The early develoment of the duck's egg, with special reference to the origin of the primitive streak

This study deals with the development of the duck embryo from the unincubated blastoderm to the 1‐to‐3‐somite stage. In the unincubated stage the thickened posterior germ wall is the most active region mitotically, contributing cells to complete the entodermal layer. The slightly thickened ectoderm of the posterior half of the pellucid area is the precursor of the embryonic shield. During incubation this ectoderm increases in height, forming the embryonic shield. The primitive streak arises in the posterior part of the shield, where tall ectodermal cells in the midline become several‐layered and proliferate cells from their ventral surface which migrate laterally, becoming mesoderm. At first short and broad, the streak becomes longer and narrower until it attains its definitive stage. The greatest proliferation of ectodermal cells occurs always at the anterior end of the streak of all stages, forming at the definitive stage a definite thickening–Hensen's node. Proliferation diminishes posteriorly along the streak. The number and orientation of mitotic figures indicate cell movements as follows: Prestreak stages–ectodermal cells migrate from posterior part of shield toward midline, thence forward in midline; early streak stages–anterior movement in streak and to sides of streak, and lateromedial movement from sides toward middle of streak; definitive streak stages–postero‐anterior and lateromedial migration of cells. The embryonic axis, of prestreak and streak stages, lies approximately at right angles to the egg axis in slightly over 50 per cent of the cases.