Day-1 chick development.

The first day of chick development takes place inside the mother hen (in utero), during which the embryo progresses from fertilization to late blastula/early gastrula formation. The salient features of developmental anatomy in this period are conserved among the sauropsids (birds and reptiles). Many of these features are also shared in prototherian (monotreme) embryos, whereas metatherian (marsupial) and eutherian (placental) embryos display significant variations. Important for understanding the evolution of early development in amniotes, the knowledge of cellular and molecular mechanisms regulating in utero chick development may also offer valuable insight into early lineage specification in prototherians and conserved features in mammalian early development. This commentary provides a snapshot of what is currently known about intrauterine chick development and identifies key issues that await further clarification, including the process of cellularization, allocation of maternal determinants, zygotic gene activation, mid-blastula transition, cell layer increase and reduction, radial symmetry breaking, early lineage segregation, and role of yolk syncytium in early patterning.