Fig 1.
Development and regression of the primitive streak in Brown Tsaiya Duck during the first 72 h of incubation.
Representative figures show the primitive streak of Brown Tsaiya Duck (BTD) embryos. Black arrowheads indicate the beginning of the primitive steak and black arrows indicate the growing or regressing end of the streak. (A) The initial streak can be observed by 10–13 h post-incubation or earlier; (B) the initial primitive streak elongates into the intermediate streak by 19–24 h post-incubation; (C) a full-length streak can be observed, with a clearly formed neural groove (thin black arrow) by 25–27 h post-incubation; (D-F) after reaching its full length (D), the streak starts to regress by 28–42 h post-incubation, along with the formation of 3–4 somite pairs, pharyngeal endoderm (thin black arrow) (E) and heart primordia (red arrowhead) with more than 6 somite pairs (F). Bright field, scale bar = 0.5 mm.
Table 1.
The emerging time of embryonic structures in Taiwan Country Chicken (TCC) and Brown Tsaiya Ducks (BTD) during the first 72 h of incubation in comparison with the HH* staging system.
Fig 2.
Developmental structures and rudiments of embryonic features during the first 72 h of incubation.
Representative figures show the structures of Taiwan Country Chicken (TCC) embryos. (A) The area opaca (arrow) and the area pellucida (arrowhead) are distinct at 5 h post-incubation; (B) the primitive streak (arrow) appears at 12 h in TCC post-incubation; (C) the headfold (arrow) becomes visible at 25 h post-incubation; (D) the initial pair of somites (arrow) and neural plate (arrowhead) first appear at 26 h post-incubation; (E) the primary optic vesicles (arrow) and the paired heart primordia (arrowhead) start to form at 30 h post-incubation; (F) the three primary brain rudiments (arrows) are visible at 33 h post-incubation along with the developing heart primordia (arrowhead) at the 13-somite stage; (G) the five neuromeres (arrows) are distinguishable at 42 h post-incubation with a relatively developed heart (arrowhead); (H) the wing (arrow) and leg buds (arrowhead) develop at 57 h post-incubation and the allantois is barely visible (thin arrow); (I) a prominently enlarged allantois (arrow) can be identified at 72 h post-incubation. Bright field, scale bar = 0.5 mm.
Table 2.
Comparison of major developmental parameters between Taiwan Country Chicken (TCC) and Brown Tsaiya Duck (BTD) embryos during the first 72 h post-incubation.
Fig 3.
Establishment of linear regression lines between incubation time and embryonic crown-tail length (ECTL).
The rates of embryo growth in Brown Tsaiya Ducks (blue dot line; y = 0.107x - 2.2185, R2 = 0.9522) and Taiwan Country Chicken (red dot line; y = 0.1486x - 2.8836, R2 = 0.9269) can be accurately predicted by each own regression line. Values are presented as mean ± SD in both species.
Fig 4.
Quadratic regression lines between incubation time and development of the primitive streak in Taiwan Country Chicken (TCC) and Brown Tsaiya Ducks (BTD).
The developing primitive streak of embryos in BTD (blue dot line; y = -0.002x2 + 0.1079x - 0.6786 R2 = 0.8716) and TCC (red dot line; y = -0.0026x2 + 0.0985x + 0.1585 R2 = 0.7266) can be well-represented with each quadratic regression line. Values are presented as mean ± SD in both species.
Fig 5.
Linear regression lines between incubation time and development of the notochord in Taiwan Country Chicken (TCC) and Brown Tsaiya Duck (BTD).
Values are presented as mean ± SD in Brown Tsaiya Ducks (blue dot line; y = 0.1008x - 2.4389 R2 = 0.8994) and Taiwan Country Chicken (red dot line; y = 0.1543x - 3.0259 R2 = 0.9539).
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.
Table 3.
Staging with key developmental features of Brown Tsaiya Duck embryos during the first 72 h of incubation.
Table 4.
Staging Taiwan Country Chicken (TCC) and Brown Tsaiya Duck (BTD) embryos by the incubation time (h) relative to Hamburger and Hamilton (HH) and Dupuy (D-) staging system during the first 72 h of incubation.