Describe the significant changes in germinal, embryonic, and fetal stages of prenatal development
What will be an ideal response?
The germinal stage is the first stage of prenatal development and refers to the 2-week period following conception. Once the ovum has been fertilized, it is called a zygote, which is a single cell that is smaller than the dot in the letter i. The zygote begins a process of repeated division and, after about a week, consists of about 150 cells. After 2 weeks, it has become a mass of cells and attaches itself to the wall of the uterus. Once the zygote is implanted, or attached to the wall of the uterus, the embryonic stage begins.
The embryonic stage is the second stage of the prenatal period and spans the 2 to 8 weeks that follow conception; during this stage, cells divide and begin to differentiate into bone, muscle, and body organs. At about 21 days after conception, the beginnings of the spinal cord and eyes appear; at about 24 days, cells differentiate to form what will become part of the heart; at about 28 days, tiny buds appear that will develop into arms and legs; and at about 42 days, features of the face take shape. Toward the end of the embryonic stage, the organism has developed a number of body organs, such as the heart. The embryo is only about 4 cm long but already has the beginnings of major body organs and limbs and begins to look somewhat human.
The fetal stage, which is the third stage in prenatal development, begins 2 months after conception and lasts until birth. At the end of the fetal stage, usually 38 to 42 weeks after conception (or roughly 9 months), birth occurs and the fetus becomes a newborn. During the fetal stage, the fetus develops vital organs, such as lungs, and physical characteristics that are distinctively human. For example, at about 6 months, a fetus has eyes and eyelids that are completely formed, a fine coating of hair, relatively well-developed external sex organs, and lungs that are beginning to function.
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