Zebrafish embryo at 75% epiboly. The view is from the animal pole. The focal plane is deep within the blastoderm. The focal plane changes as the embryo rolls during the sequence. The fluorescent lipid outlines the boundaries of every deep cell in the embryo. Deep cells undergo radial intercalation to form a coherent sheet known as the hypoblast. During embryonic shield formation, cells within the hypoblast migrate towards the embryonic midline (to the upper right of the image.) The dark centers which come into view are nuclei located within the yolk syncytial layer. The rudiment of the head (the head anlagen) emerges as the leading edge of the embryonic shield. The embryo rolls once again. The sequence ends with a cross-sectional view of the enveloping layer ensheathing the embryo. The head anlage lies between the enveloping layer epithelium and the brightly labelled yolk cell.This video made available by Mark Cooper thru Zebrafish - The Living Laboratory. The videos in this group can also be found in the Developmental Landmarks page of the Fishscope web site.