Embryonic shield formation
radial intercalation of blastomeres, formation of the hypoblast and embryonic shield, appearance of the head anlage.
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Movie#1595. 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.
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 anlage) 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.