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.