4 weeks: Forebrain telencephalon and diencephalon, midbrain, and hindbrain pons and medulla differentiate {brain divisions}.
4 weeks: Embryo floats in amniotic sac, connected to uterus wall only by umbilical cord. Embryo has large head, gills, tail, somites on notochord sides, and beating heart {head and heart development}.
4 weeks: Guided by chemical or electrical signals with no learning, brainstem and spinal-cord ventral motor cells grow axons to trunk, limb, and viscera muscles {motor cell and movement}. Fetal birds and mammals have varied movements, even before sense nerves appear.
4 weeks: Embryo has mesoderm muscle precursors {somite}| on notochord sides.
4-Zoology-Development-Stages-Gestation
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Date Modified: 2022.0225