The neural plate formation in chick embryo is due to a change in cell shape. The cubic cells (Plate I, fig. 1) become cylindrical (Plate II, fig. 3). Their adhesion increases and microtubules appear in the cytoplasm. The neural tube formation is due to the cells, which become bottle-shaped. These cells are linked at the surface of the neural plate by occludent junctions (Plate IV, fig. 8). The cells contract in the region where there are microfilaments between the junctions and expand at the opposite pole. The microtubules oriented along the cell axis (Plate II, fig. 4) seem to transport cytoplasmic material. Junctions, microfilaments and microtubules do not change their topography when optic vesicle becomes optic cup (Plate VI, fig. 11). Evident in this stage is an increase of the microfilaments (Plate VII, fig. 13) and microtubules. Evident also is the presence of the Golgi apparatus (Plate VIII, figs. 16, 17) and, during the formation of optic cup, an active pinocytosis process. At the bottom of the optical vesicles mitoses are abundant and between the cells are large intercellular spaces. The basal membrane surrounds all the nervous system rudiment and also the optic vesicles (Plate VI, fig. 12). A mechanical action of the lens rudiment seems to determine optic cup invagination (Plate V, fig. 10). Numerous microtubules oriented along the cell axis can be seen in all the cells of the optic cup (Plate VIII, fig. 15). In the first stage of neural tube formation the function of the microtubules seems to be that of transport; in stages of optic cup introflexion a mechanical function is probable.