horizon(redirected from apparent horizons)
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a specific anatomic stage of embryonic development, of which 23 have been defined, beginning with the unicellular zygote (fertilized egg) and ending 7 to 9 weeks later with the beginning of the fetal stage.
Etymology: Gk, horizein, to encircle
a specific stage of human embryonic development determined by the appearance and ultimate formation of certain anatomical characteristics. The classification comprises 23 stages, each lasting 2 to 3 days, beginning with the fertilization of the ovum and ending 7 to 9 weeks later with the initiation of the fetal period of intrauterine life.