= 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. <Reconstruction of zebrafish early embryonic development by scanned light sheet microscopy by Philipp J. Keller and others>[2008, live light-sheet imaging and nuclear tracking]{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.
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