Embryonic development

Embryonic development is the organized sequence through which an embryo forms and differentiates before the later juvenile or fetal stages defined for its species. Cell division, movement, changing gene expression and interactions among neighbouring cells jointly build tissues and body organization. Developmental stage therefore describes more than elapsed time or total cell number.

Light-sheet imaging of living zebrafish embryos enabled repeated three-dimensional observation and tracking of nuclei across early development. ^[ref] Following one developing specimen distinguishes movement and ancestry from an ordering inferred solely from fixed specimens of different ages. A lineage record still does not by itself identify the molecular signals determining every descendant cell fate.