At this stage, the periblastic ectoderm covers approximately one-quarter of the surface of the yolk (fig. 2A). One segment of germ ring has expanded and thickened form the embryonic shield (or germinal shield) which will form the embryo proper (fig. 2A). The caudal lip of the germinal shield is thickened and may be considered homologous to the dorsal lip of the blastopore in other animals (fig. 2C). The length of the germinal shield is 0.35 mm. It possesses a thickened central region which results from the proliferation and invagination of cells to form the neural keel (fig. 2B).