Cilia begin to grow from the free surface of some ectodermal cells during the neural plate stage (stage 13). Ciliary growth is not closely synchronized between cells in the same embryo, but the number of ciliated cells increases greatly during neural fold development and further growth in length of pre-existing cilia occurs. Ciliated cells are numerous and widely distributed over the surface of early tailbud stages. However, cilia do not develop on cells in the neural plate and inner sides of the neural fold, and only an occasional ciliated cell is observed on the surface of the paired suckers. The number of ciliated epidermal cells per embryo increases during tail growth (stages 18-22). Ciliated epidermal cells persist after hatching (stage 20), but regression of cilia can be detected in stage 24 larvae. By late stage 25, most of the cilia have disappeared and the morphological variations observed indicate that the process involves resorption of cilia. Non-ciliated (secretory) ectodermal cells from the neurula onward to stage 25 synthesize granules which are released to provide a mucous-like coat for the embryo and larva. The surface structure of both ciliated and non-ciliated ectodermal cells is described in sections studied with the transmission electron microscope and compared to the surface architecture of both cell types observed with the scanning electron microscope.

Upon fertilization, the zebrafish egg undergoes marked physiological and structural changes, one of which involves blastodisc formation. Before fertilization, yolk globules are rounded and the endoplasm extends throughout the oocyte. During blastodisc formation, the yolk globules become angular and the endoplasm is restricted to streamers among the yolk globules. The streamers are oriented in an anterior-posterior axis of the egg. During blastodisc formation the cytoskeleton consists of an extensive array of filamentous structures of variable width in both the cortex as well as within elongate endoplasmic streamers. Although the filamentous components in the cortex and endoplasmic streamers probably include both microfilaments and microtubules, frequently they are somewhat wider than the usual dimensions, and possible reasons for this are suggested. From their arrangement in both the cortex and endoplasm, it seems likely that the components of the cytoskeleton (e.g., microfilaments and microtubules) may provide, through contraction, the major force responsible for the streaming of the endoplasm into the forming blastodisc. It is assumed that the surface tension of the vegetal hemisphere exceeds that of the animal hemisphere, thus forcing, through differential contraction, the endoplasm to flow in the direction of the forming blastodisc. No distinct barrier between the yolk and forming blastodisc was observed. The compressed condition of the larger and many-sided yolk globules could prevent their movement into the blastodisc. Scanning electron microscopy is limited in the resolution with which it can depict the cytoskeleton, but nonetheless it provides useful information about structural interrelationships.

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