Figure 2

Early embryonic developmental stages of Heterobranchus bidorsalis . A: Fertilized egg; outer perivitelline membrane, opm; inner perivitelline membrane, ipm. B: Polar division. Anal pole, ap; Vegetal Pole, vp. C: One-cell stage. D: Two-cell stage. E: Four-cell stage. F: Eight-cell stage. G: Sixteen-cell stage. H: Thirty-two-cell stage. I: Sixty-four-cell stage. J: Morula. K: Blastula (High). L: Blastula (low). M: Enveloping layer, EVL; yolk syncytial layer, YSL; blastoderm, b. N: 50% Epiboly. O: Germ ring (shown by arrow). P: Embryonic shield (shown by arrow). Q: 75% Epiboly. R: 95% Epiboly; yolk plug, yp. S: Epiboly completed (100%); polster, p; tail bud, tb; perivitelline membrane thickens. T: Neural keel, nk; Tail bud expansion noticed. U: Formation of somite blocks; somite block, s; notochord, n; early emergence of optic primordia, op. V: Kupffer’s vesicle, kv; spinal cord, sc; sclerotome, sl; myotome, m. Earlier formed somites developed into myotome blocks and progressed caudally. Differentiation of aggregated tail bud cells forming notochord's primordia and spinal cord, marking pharygula stage; somites develop and mature cephalocaudally. W: Optic placode, pp; and otic placode, tp; with the two tiny otoliths; inward cellular movement, i, noticed. X: Hollowed embryonic membrane/pouch during hatching; chorion or perivitelline membrane, pm; area of emergence, ao. Scale bar: 1 mm; Magnification: ×40.