Fig 6. Anatomical structures of developing Brown Tsaiya Duck embryos during the first 72 h of incubation.

(A) The area opaca (arrowhead) and pellucida (arrow) are clearly distinguishable at 7 h post-incubation; no other prominent embryonic structures can be observed at this stage. (B) The primitive streak (arrow) starts budding out and becomes visible at 14 h post-incubation; (C) an intermediate or growing streak (arrow) stems from the marginal zone (arrowhead), extending out at 19 h post-incubation; (D) the streak becomes longer (arrow) and the area pellucida forms a V-shape or an ice-cream cone shape (arrowhead) at 28 h post-incubation; (E) the head process emerges (arrow) at 34 h post-incubation while the notochord extended posteriorly from the head position (thin arrow) to the Hensen’s node, and the area pellucida becomes elongated along the anterio-posterial axis (arrowhead); (F) the headfold (arrow) becomes visible and Hensen’s node repositions around the center of the embryo (arrowhead) by 31 h post-incubation; (G) the first 3 pairs of somites (arrow) and the neural fold (arrowhead) appear by 33 h post-incubation; with the lengthening of the notochord, the Hensen’s node starts to migrate caudally accompanied with the shortening of the primitive streak; (H) around 4–5 somite pairs (arrow) and the subcephalic pocket of the embryo are visible (arrowhead) at 36 h post-incubation; (I) the optical vesicles (arrowhead) and the paired heart primordia (thin arrow) are formed at 42 h post-incubation; (J) the forebrain region of three primary brains (arrow) and the omphalomesenteric veins (arrowhead) become distinguishable by 46 h post-incubation; (K) the heart loop is bent to the left (arrow) and the neuropore is not yet closed (arrowhead) 52 h post-incubation; (L) the optical vesicles (arrow) and proencephalon (arrowhead) are forming by 58 h post-incubation; (M) the head is turning to the left (arrow) and the telencephalon (arrowhead) enlarge by 60 h post-incubation; (N) the head completely turns to the left (arrow) and the auditory pit (arrowhead) is prominent 63 h post-incubation; (O) By 65 h, the optic cup was completely formed (arrow) and the amnion extended to somites 14–15 (arrowhead), while the crown-tail axis further curling up; (P) with increasing body size, the primordia wing buds first appeared as a tiny scratch (arrow) and tail bud formed a short cone-shaped structure (arrowhead) by 70 h; (Q) The body size further increased and peripheral vessels gradually filling in with blood cells (fine arrows); the leg buds are lifted off and appeared as fine crest (arrow) and tail bud appeared as a small bulge and slightly bent ventrally (arrowhead) by the end of 72 h. Numerical numbers 1–17 and HH1-HH17 represent different developmental stages in BTD and HH staging system, respectively. Bright field, scale bar = 0.5 mm.