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158 PILLAR 2 Development and LearningThe Beginnings of Life10-1 How do humans grow from single cells into newborns?If you are female, you were born with all the egg cells you%u2019ll ever have, although they were immature when you were just a baby. Only about 1 of every 5000 egg cells will mature and be released from an ovary. If you are male, you did not begin producing sperm cells until you reached puberty, but you%u2019ll keep producing them until you die. Although the rate slows with age, men produce about 2 million sperm cells in the 30 minutes it takes to eat lunch, a rate of about 1000 per second.Prenatal DevelopmentPrenatal literally means %u201cbefore (pre-) birth (-natal ).%u201d The prenatal stage of development starts at conception and ends at birth.A newly fertilized egg is called a zygote. Fewer than half of all zygotes make it past the first 14 days after being fertilized.1,2 Luckily, you and I survived that risky period. A lot happens during those 2 weeks. First, the singlecell zygote begins to divide into identical cells; one cell becomes two, and two cells become four. In the first week, after dividing about seven times, the cells start to differentiate, to specialize in function. For example, some cells start to develop into a brain, and others start to develop into lungs or a heart. Genes%u2014the biochemical units of heredity%u2014direct this process.Around the tenth day, the zygote attaches to the mother%u2019s uterine wall, where it will stay for about 37 weeks. But when the developing human organism passes the 14-day milestone, it is no longer known as a zygote; it is now an embryo, the developing human organism from about 2 weeks after fertilization through the end of the eighth week (Figure 10.1).The embryo has a noticeable heartbeat and red blood cells produced by its own liver. Most of the body%u2019s major organs will have formed by the end of the embryonic stage.At 9 weeks, the developing organism enters the fetal period. A fetus is the developing human organism from 9 weeks after conception to birth. It is unmistakably zygote A fertilized egg.genes The biochemical units of heredity that make up chromosomes.embryo A developing human organism from about 2 weeks after fertilization through the end of the eighth week.fetus A developing human organism from 9 weeks after conception to birth.Circle of Life%u201cOf all the joys that brighten suffering earth, what joy is welcomed like a newborn child?%u201d%u2014Caroline Norton (1808%u20131877).3 weeks 4 weeks 9 weeksFIGURE 10.1 Nine Weeks After ConceptionAt 9 weeks, the developing organism becomes a fetus and is unmistakably human in form. Facial features, hands, and feet are visible at this time.KidStock/Blend Images/Getty Images SciePro/SPL/Science Source Biophoto Associates/Science Source DEA/L. RICCIARINI/Getty Images%u00a9 Bedford, Freeman & Worth Publishers. For review purposes only. Do not distribute.