horizon /hôrī″zən/ [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.
