14 days: Embryo has 800 cells. Ectoderm disk lies under endoderm. Mesoderm grows between endoderm and ectoderm and splits into two sheets, one on ectoderm and one on endoderm, with a fluid cavity {coelom, development}| between sheets.
14 days: Endoderm disk has line {primitive streak}| of cells along longitudinal body-axis top. At anterior end, primitive streak becomes more specialized and has no segments, like notochord.
2 to 3 weeks: Ectoderm above notochord makes first neural plate, then neural groove, then neural folds, and then neural tube {neural tube}|.
2 to 3 weeks: Neural tube has inner ventricular zone {ependyma} around central ventricle, intermediate-zone or mantle-layer gray matter, and marginal-zone or pia white matter. Ependyma becomes CNS neurons and glia.
2 to 3 weeks: Neural-tube mantle layer becomes dorsal sense neurons {alar plate} and ventral-motor-neuron basal plate.
2 to 3 weeks: Neural-tube mantle layer becomes dorsal-sense-neuron alar plate and ventral motor neurons {basal plate}.
2 to 3 weeks: Mesoderm {neural crest}| lies next to neural tube and makes adrenal medulla, sympathetic ganglia, and dorsal-root ganglia.
2 to 9 weeks: Embryo {embryo, animal}| grows first at head and then down sides. First, disk sides curve up and around notochord at head. Then disk sides curve down to tail. Body folding makes digestive-system foregut and hindgut.
2 to 9 weeks: In vertebrates, embryos are cylindrical bodies {neurula}, with ectoderm layer on outside reaching over notochord, mesoderm layer, and inner endoderm tube.
2 to 9 weeks: Ventral-body-wall constriction makes a tube {umbilical cord}| {umbilicus} come out from belly. Tube has outer ectoderm, middle mesoderm, and inner endoderm.
4-Zoology-Development-Stages-Gestation
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Date Modified: 2022.0225