cilia appear in cilia with lengths as short as 1.75 micron. In the growing palate cilium, a disk-shaped plate is formed at the tip during the first micron of growth. As the cilium elongates to 1.5-2.0 microns, a small plate forms underneath the disk-shaped

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... that gives an asymmetrical appearance to the whole cap structure. The structure of the cap is complete in cilia longer than 2.0 microns. The hair-like structures that form the extraciliary crown appear on the membrane at the ciliary tip at the same time as the mature cap is forming. The formation of a cap structure is discussed in relation to microtubule assembly during ciliogenesis.