Zoological Science 11: 825-839 (1994) - 1 day 2 hrs Stage 18 (1 day 2 hrs) Late neurula stage (Optic bud formation): The brain and nerve cord in the arrow-shaped embryonic body codevelop as a solid rod of cells. A solid optic bud (rudimentary eye vesicle) appears on each side of the cephalic end. The beak- like cell mass is still visible. The Kufpper's vesicles enlarge somewhat. A small part of the yolk sphere still forms a blastopore at the vegetal pole. - 1 day 2 hrs