Dome-stage Zebrafish embryo. The animal pole is at the bottom of the image. The yolk syncytial layer shows two tiers of nuclei. As the sequence begins, the yolk syncytial layer narrows. The view then changes to the edge of the blastoderm. Involution at the margin of the blastoderm forms the germ ring. Deep cells at the center of the blastoderm converge toward the inner surface of the blastoderm, forming the inner layer of the hypoblast.
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.