Cleavage in Notopterus notopterus and blastula formation. (a) Stage 1: one cell (zygote) with forming blastodisc (bd), 1h:10min; (b) Stage 2: two blastomeres (b), at 2:00h; (c) Stage 3: four cells, 2h:20min; (d) Stage 4: eight cells, 2h:42min; (e) Stage 5: early morula with 16 cells, 3h:45min; (f) Stage 6: asynchronous blastomeres up to circa 32 cells, 4h:30min; (g) Stage 7: blastula with compact knob-like blastodisc, 6h:25min; (h) Stage 8: flat blastula with expanding yolk-sac syncytium (ys), roundish mound of the blastoderm on the top of the yolk, 7:40; (i) Stage 9: late blastula, the surface of blastoderm appears smooth, covering part of the yolk, but the cells are still distinct, 8h:35min. Scale bar = 1 mm.